MyHDL ... MyPWM

The PWM topic appears to be popular lately on the fpgarelated site.  This is coincidence, but I typically find the topic of modulating and demodulating signals interesting.  For digital systems it is always entertaining to play with PWMs.  The following PWM RTL description is quite a bit different than the PWM module described by Anton Babushkin.  The module presented here is a minimal PWM engine defined at design time (i.e. not run-time).

As mentioned, the following is a basic MyHDL module to generate a PWM signal.  This module is intended to be used in an FPGA.  The PWM module is configured for the application at design time and compiled (synthesized, P&R) for a particular FPGA.  Other than the PCM input to the PWM module there are no real-time configurations that occur.  The typical use of a PWM is to configure it for the application and let it run.

To learn more about PWM signals see this overview buried in a filtering example.  The following PWM module has a couple primary parameters: the system clock frequency (attribute of the clock port) and the frequency (1/period) of the output PWM (pwm_frequency).  From these two parameters the number of bits will be determined.  If the PWM number of bits is less than the input, the input will be truncated.

The following is a code snip that exercises the parameters and a summary given the parameters.

>>> from myhdl import *
>>> from myhdl_tools import Clock,Reset
>>> from pwm import m_pwm
>>> cfreq = (50e6,100e6,200e6,333e6,500e6)
>>> pfreq = (.5e3,1e3,10e3)
>>> Xmax,Xmin = (2**15,-2**15)
>>> reset = Reset(0, active=0, async=False)
>>> x = Signal(intbv(0, min=Xmin, max=Xmax))
>>> y = Signal(bool(0))
>>> ts = Signal(bool(0))
>>> for cf in cfreq:
for pf in pfreq:
clock = Clock(0, frequency=cf)
tb_dut = m_pwm(clock,reset,x,y,ts,pwm_frequency=pf)

...
~~~[PWM Module]~~~
clock frequency ................... 50.000 MHz
pwm frequency ..................... 1.000 kHz
local counter max ................. 32768
pwm number of bits ................ 15
pwm offset ........................ 32768
pwm shift ......................... 1

...

~~~[PWM Module]~~~
clock frequency ................... 500.000 MHz
pwm frequency ..................... 10.000 kHz
local counter max ................. 32768
pwm number of bits ................ 15
pwm offset ........................ 32768
pwm shift ......................... 1

...


The following is the complete code for the module.

The following is a waveform from a simulation of the module. The waveform shows the input signal (x) and the output modulated signal (y).

Previous post by Christopher Felton:
MyHDL Resources and Projects
Next post by Christopher Felton:
What do Ohio, Python, and FPGAs have in common?

[ - ]
Comment by July 30, 2018

Hi, any chance the MyHDL source for this example could be recovered, as it seems to be missing? It would make a good MyHDL learning resource, which would help me learn MyHDL as I look for useful examples.

[ - ]
Comment by July 30, 2018

The link in the post worked for me https://bitbucket.org/cfelton/examples/src/tip/pwm... ?

[ - ]
Comment by July 30, 2018

Strange, in one browser on one system I couldn't see any code at all, yet on another system I can see it plain as day. Sorry for the noise - must be something wrong with my first system.

[ - ]
Comment by July 30, 2018

BTW, I can't reply to you (or anyone) or DM you on MyHDL Discourse because my account is locked pending moderation for some reason - are you able to resolve this please?

[ - ]
Comment by July 30, 2018

Comment deleted by author

[ - ]
Comment by July 30, 2018

Yes, thank you.

To post reply to a comment, click on the 'reply' button attached to each comment. To post a new comment (not a reply to a comment) check out the 'Write a Comment' tab at the top of the comments.