msdsl

msdsl is a tool for generating synthesizable real number models (RNMs) for analog circuits for use in FPGA emulation. It is part of the mixed-signal emulation framework that include svreal and anasymod.

Installation

From PyPI

> pip install msdsl

If you get a permissions error when running the pip command, you can try adding the --user flag. This will cause pip to install packages in your user directory rather than to a system-wide location.

From GitHub

If you are a developer of msdsl, it is more convenient to clone and install the GitHub repository:

> git clone https://github.com/sgherbst/msdsl.git
> cd msdsl 
> pip install -e .

Basic example

msdsl allows you to describe analog blocks at a variety of abstraction levels. As a simple example, suppose that you want to model an RC filter with a fixed-timestep equation. That can be expressed with the following msdsl code:

from msdsl import *
from math import exp
r, c, dt = 1e3, 1e-9, 0.1e-6
m = MixedSignalModel('rc')
x = m.add_analog_input('x')
y = m.add_analog_output('y')
a = exp(-dt/(r*c))
m.set_next_cycle(y, a*y + (1-a)*x)
m.compile_and_print(VerilogGenerator())

The first thing to notice is that msdsl is imported just like any other Python package. This allows you to instantiate a MixedSignalModel object that is used to described analog behaviors and generate Verilog code. Next, an analog input (x) and analog output (y) are declared. The set_next_cycle command implements the discrete-time equation:

y[k+1] = a*y[k] + (1-a)*x[k]

Users can specify the clock and reset signals that control the initialization and updating of this equation; they default to macros CLK_MSDSL and RST_MSDSL when unspecified.

On the last line, the Verilog code is generated and printed out, resulting in something like this:

`include "svreal.sv"
module rc #(
    `DECL_REAL(x),
    `DECL_REAL(y)
) (
    `INPUT_REAL(x),
    `OUTPUT_REAL(y)
);
    // Assign signal: y
    `MUL_CONST_REAL(0.9048374180359596, y, tmp0);
    `MUL_CONST_REAL(0.09516258196404037, x, tmp1);
    `ADD_REAL(tmp0, tmp1, tmp2);
    `DFF_INTO_REAL(tmp2, y, `RST_MSDSL, `CLK_MSDSL, 1'b1, 0);
endmodule

This code makes use of macros from svreal, which is a library that provides a flexible, synthesizable real-number type.

Finally, note that although compile_and_print is used in this example, you may find it more useful to use compile_to_file in practice, since msdsl is often used to generate model files for use in building an emulator.

Building models

msdsl provides a number of convenient features for building models, such as operator overloading, synthesizable functions, and noise.

Signals

msdsl signals can be declared as analog or digital, and internal or external. Digital signals default to 1-bit, unsigned, but their width and signedness can be specified. Analog signals are real values with a specified +/- range that is used to compute fixed-point formats. It is generally only necessary to specify ranges for model I/O and state variables, since msdsl can automatically figure out the rest.

Here are some examples of signal declarations in msdsl:

from msdsl import *
m = MixedSignalModel('model')
a = m.add_analog_input('a')
b = m.add_digital_output('b', signed=True, width=8)
c = m.add_analog_state('c', init=1.23, range_=4.56)
d = m.add_digital_signal('d', width=4)

Note the optional init argument used for state variables. Although not shown here, it is also available for digital signals.

Assignments

The two basic types of assignments in msdsl are set_this_cycle and set_next_cycle. set_this_cycle acts like an assign statement in Verilog, which set_next_cycle acts like a synchronous assignment in an always block. Here are some examples:

from msdsl import *
m = MixedSignalModel('model')
a = m.add_analog_input('a')
b = m.add_analog_output('b')
c = m.add_analog_state('c', init=1.23, range_=4.56)
m.set_next_cycle(c, 0.9*c + 0.1*a)
d = m.set_this_cycle('d', 6.78*c + 7.89*a)  # create a new signal, 'd'
m.set_this_cycle(b, 0.88*d)  # assign to existing signal 

Operators

Many Python operators for arithmetic, comparison, and bitwise operations are overloaded in msdsl, allowing you to write down expressions conveniently. Currently supported operators include +, -, *, ~, &, |, ^, <<, >>, <, >, <=, >=, ==, and !=. The true division operator, /, is only partially supported: dividing by constants works, but dividing by variables does not.

Synthesizable functions

msdsl makes it possible to convert Python functions into synthesizable approximations. Here's an example where that feature is used to implement a variable-timestep RC filter:

import numpy as np
from msdsl import *
r, c = 1e3, 1e-9
m = MixedSignalModel('rc')
x = m.add_analog_input('x')
dt = m.add_analog_input('dt')
y = m.add_analog_output('y')
func = lambda dt: np.exp(-dt/(r*c))
f = m.make_function(func,
    domain=[0, 10*r*c], numel=512, order=1)
a = m.set_from_sync_func('a', f, dt)
x_prev = m.cycle_delay(x, 1)
y_prev = m.cycle_delay(y, 1)
m.set_this_cycle(y, a*y_prev + (1-a)*x_prev)
m.compile_and_print(VerilogGenerator())

func is a regular Python function, and it is converted into an msdsl Function via the make_function command. This generates a piecewise-polynomial approximation of the function over a specified domain. In this example, a piecewise-linear approximation is used (order=1) with 512 segments (numel=512) and the domain is 10 RC time constants (domain=[0, 10*r*c]). By default, inputs outside of the domain will return the value at the closer edge of the domain.

set_from_sync_func generates hardware to implement the function, meaning that make_funcion can be called once, and then applied multiple times with set_from_sync_func. This is convenient since it generates more concise output code.

msdsl also supports single-input, multi-output functions. This is important because it reduces the hardware overhead as compared to completely independent invocations of the same function. Here's an example:

import numpy as np
from msdsl import *
m = MixedSignalModel('model')
x = m.add_analog_input('x')
y1 = m.add_analog_output('y1')
y2 = m.add_analog_output('y2')
func1 = lambda t: np.sin(t)
func2 = lambda t: np.cos(t)
f = m.make_function([func1, func2],
    domain=[-np.pi, np.pi], numel=512, order=1)
m.set_from_sync_func([y1, y2], f, x)
m.compile_and_print(VerilogGenerator())  

The main difference as compared to the previous example is that make_function and set_from_sync_func are called with a list of functions/output variables.

