Watchdog pre-scaler values

Using Arduino Programming Questions
1

I am running a barebones board with a ATMEGA328P running the internal oscillator at 8mHz and 3V.
What would the watchdog pre-scaler values be?

I have only found:

   *  Setting the watchdog pre-scaler value with VCC = 5.0V and 16mHZ
   *  WDP3 WDP2 WDP1 WDP0 | Number of WDT | Typical Time-out at Oscillator Cycles
   *  0    0    0    0    |   2K cycles   | 16 ms
   *  0    0    0    1    |   4K cycles   | 32 ms
   *  0    0    1    0    |   8K cycles   | 64 ms
   *  0    0    1    1    |  16K cycles   | 0.125 s
   *  0    1    0    0    |  32K cycles   | 0.25 s
   *  0    1    0    1    |  64K cycles   | 0.5 s
   *  0    1    1    0    |  128K cycles  | 1.0 s
   *  0    1    1    1    |  256K cycles  | 2.0 s
   *  1    0    0    0    |  512K cycles  | 4.0 s
   *  1    0    0    1    | 1024K cycles  | 8.0 s

I require the micro to sleep for an ~hour - interval is not critical.

2

For which purpose? The table you've listed shows the approximate time it takes from a watchdog reset to a watchdog interrupt / system reset.

3

I will use a counter to count the number of 8 second intervals.
Using this I can run a function after XXX counts.

4

Shouldn't it be obvious that you need the 1k (last in table) prescaler then? Also be warned that the WDT is not consistent; You cannot expect 8 seconds to be exactely 8 seconds. The interval may vary quite a lot and how much depends on supply voltage and ambient temperature. But it's great fun to play with, though! :wink: