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LiteX for Hardware Engineers
This is an introduction to LiteX written by and for hardware engineers who have experience designing FPGAs using Verilog and Vivado.
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Experienced Python engineers can skip this and read the source code for documentation. |
LiteX is a Python "front-end" that generates Verilog netlists, and drives proprietary build "back-ends", such as Vivado or ISE, to create bitstreams ("gateware") for FPGAs.
LiteX is relies on a Python toolbox called Migen. In addition to a build environment, it provides a set of IP blocks. Some of the IP blocks include a DDR2/3 MIG equivalent, various softcore CPUs (lm32, openrisc, RISCV), Ethernet controller, HDMI input/output, Wishbone routing fabrics, streams, and PCI express.
LiteX naively supports Linux/x86. It requires Python3.5 or later. You’ll need to manually download, install, and provision your back-end tools (e.g.: Vivado/ISE), and you’ll also need to install a gcc cross-compiler to any softcore CPU you plan to use in your designs. Details later.
Here’s the design flow in a nutshell:
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Describe your design in Python using the migen toolbox and LiteX IP by customizing a
Moduleobject (typically by subclassingSoCSDRAM, which is a subclass ofSoCCorewhich subclasses the baseModuleclass) -
Describe your build environment by customizing a
Platformobject (typically by subclassing theXilinxPlatformclass which itself sublcasses theGenericPlatformbase class) -
Run a function which passes your
Platformobject to aBuilderobject, and invokes thebuild()method which:-
Creates a
top.vfile: a single, flat verilog netlist of your entire design modulo a few exceptions to be noted later. -
Creates a
top.xdcfile: constraints that locate pins, defines clocks, and eliminates false paths -
If a CPU is configured, generates and builds a BIOS binary to be compiled into the design
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If a toolchain is configured, creates a
top.tclfile which drives the proprietary synth/place/route/bitgen "backend" toolchain -
Attempts to run the proprietary back-end tool (Vivado will be assumed for this doc, but ISE is also supported)
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Run
makein thefirmwaredirectory, which builds your firmware binary (firmware.bin). -
Upload
top.bitto the FPGA — typically over JTAG via openOCD -
Upload
firmware.binto the FPGA — typically via UART or Ethernet, using thefltermhost-native application and theserialbootcommand -
Interact with your firmware’s REPL loop using flterm
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If you designed a
litescopeinto your design (an ILA like Chipscope), configure triggers and download traces using an analyzer script, which relies on a helper program called litex_server. Debugging occurs either through a supplementary UART or Ethernet that must be present in the hardware (either designed in or test leads connected to a header). -
Find bugs & go back to step 1!
LiteX-buildenv attempts to automate steps 3 and onwards. However, I don’t use the master script, it’s a bit too brittle yet for reliable development, so I tend to run each of the major steps one command at a time.
It’s extremely helpful to use a very featureful Python editor when coding with LiteX/migen. Just trying to code in a basic text editor will drive you nuts. I was introduced to PyCharm, and I would strongly recommend it if you don’t already have a preferred editor. In particular you want to be able to "push" into object declarations by control-clicking on the name and method name autocompletion is extremely helpful when groping around for signal names.
Furthermore, Python has several key limitations, a major one being package management. Namely, it’s not a native feature of the language. Just running setup.py on various eggs will toss all your packages into your system dist-packages directory, which can break other dependencies in your Linux system. If you ask five Python developers how to deal with this, you’ll get five different answers. Litex-buildenv I think tries to use Conda to get around this. Just be aware there’s some weird stuff going on here that’s totally obvious to the package maintainer and if you don’t match your assumptions to theirs you’ll be blindsided by a problem down the road.
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Python3 is inherently nondeterministic. This is a "security feature" which causes, among other things, dictionaries and hash iterators to be visited in a different order every time a script is run. This means your verilog netlist, register space addressing, and so forth will change with every run. Some think this is a feature, I think this is a bug. I work around this by setting the PYTHONHASHSEED variable to a fixed value, and checking the setting within the Python script. It’s not possible to change or set the variable once the program is started. You can only check it. |
One tricking bit on going to Python that always trips me up is bit ordering and array indices, so it’s getting a section of its own near the top.
Array indices in Python are foo[offset:offset+length]. So if you want to select the bottom 10 bits of a signal, you do a_python[:10] instead of a_verilog[9:0] (an empty spot has implicit meaning in Python it seems). The next 10 bits is b_python[10:20], instead of b_verilog[19:10]. You can’t "reverse" MSB/LSB in Python like you can in Verilog by changing the index order in the brackets.
This is known as the standard Python "slicing" syntax - https://www.pythoncentral.io/how-to-slice-listsarrays-and-tuples-in-python/
Smaller bit-width Signal()`s can be combined together using the `Cat() function.
Note that Cat() combines from LSB-to-MSB order (opposite of verilog), as follows:
foo = Signal(7)
bar = Signal(2)
baz = Signal()
self.comb += [
foo.eq(Cat(0, 0, bar, 0, baz, 1)),
]This is the verilog equivalent of:
wire[6:0] foo;
wire[1:0] bar;
wire baz;
assign foo = {1'b1, baz, 0, bar[1:0], 1'b0, 1'b0};Migen is the Python toolbox that is used to create a description of your hardware design. It abuses the Python’s object-oriented class and method system to create a design tree embodied as a single mega-object using a process called "finalization".
For design description, the base class is a "Module". It has five key attributes used to organize the elements that describe any hardware design:
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Comb
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Sync
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Submodules
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Specials
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ClockDomains
Each of these attributes is a list, and a design is described by appending an element to the appropriate list. Once all the lists have been populated, the submodules are collected and then finalized into a single, huge verilog netlist.
The elements that go into a design description are numerous, but the
most common one you’ll encounter is Signal(), followed distantly by
ClockDomain() and Instance().
A Signal(), as its name implies, is a named net. By default, a Signal()
has a bit width of 1. An n-bit signal is created by Signal(n). Groups of
Signals() can be bundled together in Records() and Streams(), more on
that later. A Signal() has no inherent direction, clock domain, or
meaning. It picks this all up based on how you use it: which attribute
of the Module class you’ve assigned it to, and so forth.
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In Migen, you can specify the number of bits for a Signal() by doing Signal(max=8). Counter to what you think it should do, max is exclusive (but min is inclusive), so Signal(max=8) cannot actually hold the number 8. It can hold the number 7, because it will infer only three bits for the directive max=8. I was not in the room when this was decided.
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Dangling references and signal name typos will result in Python crashes during the finalization process, and you are expected to dig through the stack trace to find the problem. Because finalization disassociates the locality of a reference from the code that created it, this can be difficult. It helps to have an IDE with a debug mode that allows you to click into a stack level and then inspect variables at various stack levels to try and identify where you went wrong. |
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Forgetting to add new objects to one of the five key Module attributes means it will not be finalized. Thus, you might see your logic in the Python code, but if it doesn’t get finalized, it might as well just be commented out as far as migen is concerned. |
Let’s look at what each of these five attributes are, one at a time.
The comb attribute is a list of "combinational" logic
operations. The verilog equivalent is everything that occurs outside an
always @(posedge) block, e.g. all your assign statements. Since comb is
a list, you append operations onto the list using Python list syntax.
self is a shortcut to your module object, and .comb is how you
reference the comb attribute:
foo = Signal() # these are all one-bit wide by default
bar = Signal()
baz = Signal()
mumble = Signal()
self.comb += [
foo.eq(bar),
baz.eq(foo & mumble), # trailing commas at the end of a list are OK in python
]This is the verilog equivalent of:
wire foo;
wire bar;
wire baz;
wire mumble;
assign foo = bar;
assign baz = foo & mumble;You’ll notice that there’s no = operator — assignment (and thus
declaration of which signal in the source and sink) is done by invoking
.eq() on the sink and putting the source as the argument for a signal.
