[sdiy] Really Low, LFO

[Tom Wiltshire]

On 3 Apr 2010, at 08:52, Simon Brouwer wrote:

> Tom Wiltshire schreef:
>> My PIC LFO goes down to 0.05Hz (20 seconds) as it stands. If you  
>> were to slash a factor of 256 out of the sample rate (to a 76Hz  
>> sample rate!), that'd go down to 0.000195Hz, 85 mins a cycle.
> To put this into perspective for Kyle, would this be any more  
> complicated than:
>
> 1) adding a variable PRESCALE (line 151)
>
> 2) adding the following 2 lines just before the SelectDDSOrNoise  
> label (line 233):
> incfsz PRESCALE, f
> goto InterruptExit

No, it's no more complicated than that.
Adding a 'PRESCALE' variable would be one way, but TIMER2 has a  
prescalers in the hardware, so you can just change the initialisation  
of TIMER2 to enable it. Unfortunately it only goes to 1:16, so we  
have to get rid of another factor of 16 somewhere. (There's a  
postscaler too, but the PWM rate isn't affected by it - that'd make  
life *too* simple!).
  Perhaps we could change the crystal from 20MHz to 1MHz too. That'd  
drop the overall frequency range by 320!

movlw   b'00000100'           ;  Enable TMR2 (prescale set to 1:1)
movwf   T2CON

becomes

movlw   b'00000111'           ;  Enable TMR2 (prescale set to 1:16)
movwf   T2CON

My line numbering doesn't match yours for some reason, but this is  
just under the 'Main' label at the start of the main code. About line  
500 for me.

In short, there are quite a few ways of achieving this, and none of  
them are particularly difficult.

Regards,
Tom