PIC Sequencer software step rate?

dugernaut · Joined: Jul 24, 2009 Posts: 41 Location: New Jersey

I've written a fairly basic 16 step sequencer for the PIC cpu, but I've been trying to use the internal timer controlled by the ADC as a clock source.

It works, but the linearity (or my perception of linearity) sucks. When I increase the input to the ADC with a linear pot, I use that value to adjust the load into the timer. If I look at the output LEDs of the ADC (on my development board) they seem to be roughly doing what I'd expect.

The result is that very little happens at the low (slow) end, but as I move the input up, the speed slope happens way too fast. It's like a two cascaded expo generators controlling a VCO.

Maybe I'm thinking about this all wrong, but I've spent hours on this. Any help would be greatly appreciated.

Blue Hell · Posted: Mon Aug 16, 2010 12:38 pm Post subject:

How to do it depends a bit on what timer you use in exactly what PIC ...

Keep in mind that timers count up for the PIC until they overflow which then generates the interrupt ... so you'll want to subtract the value from zero (or 65536).

Unless you use an 8 bit timer with a preset register, in which case you can put the value into that one to determine the period. Such a timer will count up from zero until the PR register value is hit, at which time it will generate an interrupt.

Another thing is that probably you do not want to linearly change the timer period, but its frequency rather - which is inversely proportional, meaning you'll want to put in 1 divided by the AD value. (or maybe 1024 divided by ... or whatever scaling turns out to work best).