Fractional divider calculator for Synopsys DesignWare UART (8250) v4.00a and newer

fdcalc8250.py

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

from __future__ import print_function

import sys
import os

import math

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

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

BAUDRATES = [
    # Group 0
    110,
    134,
    # Group 1
    50,
    75,
    150,
    200,
    300,
    600,
    1200,
    1800,
    2400,
    4800,
    9600,
    19200,
    38400,
    57600,
    115200,
    230400,
    460800,
    921600,
    1843200,
    # Group 2
    576000,
    1152000,
    # Group 3
    500000,
    1000000,
    1500000,
    2000000,
    2500000,
    3000000,
    3500000,
    4000000,
]

XTAL = 38400000
BAUD = 115200
DFL = 64

#
# Fuart = XTAL / (BRDi + BRDf)
# uartclk = Fuart * 16
# baud = uartclk / DLAB
#

def go(xtal, baud, dfl):
    bb = baud * 16

    # mathematical (br1 == baud)
    div1 = xtal * 1.0 * dfl / bb
    br1 = (xtal * 1.0 / (div1 / dfl)) / 16

    # algorithm in the driver
    quot = xtal // bb
    rem = xtal % bb
    # DIV_ROUND_CLOSEST(rem * dfl, bb)
    frac = int(round((rem * dfl) * 1.0 / bb))
    div2 = quot * dfl + frac
    br2 = ((xtal * dfl) // div2) // 16

    return div1, br1, div2, br2

def usage():
    sys.stdout.write("Usage %s [XTAL [DFL [BAUD]]]\n" % PROG)

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

def main(argv):
    if len(argv) > 1:
        xtal = toint(argv[1])
    else:
        xtal = XTAL

    if len(argv) > 2:
        dfl = toint(argv[2])
    else:
        dfl = DFL

    if len(argv) > 3:
        baud = toint(argv[3])
        bauds = (baud, )
    else:
        baud = BAUD
        bauds = sorted(BAUDRATES)

    for b in bauds:
        div1, br1, div2, br2 = go(xtal, b, dfl)
        ds = math.copysign(1, br2 - br1)
        dv = ds * (abs(br2 - br1) * 100.0 / b)
        if dv < 0.005 and dv > -0.005:
            sdv = ""
        else:
            sdv = "%+2.2f" % dv
        print("%-8d %-8.3f\t%-8.0f %-10d %-10d %s" % (b, div1, br1, div2, br2, sdv))

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