Depending on how long that measure can be (seconds? days? years?), you
will need:

* A timer (better 16bit one than 8bit one, you can choose depending on
it not being already allocated for a peripheral). Take a look on the
datasheet, it is straightforward to enable a timer.
* A timer interrupt, to increment a global variable that will count
how many times the timer carried over.
* A global variable. Let's say that depending on your timer, OSC
frequency and pre/postcaler, you have an interrupt every 200uS. The
variable will increment once every 200uS so depending on how many of
these ticks you need to count, this may go to a 32bits or even 64bits
variable.

2008/4/11, Dave Roseman <dave@transpower.uk.com>:
> Hi,
>
>
>
> I am using a PIC16F690 to monitor how long a power supply is active for, but
> I have no idea how to use the timer in the micro to monitor this signal.  It
> does go high when the power is on, but how to get this info to the micro, I
> don't know.
>
>
>
> Also, this reading is required to be cumulative, so has to remember the
> previous on time and append to it.
>
>
>
> I'm just starting PIC programming, so please forgive me if this is too
> simple or obvious a question.
>
>
>
> Regards
>
> Dave Roseman
>
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-- 
Ariel Rocholl
Madrid, Spain
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