Kenneth Lumia wrote:
>> There are several parts
>>
>> Timebase
>> Timekeeping
>> Alarms
>> Interface
>>
>> Timebase is the "tick" of your calendar. This can be from an
>> external source, such as mains cycles, oscillator or IC. The
>> gif I posted shows a 0.5Hz output but an alternating 0.5Hz
>> is available at the other output pin -> 1Hz if you use both.
>> If you want a less fine resolution, you can divide this down
>> with 4040(s) to get 1 pulse per minute or hour or day
>>     
>
> Easiest is a 32.768kHz crystal connected to T1 osc pins.  If 
> you select the appropriate divide downs in timer1, you can 
> get a very accurate interrupt.  If that is not an option, you could 
> use Timer0 and divide the system clock down.  This is not 
> as nice, as the binary divide down typically does not work 
> out to a 10 mS multiple.  In this case you must preset the 
> counter each interrupt, instead of allowing it to simply rollover 
> as in the 32.768kHz case.   This causes some error as the 
> timing is not consistent (depends on what else is going on) 
> and that every write to the counter causes the prescaler to be 
> cleared.  You will need to provide some sort of clock adjustment 
> in the code.
>
>   
[snip]
>
> Ken
>   

Obviously (or not), you can also choose a convenient crystal for your 
system. I have a project with a 6.5536MHz crystal (with 4xPLL) that is 
perfect for generating 10ms interrupts on timer Rollover (I use this for 
timing the shutter-release on a camera to 1/100th of a second...).

Other convenient crystals....
4.19304MHz (no PLL, and 16prescale, or PLL with 64 prescale) gives 1 
interrupt per second, or, if you have a flashing segment to indicate 
"ticks", it is really easy to get 2 interrupts per second and thus get a 
flash-on,flash-off sequence.

You can do the math for other frequency crystals.... there are a number 
of convenient ones.

I consider this to be easier than the 32.768 option because it is fewer 
parts. Power saving opportunities are less though.

Rolf

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