[PASM2] Reading & Validating an SIRCS Bit -- Looking for Input/Suggestions
JonnyMac
Posts: 8,927
I'm working on a project that uses SIRCS bits to send data, so I want to measure and validate the incoming SIRCS data stream. External sources can foul the input, and if a sensor fails the line will stay low all the time. A standard SIRCS bit can be 2400, 1200, or 600 microseconds (low from sensor), followed by a 600 microsecond idle period (high).
My goal is to call a subroutine that will measure the low-going pulse and the idle time after. The pulse can be anything up to 250ms at which point I assume the input is bad. The idle time is nominally 600us between pulses, so 550us is my check value. The routine has a timeout function so that I can wait longer for the first (start) bit than for the next bit in a stream.
If the input is held low too long (sensor failure), the routine returns -2. If another bit shows up too early (interference), the routine returns -1.
This is what I have sketched out so far. I have tested the measurement portion using an inline method and that seems to work. Thanks for any suggestions you care to offer.
get_pulse mov pulsewidth, #0
mov idletime, #0
gp_wait testp rxp wc ' sample rx
if_nc jmp #gp_lo ' if bit detected, measure
waitx ##US_001 ' hold 1us
sub timeout, #1 wz ' decrement timeout
if_z ret ' if done, abrt
jmp #gp_wait ' keep waiting
gp_lo waitx ##US_001 ' hold 1us
incmod pulsewidth, ##250_000 wc ' inc width; check for failure
if_c neg pulsewidth, #2 ' if fail, mark
if_c ret ' and abort
testp rxp wc ' resample
if_nc jmp #gp_lo ' if low, keep measuring
gp_idle waitx ##US_001 ' hold 1us
incmod idletime, ##550 wc ' increment idle timing
if_c ret ' if 500us, we're done
testp rxp wc ' resample
if_c jmp #gp_idle ' if early pulse, interference
neg pulsewidth, #1 ' mark
ret ' and abort
The calling code will validate the legal SIRCS bit widths.