Ignore all that.... and before Olin lambasts me... I found it in the 
datasheet.....

Rolf

Rolf wrote:
> Hi all.
>
> I am building a transmitter/receiver combination both of which use a 
> PIC12F675 as the primary controller. Both are battery powered (2xAA 
> lithium cells, i.e. mostly operate at about 3.5V for 90% of the cell's 
> life, and then falls off rapidly. Devices are programmed to shut down 
> below 2.5V). So, I essentially operate at between 2.5 and 3.6V.
>
> The transmitter sleeps for a period (one sleep instruction with WDT 
> enabled to wake device, and 1:128 prescalar), wakes, reads some 
> sensors (2 digital inputs), monitors the system volts (one A/D 
> reading), transmits its state (about 32ms), then goes back to sleep.
>
> The receiver uses an algorithm to basically synchronize itself to the 
> transmitter's 'period' by using multiple short sleep periods to sleep 
> most of the time, then wakes up, enables the receiver, receives the 
> transmission, then goes back to sleep.
>
> In building this system I have come to realise that the actual sleep 
> time as described by the datasheet is very different to what I 
> expected. The datasheet indicates a 'nominal' WDT wakeup happens after 
> about 18ms. With 1:128 prescalar the datasheet indicates about 2.3 
> seconds. My calculations (using a stop-watch) are that 10 transmit 
> cycles take 29 seconds, or 2.9 seconds per transmit cycle.
>
> Now, I expected variations in the sleep times, and the receiver's 
> algorithm is able to adjust itself to pretty much any sleep period, 
> within reason, but, for the moment, I have set limits on a 'valid' 
> transmission being received between 1.5 and 4 seconds.
>
> Now that it all seems to be working quite nicely, I am trying to tune 
> the power consumption by reducing the 'on' time of the receiver as 
> much as possible. In order to do so I want to 'fine-tune' the 'window' 
> in which a valid transmission can be expected. I want to make the 
> 'valid' window less than half the transmit period so that I can't 
> possibly get two transmissions in one window....
>
> Thus, the question....
>
> what sort of variance can I expect from the WDT wake-up from sleep 
> period in the PIC12F675?
>
> Currently I am seeing 25% longer than 'spec'.
> The transmitter will be mounted in an unheated building in Canada and 
> must operate year-round (temp range from -40 to +40 I guess - hence 
> the lithium batteries....).  The receiver will be in a 'climate 
> controlled' house, thus limited temp variances between +15 and +35 I 
> guess.
>
> What can I expect as the range of sleep times? where can I find such 
> data? Does the sleep time increase with temperature? Does it decrease 
> with Vcc?
>
> Thanks for any input.
>
> Rolf
>

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