On Thu, May 18, 2006 at 11:01:29PM -0400, Jesse Lackey wrote:
> Actually I wish the same thing, I have a project that doesn't need much 
> i/o but will require a bunch of logic.  I think one just says "oh well" 
> and has 70 unused FPGA pins...

Yeah, think of it as as one very mechanically secure chip. :)

I've noticed a lot of BGA packages now have heaps of unused balls on
them, connected to ground or left floating. Heck, often they use groups
of four balls her pin!

> For three PWM and I2C, why not do the PWM in software?  I'm doing 
> similar for a RGB LED driver + serial control with the PIC18LF1220, a 
> much smaller PIC could be used (10? 12? series, am not familiar with 
> them), but I want hardware serial.  PWM in software is pretty easy.  The 
> 1220/1320 is available in TSSOP, SOIC and DIP.  I'm using the TSSOP and 
> each rgb "node" is not much bigger than the chip itself.

Well given a 8mhz PIC, the fastest speed the internal oscillators work
at, you get...

2,000,000 instructions per second / 256 PWM steps / 256 hz = 30.5
instructions per cycle..

Marginal really. I've done stuff like that before, one project I did was
a three color LED in PWM on a 12f chip, when all was said and done I had
left something like 10 instructions per cycle for the non-pwm code...
That was at 120hz. But it changed the colors over the course of hours so
it worked just fine.

For this project I'm not sure I can fit software PWM and communication
with a RTC and debounced button presses in at once. Seems very tight but
I can't think of a better way to do software PWM.

-- 
pete@petertodd.ca http://www.petertodd.ca
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