Quark SoC SPI baudrate calculator

fdcalcspi.py

#!/usr/bin/env python3
# -*- 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 = 200000000

#
# Fssp = Fsys * DDS_CLK_RATE / 2**24
# Baud rate = Fsclk = Fssp / (2 * (SCR + 1))
#
# DDS_CLK_RATE either 2^n or 2^n / 5.
# SCR is in range 0 .. 255
#
# Divisor = 5^i * 2^j * 2 * k
#       i = [0, 1]      i = 1 iff j = 0 or j > 3
#       j = [0, 23]     j = 0 iff i = 1
#       k = [1, 256]
# Special case: j = 0, i = 1: Divisor = 2 / 5
#

# Baud rate, DDS_CLK_RATE, SCR
TABLE = [
    [50000000,  8388608,    0],
    [40020000,  -1,         -1],
    [40000000,  6710886,    0],
    [39980000,  -1,         -1],
    [25000000,  4194304,    0],
    [20000000,  6710886,    1],
    [16667000,  8388608,    2],
    [13333000,  6710886,    2],
    [12500000,  2097152,    0],
    [10000000,  8388608,    4],
    [8000000,   6710886,    4],
    [6250000,   4194304,    3],
    [5000000,   4194304,    4],
    [4350000,   -1,         -1],
    [4000000,   6710886,    9],
    [3333000,   -1,         -1],
    [3140500,   524288,     -1],
    [3125000,   524288,     0],
    [3109500,   524288,     -1],
    [2500000,   4194304,    9],
    [2000000,   6710886,    19],
    [1563000,   262144,     0],
    [1250000,   2097152,    9],
    [1006000,   -1,         -1],
    [1000000,   4194304,    24],
    [994000,    -1,         -1],
    [800000,    6710886,    49],
    [781250,    131072,     0],
    [625000,    2097152,    19],
    [500000,    4194304,    49],
    [400000,    6710886,    99],
    [390625,    65536,      0],
    [250000,    4194304,    99],
    [205313,    32768,      -1],
    [200000,    6710886,    199],
    [195313,    32768,      0],
    [185313,    32768,      -1],
    [125000,    1048576,    49],
    [100700,    -1,         -1],
    [100000,    2097152,    124],
    [99300,     -1,         -1],
    [50000,     1048576,    124],
    [25000,     524288,     124],
    [10066,     131072,     -1],
    [10016,     131072,     77],
    [9966,      131072,     -1],
    [5065,      131072,     -1],
    [5040,      131072,     154],
    [5015,      131072,     -1],
    [1007,      32768,      -1],
    [1002,      32768,      194],
    [997,       32768,      -1],
    [69,        -1,         -1],
    [7,         -1,         -1],
    [2,         -1,         -1],
    [1,         -1,         -1],
]

def idiv(a, b):
    return int(a // b)

def go(fref, baud):
    fr1 = idiv(fref, 2)
    fr2 = idiv(fref * 2, 5)

    q1 = idiv(fr1 + baud - 1, baud)
    q2 = idiv(fr2 + baud - 1, baud)
    q12 = q1
    q3 = idiv(fref + baud - 1, baud)

    dds12 = 2 ** 23
    if q12 > 256:
        # Use fls()
        shift = int(math.ceil(math.log(q12, 2)))
        if shift > 9:
            q12 >>= shift - 9
            dds12 >>= shift - 9
        if q12 % 2:
            q12 += 1
    # Use ffs()
    while q12 % 2 == 0:
        q12 >>= 1
        dds12 >>= 1

    arate1 = idiv(fr1, q1)
    r1 = abs(baud - arate1)

    dds2 = idiv(2 ** 24 * 2, 5)
    arate2 = idiv(fr2, q2)
    r2 = abs(baud - arate2)

    div = 23 - int(math.log(dds12, 2))
    arate12 = idiv(fr1 >> div, q12)
    r12 = abs(baud - arate12)

    dds3 = idiv(2 ** 24, q3)
    arate3 = idiv(fref * dds3, 2 ** 24)
    r3 = abs(baud - arate3)

    c1 = '%d, %d' % (q1, r1)
    c12 = '%d, %d' % (q12, r12)
    c2 = ''
    c3 = ''

#    if (r2 == 0 and r12 == 0 and dds2 > dds12) or (r2 and r2 >= r12) or q2 > 256:
    if r2 >= r12 or q2 > 256:
        arate = arate12
        dds = dds12
        q = q12
        r = r12
    else:
        arate = arate2
        dds = dds2
        q = q2
        r = r2

    if q2 <= 256:
        c2 = '%d, %d' % (q2, r2)

    if fref / 80 >= baud:
        if r3 < r:
            arate = arate3
            dds = dds3
            q = 1
            r = r3
        c3 = '%d, %d' % (q3, r3)

    choice = ' -> %06x, %d, %d' % (dds, q, r)

    print("%8d(%8d): c1: %-12s c12: %-12s c2: %-12s c3: %-12s %s" % (baud, arate, c1, c12, c2, c3, choice))

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

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

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

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