Elapsed Timer findings

[LinkMTech]

Excellent tutorial Tabsoft.
Explains why I always had to "fine tune" after the fact when setting up a timer.

[Art]

Ok I was thinking where you said you made the PWM to measure on a scope,
so had my best go at that sitting here rather than scab the code
Probably not the most effective with memory or time, but I think the frequency will be correct.
and it’s probably also more like asm pseudo code at the moment.

‘start of code somewhere:
‘
flipflip var byte


‘in ISR straight after Ticks is incremented
‘
@ btfsc flipflop ,00 		; 1/2
@ goto aaaa			; 2
@ goto bbbb			; 2
aaaa:				‘
@ bsf portb, 07			; 1
@ goto overpwm			; 2
bbbb:				‘
@ nop				; 1
@ bcf portb, 07			; 1
overpwm:			‘
@ comf flipflop 		; 1 - but timing doesn’t matter here

‘instruction time 6 for flip state, 6 for flop state,
‘7 for the code to execute no matter the status

I like the idea the PWM is almost free if you wanted it,
and there might be a situation you’d want the electrical frequency of the timer.

It might not have originally been obvious I was setting this up for a pic.. only works up to 20MHz though:

'$FFFF - (((20 / 4) * 10000) - 8) = $3CB7
‘might as well fix the constant val too

constval = $FFFF - (((f/4)*10000-7)'

Though I have not needed to, it’s pointless unless you are varying the pic’s oscillator frequency,
and also telling the pic program the new osc frequency, or figuring out some other way what frequency the pic is running at.
That PWM would indicate resolution error at runtime for different osc frequencies.

[Tabsoft]

Very similar to what I did.
I used the Elapsed Timer to generate a 1KHz 50% duty cycle signal on PORTA.4.
I wanted to test the stability of the Timer1 interrupt period using the Elapsed Timer code we have been discussing.

In the ISR I used a call to the RELOAD_TIMER macro after setting the state of the pin.
That lets the Elapsed Timer code handle things and I could check the accuracy of the output.

I was lazy though and did not setup a variable to track the state of the pin.
I just read the port and flipped the pin.
This was a test and I really didn't mind if I hit the other pins on PORTA.

500ms Timer1 ISR:

        asm
            COMF PORTA, 0   ; 1 Tcy - Complement PORTA, Store in W
            ANDLW 10h         ; 1 Tcy - Force all PORTA pins to 0 except pin 4 
            MOVWF PORTA    ; 1 Tcy - Set the new value for PORTA
            RELOAD_TIMER    ;  Reload the timer
        endasm

[Art]

There’s some fun to be had. It would be possible to rotate a byte with any value in it
and look at the value of a bit in that byte to get some more free frequency divisions.
Or if you had a 100Hz timer and wanted a 1Hz pulse, you could do similar to the timer code,
load a constant value to a byte, increment it, and watch for one of the more significant bits to be set.