AVR Freaks

Thanks - I agree - I think I'm going to give up on any sort of soft off and just put a power switch in.  I still want to reduce power consumption when it is idle, etc., just to make the battery last longer, so I'll see what I can do.  I'm going to give the tiny84a a try and see what I can do with it.

I got some tiny84a's in today and some testing has been good:

power down mode = 0.16 uA (wow) - I can use a pin change on any of my 5 buttons to wake it back up (not limited to just INT0).

This is awesome because it means I don't have to mess with a real power switch at all.  CR2032 capacity of 225 mA / .00016 would yield 160 years so the cell will die of old age certainly before that.

idle mode = 140 uA and active mode = 442 uA.

All of this tested at 3V and 1 MHz clock speed.

I have another question asking about disabling the BOD in power down mode - but I then thought - do I need to use BOD ever at all since I am using a CR2032 battery.  If voltage gets too low, the LEDs won't light before the AVR will give out anyway.

I prefer to write in gcc with the datasheet.

I'm using idle mode now as well to reduce consumption while it is waiting on any buttons to be pressed:

uint8_t inputget()
  {
    static uint8_t lastkey;
    uint8_t c1;

    for(;;)
      {
        if (inputcount)
          {
            c1=inputbuffer[inputout];
            inputout++;
            if (inputout==KEYBOARD_BUFFER_SIZE)
              inputout=0;
            ATOMIC_BLOCK(ATOMIC_FORCEON)
              {
                inputcount--;
              }

            if (c1)
              {
                lastkey=c1;
                return c1;
              }
            else return lastkey | _BV(7);
          }

        //sleep until interrupt (reduces power from 440 uA to 150 uA for a 66% power reduction)
        set_sleep_mode(SLEEP_MODE_IDLE);
        sleep_mode();
      }
  }

That is what I'm asking avrcandies - if I put a unidirectional TVS in there designed for 3V so that it conducts over 4V, then it can provide some static protection and at the same time if they reverse the cell by accident, it will be a short on the cell and possibly keep voltages from doing any damage.  I suppose shorting the cell really isn't much protection though, but the question is could it be the difference between someone seeing the mistake and flipping over the cell to a working thing vs. not a working thing.

EDIT: Then there is also the TVS + a small PTC.  Reversing the battery would cause large current and the PTC could trip.  What is the power efficiency of a PTC vs. MOSFET vs. Schottky?

That is what I am thinking, it could work against a unidirectional TVS.  Most PTC's are not so small on current, I found 0ZCM0001FF2G which says that it is 15 ohms Rmin.  If AVR load is 1mA @ 3V that would be a 3000 ohm load and a 15 ohm PTC wouldn't be so bad, right?  It allows 10mA, but trips at 30mA.  If someone put the battery in backwards the TVS would conduct and the PTC would then trip and cut off the current.