DT’s Elapsed Timer Magic Trick

[Art]

Ok, after a little experimentation today, finding the frequency without any use of EEPROM at all is really this easy:

'
DEFINE OSC 20				‘needed so the 100ms pause period is known
'
if status.bit4 = 1 then			'was not reset by watchdog
for percount = 0 to 255			'do eeprom writes until wdt reset
pause 100				'
@ MOVF	_percount	,W		;copy counter value to timer low byte 
@ MOVWF   TMR1L				;load timer with count value
next percount				'
makereset:				'failsafe to make reset occur
goto makereset				'
'
else					'hardware was reset by watchdog
@ clrwdt				;
@ MOVF	TMR1L	,W			;copy counter value to timer low byte 
@ MOVWF  _wrtcount			;reload timer with correct value
endif					'
'

Then you have a number in wrtcount that represents the crystal frequency i.e. a couple of values either side of 25 = 20MHz.
This is with the power timer also set, but I doubt that matters as the pic isn’t doing anything in power timer period.

In any case, it’s very handy to know that if you’re not using Timer1,
you have an ordinary word variable that survives WDT for certain.

If your program was going to use Timer1 after this, the values should probably be reset to zero, then the clock started.
DT’s Elapsed Timer code does have a Reset feature which probably does this as well as clearing it's real time variables.