DT’s Elapsed Timer Magic Trick

[Demon]

2015 nominee for best computer generated animations.

Robert

[towlerg]

628A has two internal clocks

[Art]

I know of the 4MHz RC clock. What’s the other?

[Art]

It got figured out on another forum. A better way actually, without writing to the on-chip EEPROM at all.

'***********************************************************************************
'*                                                                                 *
'*                    Microchip Pic 16F628 @ 8,16 or 24MHz                         *
'*                                  Art 2015                                       *
'*                                                                                 *
'*                          [email protected]                               *
'*                                                                                 *
'***********************************************************************************
'
'
DEFINE OSC 20								'
DEFINE NO_CLRWDT							'watchdog is cleared manually
'
INCLUDE "Elapsed.bas"							‘modified to take a variable
'
' execution time!
'
'
percount var byte							'
wrtcount var byte							'
clkvalue var byte							'
xdata var byte								'
'oscfreq var word							‘this is declared in Elapsed Timer file
notreset var bit							'
'
CMCON = 7									'set ports digital
trisb.4 = 0									'set led output
portb.4 = 0									'
'
percount = 0								'
clkvalue = 0								'
'
'
notreset = status.bit4						'check wdt reset occured - 0 is did reset
'
'
if notreset = 1 then						'was not reset by watchdog
portb.4 = 1									'turn led on
for percount = 0 to 255						'do eeprom writes until wdt reset
pause 100									'
write percount,$00							'
next percount								'
makereset:									'failsafe to make reset occur
@ nop										;
goto makereset								'
else									'was reset by watchdog
@ clrwdt									;clear the watchdog from now on
wrtcount = 0								'reset write counter
countwrites:								'count eeprom writes
read wrtcount,xdata							'read byte
@ clrwdt									;
pause 10									'
if xdata != $00 then						'
goto donecount								'exit counting loop
endif										'
wrtcount = wrtcount + 1						'increment count
goto countwrites							'
donecount:									'
for percount = 0 to 255						'erase the eeprom for next time
pause 10									'
@ clrwdt									;
write percount,$FF							'
next percount								'
endif										'
'
'gosub flashout								'for testing only
'
oscfreq = 0									'reset oscillator frequency
'
if wrtcount > 8 then						'11 
oscfreq = $B1E7									'set oscillator frequency
endif										'
if wrtcount > 18 then						'21
oscfreq = $63C7								'set oscillator frequency
endif										'
if wrtcount > 27 then						'30
oscfreq = $15A7								'set oscillator frequency
endif										'
'
'
gosub ResetTime								'reset time 00:00:00
gosub StartTimer							'start clock timer
'
'
'
cycle:
@ clrwdt
if Ticks > 49 then
portb.4 = 1
else
portb.4 = 0
endif
goto cycle
'
'
'
flashout:									'flash out the count of eeprom writes before wdt reset
for percount = 0 to wrtcount				'with the red led
portb.4 = 1									'flash rate will still depend on oscillator frequency
pause 250									'at this stage
@ clrwdt									'
pause 350									'
@ clrwdt									'
portb.4 = 0									'
pause 250									'
@ clrwdt									'
pause 350									'
@ clrwdt									'
next percount								'
pause 500									'
@ clrwdt									'
pause 500									'
@ clrwdt									'
pause 500									'
@ clrwdt									'
return										‘
'

[towlerg]

I know of the 4MHz RC clock. What’s the other?

Actually there are three, 4 Mhz and 48Khz internal clocks and the WDT

[Art]

I always thought the wdt clock was derived from the internal RC clock, but some Googling confirms it's not.

Fortunately it appears Timer1 does not get reset with the wdt, so the byte values in the timer register
could be used instead of writing to EEPROM.

[Tabsoft]

Pretty clever Art!

[Art]

Thanks
It wasn’t my idea to use timer1 values as RAM that survives WDT reset,
but I figure as long as you haven’t started timer1, you could just write the count into
one of the timer1 bytes instead of on-chip EEPROM and read the value after the reset.

For my clock board I have swapped the 8MHz crystal with a 20MHz crystal,
and added a dual flip flop to divide two of the clocks, so now have:
8,12,16,20 & 24MHz. One dual more flip flop, and I’ll have 6 & 10MHz as well.
It has been reliable so far with the freqs I can test, bust suspect any 1MHz increment is easy to detect.

[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.