However, most arithmetic operations are available between Signals, e.g.
~ is invert, & is and, | is or, + is add, * is multiply. I think there’s
also divide and I have no idea about signed types.
The sync attribute is the list of synchronous operations.
Items added to this list will generally infer a clocked register.
"But to what clock domain?" I hear you ask. Migen starts with a
single, default clock domain called sys. Its frequency is defined by
passing a mandatory clk_freq argument to the SoCSDRAM base class,
and it’s up to you to actually hook up a clock generator that is at the
right frequency.
You can also specify which clock domain you want registers to go to by
adding a modifier to the sync attribute. The migen methodology
prescribes not assigning a clock domain until a module is
instantiated. So if a sub-module’s design can be implemented in a
single, synchronous domain, just use the generic sync attribute. If
the sub-module requires two clock domains, it’s actually recommended to
make up a "descriptive" name for the module, such as write and
read clock domains for a FIFO. Then, when the modules are created,
the all the clocks can be renamed to be consistent with the
instantiating-module level clock names using a function called
ClockDomainsRenamer().
Clear as mud? Some examples will help.
foo = Signal()
bar = Signal()
bar_r = Signal()
self.sync += [
bar_r.eq(bar),
foo.eq(bar & ~bar_r),
]This is the verilog equivalent of
wire bar;
reg foo = 1'd0; // yes, the autogen code will use decimal constants
reg bar_r = 1'd0;
always(@posdege sys_clk) begin
bar_r <= bar;
foo <= bar & bar_r;
endAgain, sys_clk is implicit because we used a "naked" self.sync. And,
note that the "zero" initializer of every register is part of the
migen spec (so if you forget to hook up an input to an output, you get
zeros injected at the break and no warnings or errors thrown by the
verilog compiler).
If you wanted to do two clock domains, you might do something like this:
class Baz(Module):
def (self):
foo = Signal()
bar_r = Signal()
bar_w = Signal()
self.sync.read += bar_r.eq(foo) # when adding just one item to the list, you can use +=
self.sync.write += bar_w.eq(foo)This is the verilog equivalent of
wire foo;
reg bar_r = 1'd0;
reg bar_w = 1'd0;
always(@posedge read_clk) begin
bar_r <= foo;
end
always(@posedge write_clk) begin
bar_w <= foo;
endEasy enough, but where does read_clk and write_clk come from? Notice how
I encapsulated the Python in a module called Baz(). To assign them in an
upper level function, do this:
mybaz = Baz()
mybaz = ClockDomainsRenamer( {"write" : "sys", "read" : "pix"} )(mybaz)
self.submodules += mybaz # I'll describe why this is important later, but it's IMPORTANTWhat’s happened here is the the write domain of this instance of
Baz() got assigned to the (default) sys_clk domain, and the read
domain got assigned to a pix_clk domain (which presumably, you’ve
created in the ClockDomains attribute, more on how to do that later). As
you can see here, the ClockDomainsRenamer lets us go from the local
names of the function to the instance names used by the actual design,
based on a Python dictionary that has the format {"submodule1_clock"
: "actual1_clock", "submodule2_clock" : "actual2_clock", …}.
The final re-assignment of mybaz to mybaz isn’t mandatory, but since you
never want to use the original instance of it, it’s helpful to discard
any possibility of confusing yourself with the old an new versions by
re-assigning the modified object to its original name.
There’s one other trick for ClockDomainsRenamer. Quite often you’re
looking to actually rename the default sys clock to something else,
because most modules are written just adding items to the base sync
domain (and hence the default sys clock domain) This leads to this
shortcut:
myfoo = Foo()
myfoo = ClockDomainsRenamer("pix")(myfoo)
self.submodules += myfooThe one argument is automatically expanded by the ClockDomainsRenamer to
the dictionary {"sys":"lone_argument_clk"}.
Noticed how above, I was particular to include a line
self.submodules += myfoo or similar at the end of every example?
This has to do with the submodules attribute.
Designs can be hierarchical in migen. That’s a good thing, but you have
to tell migen about the submodules, or else they don’t do anything. You
tell migen about a submodule — and thus include it for flattening and
netlisting — by adding it to the submodule attribute. Forgetting to do
so will silently fail, throwing no errors and leaving you wondering why
the submodule you thought you included is outputting nothing but 0.
Here’s a simple example:
myfoo = Foo()
myfoo = ClockDomainsRenamer("pix")(myfoo)
self.submodules += myfooversus
myfoo = Foo()
myfoo = ClockDomainsRenamer("pix")(myfoo)What’s the difference? In the first one, we remembered to add our module to the submodules list. In the second one, we created the submodule, did something to it, but didn’t add it to the submodules list.
The second one is perfectly valid Python syntax; it will compile and
run, and the verilog generated will throw no errors, but if you look at
the netlist, the entire contents of the myfoo instance is missing
from the generated netlist.
In other words, it’s extremely easy to forget to add something to the submodules list, and forgetting to do so means the submodule is never flattened during the build process and thus never sent to the code generator. And because migen initializes all registers to 0, the absence of the module will result in perfectly valid verilog being generated that throws no errors.
So I try to include that line in every example, even the short ones, to save you the headache and trouble.
One other confusing bit about adding something to submodules is that
later references go through self. Easier to see code than explain:
self.submodules.myfoo += Foo()
self.comb += self.myfoo.subsignal.eq(othersignal)In the example above, you added Foo() to submodules.myfoo, but later on
you /reference/ it through self.myfoo.
Specials are how migen handles certain design elements that don’t fit into the comb/sync paradigm or have to pierce the abstraction layer and do something platform or implementation-specific.
On the Xilinx platform, these are the specials I’m aware of:
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Instantiating a verilog module or primitive
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MultiReg
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AsyncResetSynchronizer
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DifferentialInput
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DifferentialOutput
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Memory
You might be tempted to stick a special in the submodules attribute,
but that won’t work because their template class is Special, not Module.
Like all the other attributes, you add to a special by just using the +=
pattern:
self.specials += MultiReg(consume.q, consume_wdomain, "write")
self.specials += Instance("BUFG", i_I=self.pll_sys, o_O=self.cd_sys.clk)The Instance special is particularly handy. You
use this to summon blocks like BUFG, BUFIO, BUFR, PLLE2, MMCME2 and
so forth. The format of an Instance special is as follows:
Instance( "VERILOG_MODULE_NAME", ...list of parameters or ios.... )So if a verilog module has a template like this:
foo #(
.PARAM1("STRING_PARAM"),
.PARAM2(5.0)
)
foo_inst(
.A(A_THING), // output: A
.B(B_THING), // input: B
.C(C_THING), // inout: C
);The Instance format would look like this:
migen_sigA = Signal()
migen_sigB = Signal()
migen_sigC = Signal()
self.specials += [
Instance("foo",
p_PARAM1="STRING_PARAM",
p_PARAM2=5.0,
i_A=migen_sigA,
o_B=migen_sigB,
io_C=migen_sigC
),
]If you’re looking to instance a module that’s your own verilog and not part of the Xilinx primitives, you can add the verilog file with a platform command:
self.platform.add_source("full/path/to_module/module1.v")This leaves the module heirarchy intact, and you also have to add all submodules referenced by your verilog to the path as well.
Specials can also be used to refer to vendor IP. eg.
