Fractional divider for LPSS PWM

fdcalcpwm.py

#!/usr/bin/python -tt
# -*- coding: UTF-8 -*-
# vim: ts=4 sw=4 et ai si

import sys
import os
import operator

import math
import fractions

__author__ = "Andy Shevchenko"
__license__ = "GPL v2"

PROG = os.path.basename(sys.argv[0])

FSYS = 19200000

# Test date (pertiod, duty cycle)
TABLE = [
    [100,       50],
    [300,       250],
    [1000,      250],
    [50000,     10000],
    [100000,    75000],
    [300000,    50000],
    [500000,    250000],
    [20000000,  10000000],
    [1,         1],
    [53,        1],
    [104,       1],
    [256,       1],
    [FSYS - 2,  1],
    [FSYS - 1,  FSYS - 2],
    [FSYS,      1],
    [FSYS,      FSYS - 1],
]

def go(fref, period, duty):
    freq = 10**9 / period

    #
    # The equation is:
    # base_unit = round(base_unit_range * freq / c)
    #
    bur = 2**22

    freq *= bur;
    bu = freq / fref;

    if duty > 0:
        d = duty
    else:
        d = 1

    otd = 255 * d
    otd /= period
    otd = 255 - otd

    bul = bu & (bur - 1)
    r = (bul << 8) + otd

    pf = fref * bul * 1.0 / bur
    dp = otd * 100.0 / 256

    print "%d %d(%d): div: %d bu: %#x(%#x) -> %#x out freq: %.1f duty: %.2f(%.2f)%%" % (
            period, duty, d, otd, bu, bul, r, pf, dp, duty * 100.0 / period
          )

def usage():
    sys.stdout.write("Usage %s [period [duty]]\n" % PROG)

def toint(value):
    try:
        return int(value)
    except ValueError:
        usage()
        raise SystemExit("Invalid argument")

def main(argv):
    fref = FSYS
    if len(argv) > 1:
        period = toint(argv[1])
        if len(argv) > 2:
            duty = toint(argv[2])
        else:
            duty = period / 2
        go(fref, period, duty)
    else:
        for item in TABLE:
            go(fref, operator.itemgetter(0)(item), operator.itemgetter(1)(item))

if __name__ == "__main__":
    sys.exit(main(sys.argv))