jgmdesign 
I am lost here
I am trying to use the mega 8515's timer0 for PWM. it must operate all by itself and output a pulse of 1/8 of a second and be still for 1 second
Once again, my code sucks and I am ready to start over.
Any assembler suggestions?
Jim
So you want an output form of 125ms high output followed by 1000ms low output, and continuously repeat that form. Is that correct? At what speed will your device be running?
jgmdesign right now i am using the 3.68mhz on the stk, but it will eventually run at 8mhz. the key right now is to just get the thing to allow mr to vary the pulse widths etc.
Thx
Jim
The 8-bit timer will roll-over faster than your desired output frequency. At 8Mhz with a maximum 1024 prescalar and a maximum 256 count, the period of the timer will be 32.768ms.
Personally, for such a slow rate and for maximum flexibility, I'd just keep track of time in a periodic interrupt. For many of my designs, I have timer0 interrut every 5-10ms to handle background processing (or at least flag for the main routine that a process is due to be performed). Mainly, the 5-10ms comes because I tend to use that as a button debouncing period.
For your desired output frequency, you can either use the 16-bit timer1, use a slower system clock speed, or keep track of time yourself and just change the output pin after counting 125ms then again after counting 1000ms.
jgmdesign I would like to use the timer1 which is 16bit. I am using timer0 to generate a 32khz square wave to run a a/d converter
Jim
The simplest approach would be to use timer1 in CTC mode with a prescalar of 64 and a CTC value of 15625. At 8MHz, that'd give you periodic interrupts at 125.0ms. You then raise your output for 1 interrupt and keep it low for the next 8 interrupts.
If you wanted to produce your desired signal without interrupts or bit-banging the output pin, then you can use a larger prescalar, set OCR1A to a value for your 125ms high and OCR1B for half of your 1125ms overall period, then OR those two output lines. You'd have to carefully setup your PWM mode so that OC1B is high for the 125ms that OC1A will be high, but then not high again until 1125ms had passed from when it last became high.
jgmdesign This is starting to sound way more complicated than it should.
What sucks is that I am currently doing everything in assembler. Most freaks use 'c'. I am at a disadvantage
Jim
theusch 
The simplest approach would be to use timer1 in CTC mode with a prescalar of 64 and a CTC value of 15625. At 8MHz, that'd give you periodic interrupts at 125.0ms. You then raise your output for 1 interrupt and keep it low for the next 8 interrupts.
If you wanted to produce your desired signal without interrupts or bit-banging the output pin, ...
...then you can use a larger prescalar, set OCR1A to a value for your 125ms high and OCR1B for half of your 1125ms overall period, then OR those two output lines. You'd have to carefully setup your PWM mode so that OC1B is high for the 125ms that OC1A will be high, but then not high again until 1125ms had passed from when it last became high.
But that seems like a lot of work. A quick session with AVRCalc shows that with a /64 prescaler 125ms is 0x1c20 -1 = 0x1c1f, and the full 1125ms period is 0xfd20 - 1 = 0xfd1f.
If you want to use OCR1B, you can use mode 14 or mode 15. Let's say mode 14. Then you load ICR1 with the period, and OCR1A or OCR1B with the on-time. Vary the pulse width with OCR1A/B and the period with ICR1.
Nary a clock cycle for continuous operation nor any tied pins in sight.
Lee
jgmdesign Lee
I am still a tad confused here.
ICR1 sets the pulse width?
and oca/b set the time between the pulses?
mr was supposed to read "me" not 'mr' bad typing
Jim
jgrunt Koshchi ICR1 sets the pulse width?
and oca/b set the time between the pulses?
If you want to use OCR1B, you can use mode 14 or mode 15.
theusch
What sucks is that I am currently doing everything in assembler. Most freaks use 'c'. I am at a disadvantage
For excellent worked timer examples in ASM, visit www.avrbeginners.net .
Lee
jgmdesign Here is a new twist.
When I set the wgm11, and wgm10 bits the ocr1h byte is no longer writeable. even if I write to the registers before anything else, as soon as I set those wto bits, the register is cleared and that is it.
It almost seems that te software thinks the timerr is 8bit
Hee is the code:
.include "m8515def.inc"
.def temp =r16
.def tempa =r17
begin:
ldi temp,0xff
out ddrd,temp
cli
ldi temp,0x18
out tccr1b,temp
ldi temp,(1<<com1a1|1<<wgm11|1<<wgm10)
out tccr1a,temp
ldi temp,0x18
out tccr1b,temp
ldi temp,0x00
ldi tempa,0xf0
out ocr1ah,tempa
out ocr1al,temp
ldi temp,0x19
out tccr1b,temp
done: rjmp done
I have no clue at this point
Jim
theusch How do you 'know' it is going to 0?
jgmdesign when you run the code in simulator using the step function you can watch it. I noticed that there is an issue with the 16bit registers in avrstudio as a known issue. will need to look into that....gonna try the dragon tonight
Jim
