Software Reliability

[BrianT]

I build data loggers that must run unattended for a year plus. There are several aspects to reliability.

The first is electrical interference. This can be cosmic rays hitting a memory cell, static discharge or nearby lightning coming in via an I/O port, dirty power supply, etc, etc. A precaution against this is to refresh all registers every time through the main loop. Something like.

code:
'************************* Initialise ******************************
Initialise:
ADCON0 = %00000000 ' ADC turned OFF until needed
ADCON1 = %00001001 ' Configures 6 analog - balance digital
CMCON = %00000111 ' Comparators OFF
CVRCON = %00000000 ' Voltage Rev OFF
TRISA = %11111111 ' ECG, Accels input
PORTA = %00000000 '
TRISB = %10000010 '
TRISC = %11111111 ' temporary low power
TRISD = %01000000 ' TFU on D.6 ensure it is set as an input
PortD = %00000000 ' temporary low power
TRISE = %00000000
PortE = %00000000
OSCCON = %01101111 ' 4 MHz
endcode

The second problem is software integrity. Watchdog timer, error trapping, housekeeping checks, etc. Things like looking for A < B or A > B rather than A = B. This only matters with floating point code where what you expect to be an equality might turn out to have a small rounding error that defeats the A = B test.

The third is to have a second party go through your code looking for logical errors and ways to break it. I know of one software company working in mission critical real time 24/7 process control systems that has as many people in the test team as they have writing the code.

The fourth is good documentation. Give your code to a colleague and ask if they understand it and could maintain it.

A fifth is the hardware. Clean all flux off the board, or use an approved no-clean flux, don't stress your capacitors, no parts running hot, quality connectors with plating appropriate to the environment your box must work in. Don't let the box boil in direct sunlight or freeze overnight, etc.

HTH
BrianT