Too many design options, the cat understands electronics slightly more than the wife, so time to turn to the forum :P
I need to write 10 bytes to eeprom on power loss (user turns equipment off, rather than unexpected internal fault), so I need to detect power loss and have enough time/power to write the eeprom. I also want rock solid and clean startup and shutdown of the processor. I'm happy with the software side of it all...
...but I've two circuit options so far - A and B at http://www.milleners.eclipse.co....
INT0 would be a triggered on low level, and the ISR would write the EEPROM and the go into an infinite loop.
To save typing, I'll not explain the circuits - if you can understand them, then you're comments will be most appreciated 8)
The significant components in the rest of the circuit are a MAX232, a MAX7219 multiplexing 8x7-segment LEDs and a remote ATMegaXXXX mutiplexing 4x7segment LEDs using discrete transistors rather than another MAX7219
So a reasonable current draw...
On A (zener is 4V8 or 5V1, BTW), I could send the MAX7219 into shutdown mode pretty quickly within the ISR to stop it (and the LEDs) drawing too much current and give me more time but I can't do that for the remote ATMega and its LEDs
On B, the supply voltage at Vcc and the MC34064 is estimated at 4.8V. Will this be OK or is it too low?
Either circuit - use BODEN as well as MC34064? Or ditch MC34064 on both and just use BODEN
Is there a circuit C hiding somewhere?
Thanks in advance