AVR-Chat

----- Original Message ----- 
From: David D. Rea
To: avr-chat@yahoogroups.com
Sent: Saturday, December 04, 2004 5:06 AM
Subject: [AVR-Chat] ATTiny26: Using USI as UART // Issue


Hi Everyone-

First post here, so I humbly beg your pardon if this topic has been
discussed previously. I did a quick search through the message archives
and didn't turn anything up.

I'm working with the following setup:
      - ATTiny26 @ 1MHz
      - WinAVR for C development
      - ATAVRISP

I'm trying set up a simple means to get debug messages out of the micro.
Just dump the values of some ADC samples, etc. I found the Atmel appnote
regarding using the USI as a UART via Timer0 and the USI overflow
interrupt.

Sounded pretty cool, but their example with both Tx/Rx and buffering was
overkill for my needs. All I really need is a sendchar function. So I
figured I'd just implement their concept on my own, only the Tx side.
Besides, their example was written in IAR C, and I'm working with the
GNU avr-libc, so in the time it would have taken to port it, I figured I
could implement it myself.

Got quite a ways along with it, but I've got one problem (thus far at
least). The Timer0 counter register needs to be seeded with a value to
set the baud rate after each bit time. Got that working great, but as
soon as I enable the USI by setting its clock source to be the Timer0
overflow, it up and clears the interrupt flag and I've got no Timer0 ISR
to seed the timer any more! :(

The appnote doesn't seem to address this; it's written (evidently) under
the assumption that Timer0 can both clock the USI *and* fire off its own
ISR. Which must be the case... I'm just missing it somewhere in my
implementation.

Has anyone run across this? Or something similar?


I was thinking of doing something similar with a Tiny11. I'd just use a 
simple software UART written in assmbler.

Leon