slow down timer

[Guest]

Hello, I would like to slow down a timer so that I can measure longer time interval.
Is the following idea feasible:
Use (for example) PWM to output divided crystal frequency, and feed this signal back to timer external input.
So my idea is:

[kisslove]

[Ttelmah]

Obviously not timer2. This doesn't have an external input, and with what you are doing, would be the clock 'source' for the other timer(s).

Yes, this is perfectly acceptable.

The 'sync' setting, ensures that the timer actually updates _in sync_ with the internal clock. Shouldn't actually matter, since your clock is in sync already.

What you show though, could be done by using Timer0. with no extra pins.

Timer1, only supports 1,2,4 & 8 'prescalers'. Timer0, allows 2,4,8,16,32,64,128, & 256 prescalers, easily going beyond what your extra division is doing...

Best Wishes

[Guest]

Thank you guys.
@Ttelmah:

[kisslove]

[quote=:
I want to run the PIC at 20 or 24MHz crystal (I can't go lower because of plenty of calculations to be done - "CPU" load), the timespan I want to measure is about 6 seconds.
Hence, I reckon (I hope I am right) that using Timer0 with 256 prescaler yields only 3,3 seconds.
So that is the reason why I want to slow down the timer.
[/quote]

Just arter T0 overflow write 5900 to it, it will give you 3 seconds, then use flag to count periods. If you have T0 interrupt and your flag=1, then 6 seconds passed.

[Ttelmah]

The slower your timer clock, the lower the resolution.
Now, it is very easy to just _count_. This is how time measurement/keeping is done in most systems. All you do is have a timer/interrupt at some fast interval, that is easy to setup, and have it count up or down, till a target is reached. Hence (for example), on the older PC's, though there is a real time clock chip, this is not actually read to generate 'time', excpet during boot up. nstead the system, just counts using an interrupt at 18mSec intervals. On modern PC's, the RTC, is checked occassionally, and the 'tick' interval is now faster, but the basic time handling is still done this way.

Now it is not clear from what you have said so far, if you want to _measure_ a time, or just have a 'trigger' at a particular interval.
The latter can be done using your hardware idea. However you won't get absolute precision. The problem is that the interval you need, is not exactly divisible by 65536, which is the basic 'count' from any of the 16bit timers. Unfortunately, though you can load a counter 'to' a value, or advance a counter (the latter is more accurate if working at any speed), doing so, _resets the counter's prescaler_, leading to cumulative errors...
There is no 'need' or point to using a 50% duty cycle. The clocks work on one _edge_ of the arriving signal, so couldn't care what the mark/space ratio actually is.

So, start wth your master clock - say 24MHz.
The 'system clock' that feeds any timer, is this /4. 6MHz.
Then for 6 seconds, you would want to count 36000000.
Divide this by 65536. 549.3. So you need a 'prescale' of 549 to get as close as you can. Biggest division available from timer2 itself, is 256, so you will need to use it's prescaler. To get below 256. Use the smallest one that mees this criterion. /4. This then gives you needing to use a division by 137, to give 548 as the nearest possible counter using this approach. A timeout (from the 24MHz oscillator) of 6000000/(4*137*65536) = 0.167 times per second. 5.985 seconds.
Downside, using two timers, two hardware pins, and still not that accurate.

One (of many) alternatives, using software.
Look for a nice 'decimal' divider of the master clock. So Timer2, has a 'postscaler' supporting /1 to /16, binary prescalers of /1, /4, and /16. Now, start again with 24Mhz, and the _largest_ available decimal multiplier possible from this timer, is: /16 /250 /16 = 64000. However 60000, is a nicer fit to the 6MHz clock, so /16, /250, /15. With this, the timer will interrupt every 1/100th second, requiring a simple count to 600, for exactly 6 seconds.
Now, the big advantage of this, is that most systems _will_ require other timekeeping functions. Things like keyboard scanning, debouncing etc., all require delays. 1/100th second, is a nice 'housekeeping' tick, to use for all these jobs, and gives you time for your required application as well.

[krey]

thanx to both of you.

[krey]

It should also be noted that the resolution can be as bad as 1ms, and that I really need to run the PIC at at least 20Mhz since it has plenty of other things to do.
cheers

[kisslove]