Pseudorandom noise

msdsl provides a means for generating different kinds of pseudorandom noise: uniform, Gaussian, and arbitrary distributions:

from msdsl import *
m = MixedSignalModel('model')

# uniform noise
m.set_this_cycle('a', m.uniform_signal(min_val=-1.23, max_val=4.56))

# Gaussian noise
m.set_gaussian_noise('b', std=1.23, mean=4.56, gen_type=...)

# arbitrary noise distribution
# specify via the inverse cumulative distribution function
inv_cdf = lambda x: ...
inv_cdf_func = m.make_function(inv_cdf, domain=[0.0, 1.0])
m.set_this_cycle('c', m.arbitrary_noise(inv_cdf_func))

All noise generators depend on a pseudorandom integer generator, which can be specified with the optional gen_type parameter ('lcg', 'mt19937', 'lfsr'). The highest-quality generator is MT19937, but it is more resource-intensive, and takes many emulator cycles to start up. The linear congruential generator (LCG) is an exact implementation of the random number generator called for in the Verilog-2001 specification. The default is a simple linear feedback shift register ('lfsr'). Random seed values are automatically generated for random number generators, but can be explictly specified as well.

Abstraction levels

Up until this point, we have considered the low-level features provided by msdsl to implement modeling strategies, but msdsl also has a bunch of common model abstractions that are ready to use.

Differential equations

msdsl provides a method for writing down symbolic systems of linear differential equations, which are solved at compile time (i.e., through matrix exponentiation) to produce a simple FPGA implementation.

from msdsl import *
r, c = 1e3, 1e-9
m = MixedSignalModel('rc', dt=0.1e-6)
x = m.add_analog_input('x')
y = m.add_analog_output('y')
m.add_eqn_sys([c*Deriv(y) == (x-y)/r])
m.compile_and_print(VerilogGenerator())

Switched systems

msdsl supports an extension to systems of linear differential equations, in which expressions can take one of several forms, depending on digital inputs. These expressions are built with the eqn_case command. In the example below, a resistor is shorted out by a switch when the digital signal s is one. The number of cases can be greater than 2, if the digital select signal has multiple bits.

from msdsl import *
r, rsw, c = 1e3, 100, 1e-9
m = MixedSignalModel('rc', dt=0.1e-6)
x = m.add_analog_input('x')
s = m.add_digital_input('s')
y = m.add_analog_output('y')
g = eqn_case([1/r, 1/r+1/rsw], [s])
m.add_eqn_sys([c*Deriv(y) == (x-y)*g])
m.compile_and_print(VerilogGenerator())

"Netlist"

msdsl supports a type of "netlist", where analog components like resistors, capacitors, and inductors are instantiated. Internally, this generates symbolic KCL and KVL equations, which are solved at compile time using the previously-described differential equation interface. Switches and diodes are supported in a limited way, in which they are either "ON" or "OFF", changing their Thevenin equivalent representation. For diodes, additional logic is generated to determine whether the diode is on or off during each timestep.

The example below is a netlist-style description of an RC filter whose resistor can be shorted out with a switch.

from msdsl import *
r, rsw, c = 1e3, 100, 1e-9
m = MixedSignalModel('rc', dt=0.1e-6)
x = m.add_analog_input('x')
s = m.add_digital_input('s')
y = m.add_analog_output('y')
circ = m.make_circuit()
gnd = circ.make_ground()
circ.capacitor('net_y', gnd, c,
    voltage_range=RangeOf(y))
circ.resistor('net_x', 'net_y', r)
circ.switch('net_x', 'net_y', s, rsw)
circ.voltage('net_x', gnd, x)
circ.add_eqns(AnalogSignal('net_y') == y)
m.compile_and_print(VerilogGenerator())

Transfer function

msdsl also allows users to specify analog dynamics with transfer functions. The user provides the coefficients of numerator and denominator polynomials, using the same style as cont2discrete.

from msdsl import *
r, c = 1e3, 1e-9
m = MixedSignalModel('rc', dt=0.1e-6)
x = m.add_analog_input('x')
y = m.add_analog_output('y')
m.set_tf(x, y, [[1], [r*c, 1]])
m.compile_and_print(VerilogGenerator())

Templates

We are currently adding higher-level abstractions that represent the function of entire blocks. This includes a saturation nonlinearity (SaturationModel), a lossy-channel, specified from S-parameters (S4PModel), and a continuous-time linear equalization model (CTLEModel), which is specified by pole/zero values. These models are implemented using a new variable-timestep discretization developed at Stanford.

from msdsl.templates.saturation import SaturationModel
m1 = SaturationModel(-1, 'dB', module_name='m1')

from msdsl.templates.channel import S4PModel
m2 = S4PModel(s4p_file='myfile.s4p', dtmax=100e-12, module_name='m2')

from msdsl.templates.lds import CTLEModel
m3 = CTLEModel(fz=1e9, fp1=2e9, fp2=10e9, dtmax=100e-12, fmodule_name='m3')