Python script which allow to compute TWI configuration register values (TWBR and TWPS) for an ATmega328P.

pyTWI.py

#!/usr/bin/env python
# encoding: utf-8
 
"""
Copyright (C) 2012  Julien Le Sech - www.idreammicro.com
 
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.
"""
 
import argparse
import sys

"""
TWI bus max frequency.
"""
TWI_MAX_FREQUENCY = 400000

"""
Prescaler bits in TWPS register.
"""
TWPS0 = 0
TWPS1 = 1

"""
Prescaler configuration.
"""
class PrescalerConfiguration:

    def __init__(self, prescalerValue, prescalerBits):
        self.value = prescalerValue
        self.bits = prescalerBits

"""
Available prescaler configurations.
"""
prescalerConfigurations = [
    PrescalerConfiguration(1, 0),
    PrescalerConfiguration(4, (1 << TWPS0)),
    PrescalerConfiguration(16, (1 << TWPS1)),
    PrescalerConfiguration(64, ((1 << TWPS1) | (1 << TWPS0)))
]

"""
Compute bit rate.
"""
def ComputeBitRate(f_cpu, f_scl, prescalerValue):
    bitRate = (f_cpu - 16 * f_scl) / (2 * f_scl * prescalerValue)
    return bitRate

"""
Main function.
"""
def main():    
    # Parse arguments.
    parser = argparse.ArgumentParser()
    parser.add_argument(
        "-f_cpu", dest = "f_cpu", type = int, default = 16000000,
        help = "CPU frequency in Hz")
    parser.add_argument(
        "-f_scl", dest = "f_scl", type = int, default = 400000,
        help = "SPI frequency in Hz")
    args = parser.parse_args()
     
    print "F_CPU = %d Hz" % (args.f_cpu)
    print "F_SCL = %d Hz" % (args.f_scl)

    # Check the preconditions.
    assert(args.f_scl <= TWI_MAX_FREQUENCY)

    bitRate = 65535
    index = 0

    for index in range(len(prescalerConfigurations)):
        bitRate = ComputeBitRate(args.f_cpu, args.f_scl, prescalerConfigurations[index].value)
        if bitRate <= 255:
            break

    # TODO: check bitrate value.

    print "Bit rate (TWBR) = %d" % (bitRate)
    print "Prescaler value = %d => prescaler bits (TWPS) = %d" % (prescalerConfigurations[index].value, prescalerConfigurations[index].bits)

"""

"""
if __name__ == '__main__':
    main()