# ila
platform.add_source("ila_0/ila_0.xci")
probe0 = Signal(6)
self.comb += probe0.eq(Cat(spi_pads.clk, spi_pads.cs_n, spi_pads.wp, spi_pads.hold, spi_pads.miso, spi_pads.mosi))
self.specials += [
Instance("ila_0", i_clk=self.crg.cd_sys.clk, i_probe0=probe0),
]
platform.toolchain.additional_commands += [
"write_debug_probes -force {build_name}.ltx",
]MultiReg is a one-bit synchronizer for crossing
asynchronous domains. By default, it creates two registers that go into
a sys clock domain, but you can change which domain it goes to by
specifying an odomain parameter:
self.specials += MultiReg( input_domainA, output_domainB, "pix" )Will take signal input_domainA, instiate two registers in the pix
domain, and the output_domainB will be synchronized accordingly. The
reason this is in a special block is there are some attributes added to
prevent retiming optimization from modifying the synchronizer structure:
presumably if you did this just using self.sync operations you might not
get the expected outcome after optimizations.
Migen includes a whole bunch of clock-domain crossing tools, including a
PulseSynchronizer and Grey counters. Take a look inside the
migen/genlib/cdc.py file for some ideas.
Migen supports a native syntax for creating FSMs. You can create an FSM in the current module by invoking the FSM() function, and then using .act() accessors to delineate new states within the FSM. Here’s a basic example of how this works.
fsm = FSM()
self.submodules.fsm = fsm # need this to enable litescope debugging
fsm.act("WAIT_SOF",
reset_words.eq(1),
If(self.address_valid &
self.frame.sof,
NextState("TRANSFER_PIXELS")
)
)
fsm.act("TRANSFER_PIXELS",
self.transfer_enable.eq(1),
If(self.address_count == self.frame_length,
NextState("EOF")
)
)
fsm.act("EOF",
If(~dram_port.wdata.valid,
NextState("WAIT_SOF")
)
)This FSM creates three states, WAIT_SOF, TRANSFER_PIXELS, and EOF, and cycles between them based on the conditions coded in the If() statements.
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Direct vs. NextValue There are two ways to set outputs within an FSM: direct, and NextValue(). The example above only uses the direct method. Direct value settings clear to zero in every state where they are not mentioned. Thus, the statement "self.transfer_enable.eq(1)" inside of "TRANSFER_PIXELS" only sets transfer_enable to 1 during that state only, and in all other states, the value is zero. Furthermore, transfer_enable is a result of an purely combinational computation on the state bit, so it changes to 1 upon entering TRANSFER_PIXELS. Alternatively, one can use NextValue(signal, value), as in, "NextValue(self.transfer_enable, 1)". This macro does two things: (1) the value is latched, so it persists even after leaving the state (it does not clear to zero unless explicitly cleared) and (2) the value will not change until one clock edge later. |
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It appears one can use both direct and NextValue at the same time without triggering a compilation error. The behavior, however, depends on if there is a previous NextValue() that could conflict with the direct setting. Which one "wins" depends upon the ordering of the two statements in the top.v generate by Migen. At least in one version, the synchronous statements are lower in the file than the asynchronous statements, and thus the NextValue() call would override the direct call in the case of a conflict. However, if there is no conflict between NextValue and direct, this will cause the value to change upon entering the state and it will persist until cleared. |
By convention, the first FSM.act() entry is also the reset state of the FSM. This is because as far as I can tell the state bits are encoded staring from 0 going up with each successive FSM.act() call, and FPGAs by default initialize their registers to 0. If you want to explicitly designate a reset state, use the "reset_state=" argument when creating the FSM object, e.g.:
fsm = FSM(reset_state = "WAIT_SOF")The default clock domain of an FSM is, as always, "sysclk". You can remap this using the ClockDomainsRenamer:
fsm = ClockDomainsRenamer("new_clk_domain")(FSM())Alternatively if you want the entire module to be synchronous and in a different domain, don’t rename the FSM immediately upon creation, but rename the entire module at the point where it is instantiated (e.g. allow all the self.sync’s to be default (sysclk) and then remap sysclk for the whole domain using the ClockDomainsRenamer at one level up the tree).
To be written — how to use Vivado to view timing reports and schematics.
Configuration and status registers are how you get a softcore to "peek" and "poke" memory. They map addresses to lines that you can wiggle or observe.
The nomenclature of migen is:
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"CSRStorage" = "output" (from CPU’s perspective) = "write" or "stores"
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"CSRStatus" = "input" (from CPU’s perspective) = "read" or "loads"
There’s also a "generic" CSR which is both read and write. You can use this, but the width is limited to less than the CSR bus width.
You can add CSRs to modules (but not the top level SoC instantiation), because CSR C-code APIs are auto-generated based on the module’s name. No name, no API.
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CSRs are a bit odd, by default they are byte-wide registers that are on 32-bit word boundaries. So a "32-bit" CSR is actually broken into four bytes spanning a total address space of 16 bytes. You can zpecify 32-bit wide CSRs but you’ll probably run into compatibility issues with other IP librariers that have hard-coded the 8-bit assumption. |
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If you allocate too many CSRs, you can overflow the CSR address space width without warning. If you find your CPU isn’t booting after a recompile, try adding the line "csr_address_width=15" to your BaseSoC arguments. The default width is 14 bits. |
Here’s a very simple example of how to use CSRs to talk to an external IP block written in verilog.
class I2Csnoop(Module, AutoCSR):
def __init__(self, pads):
self.edid_snoop_adr = CSRStorage(8)
self.edid_snoop_dat = CSRStatus(8)
reg_dout = Signal(8)
self.An = Signal(64)
self.Aksv14_write = Signal()
self.specials += [
Instance("i2c_snoop",
i_SDA=~pads.sda,
i_SCL=~pads.scl,
i_clk=ClockSignal("eth"),
i_reset=ResetSignal("eth"),
i_i2c_snoop_addr=0x74,
i_reg_addr=self.edid_snoop_adr.storage,
o_reg_dout=reg_dout,
o_An=self.An,
o_Aksv14_write=self.Aksv14_write,
)
]
self.comb += self.edid_snoop_dat.status.eq(reg_dout)Other sections talk more about using self.specials to create an external verilog block, but basically, there is a verilog module called i2c_snoop.v that’s instantiated here, and the CPU is wired up to the snoop module to query what data has been captured by the snooper from a given address. So, edid_snoop_adr is a CSRStorage(8) — it’s an "output" of the CPU that’s 8 bits wide driving into the verilog block. And edid_snoop_dat is a CSRStatus(8) — it’s an "input" of the CPU that’s 8 bits wide that reads the data presented by the verilog block. Note that all signals are assumed synchronous to the "sys" clock domain, but in this case i2c_snoop is plugged into the "eth" clock domain. For this purpose, it’s OK because we guarantee at the firmware level we don’t read the I2C block when the data is changing, but you will need to add MultiRegs or other forms of synchronizers if whatever you’re driving from the CPU isn’t in the "sys" clock domain.
In order to trigger the auto-generation of the CSR code, you have to add it to the csr_peripherals block of your SoC. This is usually up near the top of your SoC definition, a bit like this:
class VideoOverlaySoC(BaseSoC):
csr_peripherals = [
"i2c_snoop", # if this doesn't exist, the APIs won't get generated
"analyzer",
]
csr_map_update(BaseSoC.csr_map, csr_peripherals)
def __init__(self, platform, *args, **kwargs):
BaseSoC.__init__(self, platform, *args, **kwargs)
platform.add_source(os.path.join("overlay", "i2c_snoop.v"))
self.submodules.i2c_snoop = i2c_snoop = I2Csnoop(hdmi_in0_pads) # the submodule name here must match the csr_peripherals stringYou’ll end up getting a set of CSR helper functions located in the csr.h file. You want to use the helper functions because they hide the wart CSR space being byte-wide data strided on word boundaries.
/* i2c_snoop */
#define CSR_I2C_SNOOP_BASE 0xe000b000
#define CSR_I2C_SNOOP_EDID_SNOOP_ADR_ADDR 0xe000b000
#define CSR_I2C_SNOOP_EDID_SNOOP_ADR_SIZE 1
static inline unsigned char i2c_snoop_edid_snoop_adr_read(void) {
unsigned char r = MMPTR(0xe000b000);
return r;
}
static inline void i2c_snoop_edid_snoop_adr_write(unsigned char value) {
MMPTR(0xe000b000) = value;
}
#define CSR_I2C_SNOOP_EDID_SNOOP_DAT_ADDR 0xe000b004
#define CSR_I2C_SNOOP_EDID_SNOOP_DAT_SIZE 1
static inline unsigned char i2c_snoop_edid_snoop_dat_read(void) {
unsigned char r = MMPTR(0xe000b004);
return r;
}
///// included here to illustrate the CSR space byte-to-word weirdness
#define CSR_HDMI_IN1_DMA_SLOT1_ADDRESS_ADDR 0xe00088f8
#define CSR_HDMI_IN1_DMA_SLOT1_ADDRESS_SIZE 4
static inline unsigned int hdmi_in1_dma_slot1_address_read(void) {
unsigned int r = MMPTR(0xe00088f8);
r <<= 8;
r |= MMPTR(0xe00088fc);
r <<= 8;
r |= MMPTR(0xe0008900);
r <<= 8;
r |= MMPTR(0xe0008904);
return r;
}
static inline void hdmi_in1_dma_slot1_address_write(unsigned int value) {
MMPTR(0xe00088f8) = value >> 24;
MMPTR(0xe00088fc) = value >> 16;
MMPTR(0xe0008900) = value >> 8;
MMPTR(0xe0008904) = value;
}With these helper functions, dumping the memory space of the I2C snooper is quite easy:
int i ;
for( i = 0; i < 256; i++ ) {
if( (i % 16) == 0 ) {
wprintf( "\r\n %02x: ", i );
}
i2c_snoop_edid_snoop_adr_write( i );
wprintf( "%02x ", i2c_snoop_edid_snoop_dat_read() );
}In addition to providing convenient APIs on the C-code firmware side, CSRs also provide some convenience on the hardware Python side.
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You can specify the reset value by passing the reset=value parameter (for both Storage and Status)
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.reis a "write" strobe — the .re attribute provides a single-cycle pulse when a CSRStorage is updated -
.weis a "read" strobe — the .we attribute provides a single-cycle pulse when a CSRStatus is read -
yes, you read those two lines above correctly. in Litex, .re is a write, and .we is a read. I was not in the room when this was decided.
-
if write_from_dev=True is passed as a parameter to CSRStorage, the device can flip the storage bit (allowing it to work as an input, oddly enough), by providing data on .dat_w, and strobing .we. Difference between this and CSR is reads are not guaranteed atomic when CSRStorage is made writeable.
If you’re using a straight-up CSR (not a Storage or Status), the accessors for the stored value is the .r attribute, and the data you’re sending back to the CPU is connected via the .w attribute.
Interrupts are generated using the EventManager module. There’s a few ways to use it, but here’s one of the most straightforward methods I know of.
To add an interrupt to a module, you will need an EventManager() submodule, plus one or more EventSourcePulse(), EventSourceProcess(), or EventSourceLevel() modules.
EventSourcePulse() is a rising-edge triggered event. When a rising edge comes in, the corresponding .pending bit is set high. Write a 1 to .pending to clear the edge triggered event.
EventSourceProcess() is a falling-edge triggered event. When a falling edge comes in, the corresponding .pending bit is set high. Write a 1 to .pending to clear the edge triggered event.
EventSourceLevel() is a level-sensitive event. The CPU continues to receive the level-sensitive interrupt until the source causing the event is rectified (there is no "clear event" option — if you don’t lower the level, the CPU will jump right back into the ISR once you exit).
Each EventSourceXXX() module is capable of taking in a trigger that results in an interrupt being dispatched to the CPU. The Python code looks a bit like this.
class MyModule(Module, AutoCSR):
def __init__(self):
self.submodules.ev = EventManager()
self.ev.my_int1 = EventSourceProcess()
self.ev.my_int2 = EventSourceProcess()
self.ev.finalize()
self.comb += self.ev.my_int1.trigger.eq(falling_edge_interrupt_signal1)
self.comb += self.ev.my_int2.trigger.eq(falling_edge_interrupt_signal2)
class MySoC(BaseSoC):
interrupt_map = {
"my_module" : 4,
}
interrupt_map.update(BaseSoC.interrupt_map)
def __init__(self, platform, *args, **kwargs):
self.submodules.my_module = my_module = MyModule()This creates a module my_module which occupies a single interrupt vector (4) on the CPU with two sub-events that can be read out and handled by the firmware code.
In the firmware, first you must add an ISR dispatch to your ISR table. There’s typically a file called isr.c that has something like this in there:
void isr(void)
{
unsigned int irqs;
irqs = irq_pending() & irq_getmask();
if(irqs & (1 << UART_INTERRUPT))
uart_isr();
#ifdef MY_MODULE_INTERRUPT
if(irqs & (1 << MY_MODULE_INTERRUPT))
my_module_isr();
#endif
}It seems at least on lm32 and vexrisc SoC’s, there’s just a single interrupt line to the CPU, and this expands to one of 32 bits in an interrupt source register. This maps to the interrupt_map number provided in the Python code. The isr() routine is thus responsible for searching through the bits and dispatching accordingly.
You also want to enable the interrupt, in some sort of init function:
void my_module_init(void) {
// unmask the interrupts for MY_MODULE
unsigned int mask;
mask = irq_getmask();
mask |= 1 << MY_MODULE_INTERRUPT;
irq_setmask(mask);
my_module_ev_enable_write(1); // in addition to unmasking irq, you also need to enable the event handler
}Handling the isr itself looks a bit like this:
void my_module_isr(void) {
unsigned int status;
status = my_module_ev_pending_read(); // you don't need to do this if you just have one interrupt source
// my_module_ev_pending_write(1); // You'd do this if you just had one interrupt
if( status & 1 ) {
printf("Hi! I got interrupt 1\n");
my_module_ev_pending_write(1); // clear the interrupt so it doesn't keep on firing and wedge the CPU
} else if( status & 2 ) {
printf("Hi! I got interrupt 2\n");
my_module_ev_pending_write(2);
}
my_module_ev_enable_write(1); // re-enable the event handler so we can catch the interrupt again
}To be written — simple walk-through of the basic stuff needed to implement an lm32 CPU with a clock generator
Memory is a special in migen, as in, it doesn’t fit into self.comb
or self.sync, and requires platform-specific code to support. This one
actually has some documentation in the Migen manual
(https://m-labs.hk/migen/manual/fhdl.html#memories), but we expand
upon it here with examples.
The Memory() function takes the following arguments:
-
width (mandatory) - width in bits
-
depth (mandotory) - depth in width-wide units (so width=32, depth=16 is 64 bytes of memory)
-
init (optional) - an array of numbers which sets the initial value of the memory
-
name (optional) - override the default name of "mem" for the memory
For example, this will make a 16-entry, 32-bit wide memory:
self.specials.mem = Memory( 32, 16 )If you just included this one line of code in your design, it creates a memory object that has an implicit and automatically generated read/write interface to the CPU via wishbone, sharing a portion of CSR register space. You should find a reference to the memory inside build/software/include/generated/csr.h, and the memory will be automatically placed in the lowest CSR bank.
