[sdiy] LCDs, encoders, and interrupts

[Colin f]

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Tom 
> Wiltshire
> Sent: 27 February 2010 10:28
> To: synth-diy DIY
> Subject: [sdiy] LCDs, encoders, and interrupts

> The problem I've got is that the encoders and the LCD share a 
> databus. This means that the 'read encoder' interrupt can 
> occur whilst the LCD is in the middle of trying to update the 
> value - indeed, because LCDs are so slow, this is quite 
> likely. I can save and restore the state of the databus and 
> control lines to the LCD along with other variables going 
> into the interrupt routine, but this on its own isn't enough. 
> If I'm halfway through a LCD data transfer, it will get screwed up.
> 
> What to do? Has anyone else had to think about this? I don't 
> want to waste time reinventing the wheel if there's a good 
> solution out there already.

One solution, if you have the spare RAM, is to buffer writes to the LCD.
Have the main program write LCD data to the RAM buffer, then the interrupt
routine that scans the encoder also checks the LCD buffer for changes, and
incrementally updates the LCD.
This gives no conflict, and also greatly speeds up LCD access, since you
only make slow writes for data that has actually changed.
It can make your main code section much more responsive.
If you use a count variable to regulate the writing of bytes to the LCD, you
can ensure each LCD write is complete before the next one is attempted, so
you don't even have to check the LCDs busy flag.

Cheers,
Colin f