setbaud.py

# Generate baud rate constants
#
# This can't be done in Pasmo, as Pasmo only uses 16 bit maths.
#

# Required baud rate. Set if here.
baud = 3000

# Z80 clock speed
#
#cpuClock = 3500000 # ZX Spectrum 16/48K, ZX80, ZX81
#cpuClock = 3546900 # Spectrum 128K, +2/2A/3
cpuClock = 3523372 # Geometric mean, minimum error on all Spectrums
#cpuClock = 4000000 # Amstrad CPC

# Constants related to boot2.asm
#
loopOverhead = 738  # Fixed loop delay, in T-states
cycleTime = 13      # One loop of a DJNZ delay, in T-states

# Generate baud.asm file
#
outfile = open("setbaud.asm","w")
outfile.write("; This file is autogenerated by setbaud.py\n")
outfile.write("; Do not make changes to this file "
                "as they will be erased on the next build.\n")
outfile.write("; Set rate to {} baud\n".format(baud))
outfile.write("BAUD equ {}\n".format(baud))

bitPeriod = cpuClock/baud
sampleTime = bitPeriod * 0.75
maxBaud = cpuClock/(loopOverhead + cycleTime)*0.75
assert sampleTime > (loopOverhead+cycleTime),\
    "Baud rate too high. Maximum is {}.".format(maxBaud)
baudLoops = int(round((sampleTime-loopOverhead)/cycleTime))
outfile.write("BAUDLOOPS equ {}\n".format(baudLoops))
outfile.close()