This is pretty handy if you just want some scratchpad ram for your CPU, but more typically you want to interact with the memory from your own hardware. There’s three typical configurations for this:
-
Wishbone r/w, local logic read-only
-
Wishbone read-only, local logic write-only
-
No wishbone interface, just local logic read/write
Basically, you’re allowed just one write port without also inferring an arbiter. Although the RAMB36E1 primitive supports two write ports, the verilog template used to infer them is rejected by Vivado (as of 2018.2).
In general, to gain access to a Memory object from logic, you will
call the get_port() function on the logic. This returns a "port"
object, which contains the signals you need to connect to the memory.
Memory.get_port() takes the following arguments:
-
adr
Signal(aw): input address to the RAM. Up to the caller to guarantee correct width. -
dat_r
Signal(dw): output data from the RAM. Up to the caller to guarantee correct width. -
write_capable boolean (default: False) - set to True if you need the port to have write capability.
-
async_read boolean (default: False) - The master docs claim this would infer an asynchronous read port, but block RAM doesn’t support this. So perhaps this uses distributed RAM, which would make it resource-inefficient?
-
has_re boolean (default: False) - adds a read clock enable, ignored for asynchronous ports
-
we_granularity integer (default: 0) - infer write enable masks. The width of the "we" signal grows automatically to be data width / we_granularity. If zero, the "we" signal will write the entire word width.
-
mode range (default: WRITE_FIRST) - In synchronous memory, defines behavior of simultaneous read/writes. One of WRITE_FIRST, READ_FIRST, NO_CHANGE. WRITE_FIST means the simultaneously written value is returned. READ_FIRST means the previous value is returned. NO_CHANGE means that during writes, the data signal holds its previous value. Presumably if the address changes during a write the read value would not update to reflect the new address.
-
clock_domain clockstring (default: "sys"): clock domain of the returned port object.
The returned port object has the following signals:
-
self.adr
Signal(aw)implicit: the adr argument -
self.dat_r
Signal(dw)implicit: the dat_r argument -
self.we
Signal(dw/we_granularity)implicit: write enable. Width depends on the we_granularity parameter, defaults to a single bit. -
self.dat_w
Signal(dw)implicit: data to write to RAM -
self.re
Signal()implicit: read enable signal, if synchronous
This design pattern is fairly straightforward. Create the Memory() object,
call get_port() on it with no arguments to create a MemoryPort() object,
and then acess the port using the .adr and .dat_r Signals().
self.specials.mem = Memory( data_width, depth )
self.specials.rdport = self.mem.get_port()
self.comb += self.rdport.adr.eq( some_address_signal )
self.comb += data_destination.eq( self.rdport.dat_r )This creates a memory object in the CSR space that the CPU can write data to, and then your local logic can retrieve parameters out of this memory as needed by indexing into the address.
This design pattern is a bit more complicated. Here, we fail to
add Memory() as a "specials", but just keep the variable local
to the function. This prevents it from being caught in the CSR
"finalizer" sweep, so no wishbone CSR port is inferred, clearing
the way for you to add your local logic port.
Here’s how you would make the memory in the logic module:
class Logic_with_memory(Module):
mem = Memory( data_width, depth )
self.specials.port = mem.get_port( write_capable=True )
# connect the write port to local logic
self.comb += [
self.port.adr.eq( local_adr ),
self.port.dat_w.eq( logic_write_data ),
self.port.we.eq( logic_write_enable_signal )
]
# attach a wishbone interface to the Memory() object, with a read-only port
self.submodules.wb_sram_if = wishbone.SRAM(mem, read_only=True)
# get the wishbone Interface accessor object
self.bus = wishbone.Interface()
# build an address filter object. This is a migen expression returns 1
# when the address "a" matches the RAM address space. Useful for precisely
# targeting the RAM relative to other wishbone-connected objects within
# the logic, e.g. other RAM blocks or registers.
#
# This filter means the RAM object will occupy its entire own CSR block,
# with aliased copies of the RAM filling the entire block
def slave_filter(a):
return 1
# This filter constrains the RAM to just its own address space
decoder_offset = log2_int(sram_depth, need_pow2=False)
def slave_filter_noalias(a):
return a[decoder_offset:32 - decoder_offset] == 0
# connect the Wishbone bus to the Memory wishbone port, using the address filter
# to help decode the address.
# The decdoder address filter as a list with entries as follows:
# (filter_function, wishbone_interface.bus)
# This is useful if you need to attach multiple local logic memories into the
# same wishbone address bank.
wb_con = wishbone.Decoder(self.bus, [(slave_filter, self.wb_sram_if.bus)], register=True)
# add the decoder as a submodule so it gets pulled into the finalize sweep
self.submodules += wb_conAnd to integrate the logic module into your top level SoC code:
# Now we are at the top level SoC module, wherever that may be
class MySoC(SoCCore):
mem_map = {
"logic_memory": 0x30000000, # this just needs to be a unique block
}
mem_map.update(SoCCore.mem_map)
def __init__(self, **kwargs):
# top-level logic stuff
# infer the logic block with memory
self.submodules.logicmem = Logic_with_memory() # the above function
# define the memory region size/location
# memdepth is the depth of the memory inferred in your logic block
# the length is in bytes, so for example if the data_width of your memory
# block is 32 bits, the total length is memdepth * 4
self.add_memory_region("logic_memory", self.mem_map["logic_memory"], memdepth * data_width//8, type='cached')
# add the wishbone slave mapping to the mem_map entry made above
self.add_wb_slave(self.mem_map["logic_memory"], self.logicmem.bus)This design pattern is fairly straightforward. Just create the Memory()
object, don’t add it as a special, but create local read and write ports
using get_port().
storage = Memory(width, depth)
self.specials += storage
wrport = storage.get_port(write_capable=True)
self.specials += wrport
self.comb += [
wrport.adr.eq(produce),
wrport.dat_w.eq(self.din),
wrport.we.eq(1)
]
rdport = storage.get_port()
self.specials += rdport
self.comb += [
rdport.adr.eq(consume),
self.dout.eq(rdport.dat_r)
]A collection of design patterns enabled by the migen toolbox.
Timing delays — inserting pipeline registers to equalize delays between control and data paths — is a common task. There’s a few ways to do it in Migen. Here’s some examples.
The simplest way to create a delay is to make it manually:
sig = Signal()
sig1 = Signal()
sig2 = Signal()
sig3 = Signal()
self.sync += [
sig3.eq(sig2), # three clock cycles delay
sig2.eq(sig1),
sig1.eq(sig),
]This can get cumbersome for busses. Here’s an example of creating a record that defines a bus, and then using a parameterizeable function that builds the delay pipe with a for loop.
rgb_layout = [ # define the bus layout as a record
("r", 8),
("g", 8),
("b", 8)
]
class TimingDelayRGB(Module):
def (self, latency):
self.sink = stream.Endpoint(rgb_layout) # "inputs"
self.source = stream.Endpoint(rgb_layout) # "outputs"
for name in list_signals(rgb_layout):
s = getattr(self.sink, name)
for i in range(latency):
next_s = Signal(len(s))
self.sync += next_s.eq(s) # self.sync means this module by default is using "sys" clock
s = next_s
self.comb += getattr(self.source, name).eq(s)
class MyModule(Module):
def (self):
timing_rgb_delay = TimingDelayRGB(4)
timing_rgb_delay = ClockDomainsRenamer("pix_o")(timing_rgb_delay) # remap the default "sys" clock to local "pix_o" domain
self.submodules += timing_rgb_delay # if you forget this line, the timing delay won't be generated in the verilog netlist
self.hdmi_out0_rgb = hdmi_out0_rgb = stream.Endpoint(rgb_layout)
self.hdmi_out0_rgb_d = hdmi_out0_rgb_d = stream.Endpoint(rgb_layout)
self.comb += [
hdmi_out0_rgb.b.eq(core_source_data_d[0:8]), # wire up the input record
hdmi_out0_rgb.g.eq(core_source_data_d[8:16]),
hdmi_out0_rgb.r.eq(core_source_data_d[16:24]),
hdmi_out0_rgb.valid.eq(core_source_valid_d),
timing_rgb_delay.sink.eq(hdmi_out0_rgb), # wire the input record to the timingdelay element
hdmi_out0_rgb_d.eq(timing_rgb_delay.source) # hdmi_out0_rgb_d is 4 cycles delayed from hdmi_out0_rgb
]So this uses a record with r,g,b fields, takes a latency parameter,
and automatically iterates through the latency depth and creates a set
of daisy-chained registers.
