??? 09/18/11 00:43 Read: times |
#183794 - Already answered Responding to: ???'s previous message |
You have already been given a good answer.
In short: With the current crystal, your timer can't be slowed down enough for 2Hz (i.e. 4 interrupts/second). But you can configure the timer to produce interrupts at a higher frequency, and in the ISR use a software counter to decide that you toggle the pin every n interrupts. So if you configure the timer for 10ms (which I think was the suggested frequency - complete with timer parameters - in a previous post), then you should toggle the pin every 25 interrupts. 25 * 10ms = 250ms. And 250 ms * 2 = 500 ms period time = 2 Hz frequency. It is standard that software programs needs to use software counters internally, for the delays. A 32-bit processor might be lucky enough to have 32-bit timers. But if ticking at 48MHz, such a timer would still be limited to 83 seconds, unless there is a prescaler. Next thing - few processors have more than 4-8 timers. Many have 1 or 2. But a program may need to handle hundreds of time-controlled tasks internally. So every task can't have a dedicated hardware timer. The solution? The one suggested above. Set up a timer ticking at 10ms. Every 100 tick (as counted with a variable in the ISR), one second will have passed. Every 60 seconds, a minute will have passed. Suddenly, it becomes trivial to handle times way longer than a timer can span. Or to handle many more events than there are hardware timers. With timers that can toggle output pins, it is often possible to make use of this hardware functionality even if the toggle frequency is slower than the timer period. This may be done by activating the pin-toggle function at the interrupt tick before the one where the pin should toggle. On next interrupt deactivate this feature again while waiting for time to prepare next toggle. Why do that, instead of manually toggling the pin in the ISR? Because the ISR has a little bit of jitter because of the interrupt response time in relation to other operations and currently processed instruction. Manually turning pin-toggle on/off can allow the pin to be toggled with zero jitter by the hardware. It's too long since I used the timer function of 8052 chips so it may not be applicable to them. But it is a good trick to remember if it is important to have very low jitter on the generated signal while still have longer periods than the timer can handle in a single delay. |