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README: Core Logic, Bus, Bank

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sbourdeauducq committed Dec 19, 2011
1 parent 7774ace commit d9dc604c99984355634898a939f9e9f2cf99ee83
Showing with 72 additions and 14 deletions.
  1. +72 −14 README
@@ -78,8 +78,8 @@ cluttered syntax at times when writing descriptions in FHDL, but we
believe this is totally acceptable, particularly when compared to VHDL
-Migen is made up of several related components, which are described
+Migen is made up of several related components, which are briefly
+described below.
Migen FHDL
@@ -237,16 +237,16 @@ For convenience, there is also a Elif() method.
If(tx_count16 == 0,
- + 1),
- If(tx_bitcount == 8,
- ).Elif(tx_bitcount == 9,
- ).Else(
-[1:], 0))
- )
+ + 1),
+ If(tx_bitcount == 8,
+ ).Elif(tx_bitcount == 9,
+ ).Else(
+[1:], 0))
+ )
Case statement
@@ -317,20 +317,77 @@ autofragment module.
Migen Core Logic
+Migen Core Logic is a convenience library of common logic circuits
+implemented using FHDL:
+ - a multi-cycle integer divider.
+ - a round-robin arbiter, useful to build bus arbiters.
+ - a multiplexer bank (multimux), useful to multiplex composite
+ (grouped) signals.
+ - a condition-triggered static scheduler of FHDL synchronous statements
+ (timeline).
Migen Bus
+Migen Bus contains classes providing a common structure for master and
+slave interfaces of the following buses:
- Wishbone [5], the general purpose bus recommended by Opencores.
- CSR-NG, a low-bandwidth, resource-sensitive bus designed for
accessing the configuration and status registers of cores from
- FastMemoryLink-NG, a split-transaction bus optimized for use with a
high-performance, out-of-order SDRAM controller. (TODO)
+It also provides interconnect components for these buses, such as
+arbiters and address decoders. The strength of the Migen procedurally
+generated logic can be illustrated by the following example:
+ wbcon = wishbone.InterconnectShared(
+ [cpu.ibus, cpu.dbus, ethernet.dma, audio.dma],
+ [(0, norflash.bus), (1, wishbone2fml.wishbone),
+ (3, wishbone2csr.wishbone)])
+In this example, the interconnect component generates a 4-way round-robin
+arbiter, multiplexes the master bus signals into a shared bus, determines
+that the address decoding must occur on 2 bits, and connects all slave
+interfaces to the shared bus, inserting the address decoder logic in the
+bus cycle qualification signals and multiplexing the data return path. It
+can recognize the signals in each core's bus interface thanks to the
+common structure mandated by Migen Bus. All this happens automatically,
+using only that much user code. The resulting interconnect logic can be
+retrieved using wbcon.get_fragment(), and combined with the fragments
+from the rest of the system.
Migen Bank
+Migen Bank is a system comparable to wishbone-gen [6], which automates
+the creation of configuration and status register banks and
+(TODO) interrupt/event managers implemented in cores.
+Bank takes a description made up of a list of registers and generates
+logic implementing it with a slave interface compatible with Migen Bus.
+A register can be "raw", which means that the core has direct access to
+it. It also means that the register width must be less or equal to the
+bus word width. In that case, the register object provides the following
+ - dev_r, which contains the data written from the bus interface.
+ - dev_re, which is the strobe signal for dev_r. It is active for one
+ cycle, after or during a write from the bus. dev_r is only valid when
+ dev_re is high.
+ - dev_w, which must provide at all times the value to be read from the
+ bus.
+Registers that are not raw are managed by Bank and contain fields. If the
+sum of the widths of all fields attached to a register exceeds the bus
+word width, the register will automatically be sliced into words of the
+maximum size and implemented at consecutive bus addresses, MSB first.
+Field objects have two parameters, access_bus and access_dev, determining
+respectively the access policies for the bus and core sides. They can
+take the values READ_ONLY, WRITE_ONLY and READ_WRITE.
+If the device can read, the field object provides the dev_r signal, which
+contains at all times the current value of the field (kept by the logic
+generated by Bank).
+If the device can write, the field object provides the following signals:
+ - dev_w, which provides the value to be written into the field.
+ - dev_we, which strobes the value into the field.
Migen Flow
@@ -344,6 +401,7 @@ References:
[4] p30-35
Practical information

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