Note that in the TimingDelayRGB() module, we’re iterating through and
using the same variable name, next_s over and over again. It would
seem that this wouldn’t make a delay, but rather a whole bunch of wires
all tied to the same signal. However, next_s is just a temporary
variable name, and the Signal() object assigned to it is always
unique because every call to Signal() creates a brand new Signal()
object.
Breaking it down step by step:
next_s = Signal(len(s))Is creating a new Signal() object, with a globally unique ID, and
temporarily binding it to next_s.
self.sync += next_s.eq(s)This adds the next_s Signal to the sync list. What happens is migen
automatically sees that the object referenced by next_s is unique, and
resolves this by internally appending a unique number to next_s to make
the instance unique. If you look at the generated verilog, you’ll see
next_s1, next_s2, next_s3, … and so forth as it "uniquefies" the
instances added to the sync attribute list.
s = next_sThis line just stashes the reference to the Signal so the next iteration of the loop can wire up the daisy chain.
If instead of creating a new Signal() object and assigning it to next_s,
but instead referencing an existing signal with the same globally unique
ID, you would in fact have a whole series of `Signal`s just wire-OR’d
together.
Here’s another design pattern for doing timing delays.
for i in range(rgb2ycbcr.latency + chroma_downsampler.latency):
next_de = Signal()
next_vsync = Signal()
self.sync.pix += [
next_de.eq(de),
next_vsync.eq(vsync)
]
de = next_de
vsync = next_vsyncThis is an in-line approach to creating the delays, reasonably compact and doesn’t require templates to be defined for every signal group.
A final design pattern is to implement a synchronous buffer using a memory element to implement a delay:
class _SyncBuffer(Module):
def (self, width, depth):
self.din = Signal(width)
self.dout = Signal(width)
self.re = Signal()
produce = Signal(max=depth)
consume = Signal(max=depth)
storage = Memory(width, depth)
self.specials += storage
wrport = storage.get_port(write_capable=True)
self.specials += wrport
self.comb += [
wrport.adr.eq(produce),
wrport.dat_w.eq(self.din),
wrport.we.eq(1)
]
self.sync += _inc(produce, depth)
rdport = storage.get_port(async_read=True)
self.specials += rdport
self.comb += [
rdport.adr.eq(consume),
self.dout.eq(rdport.dat_r)
]
self.sync += If(self.re, _inc(consume, depth))This uses the "Memory" paradigm plus pointer arithmetic. It has the
advantage that the delay can be varied dynamically (not at compile time)
and can also be more efficient for long delays, since instead of eating
FD’s for delays it’s using a block RAM. It does require some additional
logic to wrap around the SyncBuffer to let it "fill" first to the
depth you need for the delay before draining it.
Litescope is the equivalent of the Xilinx ILA for Litex. It samples a set of signals into holding registers that can be read out via wishbone. Because it’s wishbone-based, the data read out can occur via any wishbone bridge — UART, ethernet, or PCI.
Only simple trigger conditions are supported (signal equals 1 or 0, no edges or compound statements)
So, the architecture of a litescope instantiation consists of two parts: the sampler, and the wishbone readout bridge.
You’ll need to modify three sections in your SoC description to add an analyzer. See below for the three sections called out:
class MySoC(BaseSoC):
csr_peripherals += "analyzer" ## 1. need this to create the wishbone interface
csr_map_update(BaseSoC.csr_map, csr_peripherals)
def __init__(self, ...):
# 2. add this inside your "init" function of your base SoC
from litescope import LiteScopeAnalyzer
analyzer_signals = [
signal1,
signal2,
]
analyzer_depth = 128 # samples
analyzer_clock_domain = "sys"
self.submodules.analyzer = LiteScopeAnalyzer(analyzer_signals,
analyzer_depth,
clock_domain=analyzer_clock_domain)
# 3. Add this function to your SoC definition to generate the analyzer definition file.
builder = Builder(soc, output_dir="build",
compile_gateware=not args.nocompile_gateware,
csr_csv="test/csr.csv")
vns = builder.build()
soc.analyzer.export_csv(vns, "test/analyzer.csv") # Export the current analyzer configurationBasically, you assign the signals to the analyzer_signals domain, and then instantiate the LiteScopeAnalyzer(). Here’s the arguments to LiteScopeAnalyzer:
-
analyzer_signals — the array of signals to be sampled
-
depth — in this case 128. Depth is limited by the capacity of your FPGA (so it’s width of analyzer_signals * depth < available memory)
-
sampler domain — the name of tho clock domain that your signals are coming from.
sysby default.
You also need to hook do_exit() of your SoC description to generate the
analyzer.csv file. You should change the path to wherever your analyzer
readout script is located (couple sections down for more on that one).
You also need to add analyzer to the CSR peripherals list so it
shows up in the firmware address space. This function gets called
automatically if it exists.
You have many choices to extract data from the lightscope sampler. It’s just another etherbone peripheral, so you could use the local softcore CPU to read out data. Or you can send commands over a bridge that translates e.g. UART, PCI express, or Ethernet to wishbone addresses and vice versa.
Here’s an example of a UART bridge:
# 1. define the pins
_io += [
("serial", 1,
Subsignal("tx", Pins("B17")),
Subsignal("rx", Pins("A18")),
IOStandard("LVCMOS33")
),
]
# 2. instantiate the bridge
from litex.soc.cores.uart import UARTWishboneBridge
self.submodules.bridge = UARTWishboneBridge(platform.request("serial",1), 100e6, baudrate=115200)
self.add_wb_master(self.bridge.wishbone)In this case, the first argument are the pads, the second is the sys clock frequency, and the third is the baud rate of the serial port. Apparently only 115200 is well-tested. You can try higher baud rates but you might have some bit errors.
Here’s an example of an Ethernet bridge:
# 1. define the pins
_io += [
# RMII PHY Pads
("rmii_eth_clocks", 0,
Subsignal("ref_clk", Pins("D17"), IOStandard("LVCMOS33"))
),
("rmii_eth", 0,
Subsignal("rst_n", Pins("F16"), IOStandard("LVCMOS33")),
Subsignal("rx_data", Pins("A20 B18"), IOStandard("LVCMOS33")),
Subsignal("crs_dv", Pins("C20"), IOStandard("LVCMOS33")),
Subsignal("tx_en", Pins("A19"), IOStandard("LVCMOS33")),
Subsignal("tx_data", Pins("C18 C19"), IOStandard("LVCMOS33")),
Subsignal("mdc", Pins("F14"), IOStandard("LVCMOS33")),
Subsignal("mdio", Pins("F13"), IOStandard("LVCMOS33")),
Subsignal("rx_er", Pins("B20"), IOStandard("LVCMOS33")),
Subsignal("int_n", Pins("D21"), IOStandard("LVCMOS33")),
),
]
# 2. instantiate the bridge
from liteeth.phy.rmii import LiteEthPHYRMII
from liteeth.core import LiteEthUDPIPCore
from liteeth.frontend.etherbone import LiteEthEtherbone
self.submodules.phy = phy = LiteEthPHYRMII(platform.request("rmii_eth_clocks"), platform.request("rmii_eth"))
mac_address = 0x1337320dbabe
ip_address="10.0.11.2"
self.submodules.core = LiteEthUDPIPCore(self.phy, mac_address, convert_ip(ip_address), int(100e6))
self.submodules.etherbone = LiteEthEtherbone(self.core.udp, 1234, mode="master")
self.add_wb_master(self.etherbone.wishbone.bus)|
🔥
|
Etherbone only works with a direct network connection between the FPGA and the host. NAT traversal seems to be broken, so if you’re using a VM to hold your litex build environment, try plugging a USB ethernet dongle in and associating that directly with your VM, so you don’t have to traverse a NAT. |
The code above puts the ethernet bridge into the sys domain, which
defaults to 100MHz. Because the etherbone packet engine contains a full
stack for unpacking and responding to packets, timing might be tough to
close at 100MHz. Here’s an example of how to instatiate a
reduced-frequency bridge, which seems to work just as well as the above
code but doesn’t have the timing closure issues. This assumes that the
eth domain is set at 50MHz. In this design, the master PLL was
modified to add a 50 MHz tap driving a BUFG to create the clk_eth
domain.
from liteeth.phy.rmii import LiteEthPHYRMII
from liteeth.core import LiteEthUDPIPCore
from liteeth.frontend.etherbone import LiteEthEtherbone
phy = LiteEthPHYRMII(platform.request("rmii_eth_clocks"), platform.request("rmii_eth"))
phy = ClockDomainsRenamer("eth")(phy)
mac_address = 0x1337320dbabe
ip_address="10.0.11.2"
core = LiteEthUDPIPCore(phy, mac_address, convert_ip(ip_address), int(50e6), with_icmp=True)
core = ClockDomainsRenamer("eth")(core)
self.submodules += phy, core
etherbone_cd = ClockDomain("etherbone")
self.clock_domains += etherbone_cd
self.comb += [
etherbone_cd.clk.eq(ClockSignal("sys")),
etherbone_cd.rst.eq(ResetSignal("sys"))
]
self.submodules.etherbone = LiteEthEtherbone(core.udp, 1234, mode="master", cd="etherbone")
self.add_wb_master(self.etherbone.wishbone.bus)There’s no architectural reason why you can’t have both a UART bridge and an etherbone bridge master in the same design. You could leave both in and just choose the interface you like to debug the chip.
However, the extra hardware and complication in the wishbone fabric can cause timing closure and resource consumption issues.
OK, now you’ve got an analyzer and a bridge. How do you actually pull
the data out? There is a helper program called litex_server which is
meant to be run on your host — either on the computer with the UART
adapter, or the other side of the ethernet connection. litex_server can
drive a multiplicity of bridge interfaces, as specified by command line
arguments:
-
litex_server --udp --udp-ip 10.0.11.2 &would start an ethernet server for the above example -
litex_server --uart --uart-port /dev/ttyUSB0 --uart-baudrate 115200 &would start a UART server, assuming an FTDI available on/dev/ttyUSB0
Once you’ve got the server running in the background, you can connect to it with a wishbone client program. For example, you can read not just the litescope ILA, but you can read out anything on the wishbone, such as the XADC if you have it instantiated in your SoC:
#!/usr/bin/env python3
from litex.tools.litex_client import RemoteClient
wb = RemoteClient()
wb.open()
print("Temperature: ")
t = wb.read(0xe0005800)
t <<= 8
t |= wb.read(0xe0005804)
print(t * 503.975 / 4096 - 273.15, "C")
wb.close()To read out the analyzer, you can use this script:
from litex.tools.litex_client import RemoteClient
from litescope.software.driver.analyzer import LiteScopeAnalyzerDriver
wb = RemoteClient()
wb.open()
analyzer = LiteScopeAnalyzerDriver(wb.regs, "analyzer", debug=True)
analyzer.configure_subsampler(1) ## increase this to "skip" cycles, e.g. subsample
analyzer.configure_group(0)
# trigger conditions will depend upon each other in sequence
analyzer.add_falling_edge_trigger("soc_videooverlaysoc_hdmi_in0_timing_payload_vsync")
analyzer.add_rising_edge_trigger("soc_videooverlaysoc_hdmi_in0_timing_payload_de")
analyzer.add_trigger(cond={"soc_videooverlaysoc_hdmi_in0_timing_payload_hsync" : 1})
analyzer.run(offset=32, length=128) ### CHANGE THIS TO MATCH DEPTH
analyzer.wait_done()
analyzer.upload()
analyzer.save("dump.vcd")
wb.close()Note that this assumes the files analyzer.csv and csr.csv are in
the same directory. They are both kicked out by the Litex build
environment, and analyzer.csv contains the fully specified names of the
signals you’re monitoring, which you should use to set trigger
conditions.
The same analyzer wishbone readout script works regardless of the bridge
interface you’re using. The litex_server takes care of all of that.
Once you’ve got your dump.vcd file, you can view it with a program like
gtkwave.
FSM support is relatively new as of July 2018. See this commit:
Note that for FSM support to work, the FSM has to be explicitly named as a submodule so you can instantiate it in the analyzer section. In other words, this does not work:
fsm = FSM()
self.submodules += fsmBecause in this case, there’s no explicit name for the FSM in the submodules tree, and referring to the "fsm" element of the submodule won’t resolve reliably. However, this works:
fsm = FSM()
self.submodules.fsm = fsmIn this case, you can refer to the fsm by name because you’ve given it the name "fsm" in the submodule tree.
Docs about the IP cores.
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Migen has a terrible abstraction layer for ports, in that it doesn’t. Python offers a perfectly sensible way to define the inputs and outputs of a function, as in, f(a, b, c) would make you think the ports to a function f might just be a, b, and c. However, Migen coders severely abuse the ability in Python to, post-facto, reach through the function call abstraction and manipulate local variables within a Migen instance. Migen coders think this is a "feature" because it saves you the hell of modifying layers of Verilog function call templates to break out a deeply buried signal for debugging purposes or when refactoring code. However, it makes figuring out exactly what you can or can’t do with IP in migen extremely hard, and most Migen coders make no attempt at all to document what the inputs and outputs of their IP is actually intended to be — mostly by unwritten convention, familiar mostly to the authors of the IP. |
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But wait there’s more. When specifying clocks, names can be global and referred to explicitly within implementations, or overridden through a completely separate mechanism using clock domain renamers. Or they may not be specified at all in which case they default to "sys" |
Here we try to unwind some of that, bit by bit. However, for the "ports" specification, I will refer to only the "typical" variables one might manipulate inside an IP core. Remember, technically, every signal inside an IP core can be manipulated using Migen (feature not bug, supposedly).
So:
-
For "Ports", if listed as a simple name, then it’s specifiable as a function parameter. Of course "Ports" are untyped in Python, thus since every type of object can be passed, every type of object has been passed. Sometimes it’s a
Signal(), sometimes it’s a clock string, sometimes it’s just a configuration flag. If not mentioned, it’s aSignal(), but clarified on a case by case basis. -
If listed as "implicit", then you need to access the port by reaching into the instantiated object, that is:
self.submodules.foo = FooModule(in_port, out_port)
self.comb += self.foo.implicit_signal.eq(1) # set self.implicit_signal inside "foo" to 1-
If listed as "clock", then you need access via the clock & reset domains mechanism, that is:
foo = FooModule(in_port, out_port) # in the case of two clocks inside foo named module_clock1, and module_clock2 that need to be # mapped to external_clock1 and external_clock2, respectively: foo = ClockDomainsRenamer({"module_clock1":"external_clock1", "module_clock2":"external_clock"})(foo) # in the case of a single internal named clock called "myclock" that you want to connect to "fastclock": foo = ClockDomainsRenamer({"myclock:fastclock"}) # in the case of no internally specified clock, but you want to assign a clock to the module, say, "extclk". # The internal name defaults to "sys". So this is how to do it explicitly: foo = ClockDomainsRenamer({"sys:"extclk"}) # But more typically, the "sys" is dropped and it's done just like this: foo = ClockDomainsRenamer("extclk") # don't forget to add the module to submodules after doing all the clock stuff! self.submodules += foo
Instantiate 2 or more flip flops in a chain to synchronize between clock domains
Class: specials
Ports:
-
i
Signal(): input (from an asynchronous clock domain) -
o
Signal(): output -
odmain clockstring (default: "sys"): output clock domain
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n integer (default 2): parameter, depth of flip flop chain
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reset integer (default 0): parameter, reset value
Attempt to synchronize signals between two disparate clock domains. Works only for clocks of similar frequencies.
Class: submodules
Ports:
-
idomain clockstring: input clock domain (typicaly a string, like "sys" for cd_sys)
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odomain clockstring: output clock domain (also a string)
-
self.i
Signal()implicit: input signal to synchronize -
self.o
Signal()implicit: output signal to synchronize
Notes:
I believe this block was designed to synchronize signals between similar-frequency, but asynchronous domains. That is, two 100 MHz clocks, but originating from different crystals. Generally the block’s function make sense if the ratio of frequencies is within a factor of 2.
However, if the idomain and odomain clocks are of very different frequencies (e.g. 20MHz to 100MHz), the following caveats have been observed:
-
idomain faster than odomain: short idomain pulses are lost. pulsing behavior of the output depends on relative timing of the input pulse to the output pulse
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odmain faster than idomain: incoming pulses get turned into a pulse train toggling at the rate of the incoming pulse. e.g. even if you have a single idomain-synchronized pulse, at a minimum you will always get two odomain-synchronized pulses.
Synchronize a group of signals between clock domains.
Class: submodules
Ports:
-
width integer: parameter, width of the bus to synchronize
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idomain clockstring: input clock domain (typicaly a string, like "sys" for cd_sys)
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odomain clockstring: output clock domain (also a string)
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timeout integer: parameter, max idomain cycles to wait before just declaring the bus is synchronized
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self.i
Signal(width)implicit: input signal to synchronize -
self.o
Signal(width)implicit: output signal to synchronize
Notes:
If width is 1, falls back to a simple MultiReg.
Use a pair of PulseSynchronizers to create a ping/pong signal that frames a sampling window to update the bus that’s away from the odomain’s rising edge.
There’s a few hacks (like the timeout parameter) to try and get it to work with very disparate clock speeds and problems like reset glitches. I’ve had at least one difficult bug due to a failure of the module to work properly, but generally pretty handy especially if the input and output clock domains are close in frequency.
Instantiate a PWM controller
Class: submodules
Project: https://github.com/timvideos/litex-buildenv
Import: from gateware.PWM import pwm
Ports:
-
pwm
Signal(): PWM output signal -
CSR enable (wo, 1) - write 1 to enable PWM output
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CSR width (wo, 32) - width of PWM "high" in PWM clock units
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CSR period (wo, 32) - width of total PWM cycle in PWM clock units
Notes:
The default clock units for the PWM module is whatever the "sys" domain is,
but you can use a ClockDomainsRenamer() to change that.
Output is guaranteed to be 0 when enable is not set.
Convenience macro for connecting a GPIO pin to a CSR (often useful for status LEDs)
Class: submodules
Project: https://github.com/timvideos/litex-buildenv
Import: from gateware import pwm
Ports:
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pads PadGroup - PARAMETER, may be a variable-width
Signal() -
CSR out_write (wo, N) - automatic CSR to control GPIOs, based on width of PadGroup Notes:
Connects the pads in the PadGroup to a CSR. Note that the mapping of pads
to bits goes from "LSB … MSB". The pads are typically derived by calling
platform.request() to return the respective Signal().
There are many simulation flows available in migen/litex. I’ve only used one, which relies on xvlog from the native Xilinx toolchain. I prefer this one because I have greater confidence that it simulates internal hard IP macros (like SERDES and PLL) correctly.
I’ve prepared a simple template you can use to run simulations at https://github.com/AlphamaxMedia/netv2-fpga/tree/master/sim/sim_pulsesync.py
This template simulates the PulseSynchronizer primitive that’s part of the Migen CDC suite. It demonstrates how to create multiple clocks, connect them, and draw test vectors out of a Python array. Finally, the system automatically starts a GUI so you can run the simulation and browse the results in the native Vivado waveform environment.
I did my best to incorporate documentation into the example file itself. Please note that it has the following external dependencies:
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lxbuildenv_sim.py — this is needed to force the Python runtime enviroment into a sane state
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glbl.v — this is Xilinx-specific and needed to setup the FPGA’s internal global state
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run/ — this is where the actual run data is stored. The sim_pulsesync.py script will create a top.v and top_tb.v file in here, and invoke the simulator in this directory. Any data in this directory should be considered temporary.
It’s worth noting that one particular advantage of migen/litex "native" simulators is a shorter startup time. It takes about 20 seconds to start the Xilinx simulator on my system, plus you need to configure the GUI to run the simulation; but the native Python simulators are fully scriptable and can start generating results nearly instantaneously for small modules. So if you plan to go the route of success-through-simulation and iterate your way to a final piece of code, you may want to look into the native toolflows to speed your work flow.
LiteX/migen has the neat trick of being able to
configure a SPI flash memory via JTAG, using the
SPI programming via
boundary scan repo. Basically, it’s a set of bitfiles that instantiate
a BSCANE2 block, couple it with a small state machine, and uses that to
drive the SPI pins. On 7-series devices, the CCLK is dedicated, so it
also instantiates a STARTUPE2 block to drive the CCLK. It does a weird
trick where it relies on the pad bond-outs to the SPI and JTAG pins to
be invariant in terms of the on-die pads, so if you look at the code the
pinout may not match your package but it doesn’t matter since both SPI
and JTAG are reserved pins that are invariant across all package options
of a certain die type. One thing that is slightly suspect, however, is
it calls for a 2.5V I/O. Haven’t validated this thoroughly but it does
seem to make the programming process a bit fussy; probing the SPINOR
while programming, for example, might cause a bitstream error.
Unfortunately, the design requires an older version of the bscan-spi
protocol, so it doesn’t work with the latest openocd. You will need to
download and compile the version of
openocd maintained by m-labs until the bscan_spi_bitstreams repo is
updated.
Have a question or want to get in touch? Our IRC channel is #litex at irc.libera.chat.
- Welcome to LiteX
- LiteX's internals
- How to
- Create a minimal SoC-TODO
- Add a new Board-TODO
- Add a new Core-WIP
- Add a new CPU-WIP
- Reuse-a-(System)Verilog,-VHDL,-Amaranth,-Spinal-HDL,-Chisel-core
- Use LiteX on the Acorn CLE 215+
- Load application code the CPU(s)
- Use Host Bridges to control/debug a SoC
- Use LiteScope to debug a SoC
- JTAG/GDB Debugging with VexRiscv CPU
- JTAG/GDB Debugging with VexRiscv-SMP, NaxRiscv and VexiiRiscv CPUs
- Document a SoC
- How to (Advanced)