How to build a multi-threading project.

[young]

I am designing a control system. the system measure the speed ( I am thinking using the timer1 interrupt) and with different speed, I should send out different pulses (very low frwquency, 20-100hz), any suggestion how to do it.

[young]

what I am thinking right now is using the timer1 interrupt to measure the speed by counting pulses from a encoder, and send out high and low output every delays (ms delay), however, I am afraid that while I am send high and low pulses by using delay, I will lose control of measuring the encoder input time. will this be a problem?

[libor]

If you're counting pulses in the interrupt service routine, it will be triggered if enabled no matter what you are doing in your main loop. So you will not lose pulses. The only problem is that the instruction under execution in tha main block gets delayed, you should consider if this small jitter in the length of you output pulses cause problem. You can eliminate most of it if you use a counter to set the length of the output pulses in the main block, and you adjust this counter in the ISR catch up with the time the processor spends in the ISR.

You van do it using separate timers for both tasks, in this case you might get only a small jitter in your output pulses if both events fire at the same time.
You can use the 18F two-level interrupt system also for this.

Another option would be to use those PICs with built-in encoder inputs. Those do the quadrature counting in hardware.

[valemike]

I'm doing the same thing right now: rb0 is an interrupt, and i send out pulses, whose varying duty cycle will control the motor speed.

You have to use PWM with the CCP module. That way, the rb0 ISRs won't interfere with your pulses' duty cycle. If you don't use PWM, it gets even more indeterminate if you have other ISRs running such as timer isrs.

In fact, if you only have rb0 interrupts, then you won't be able to 'latch' the timer1 value automatically. You'd have to save in in your rb0 isr. Think of the interrupt latency involved. If you only have 1 isr, then your interrupt latency will be consistent. If you have other isrs, and you are servicing one of them, and all of a sudden an rb0 interrupt hits, there is still the latency of finishing off the current isr, restoring registers, then the pending rb0 isr gets serviced, saves registers, runs the isr, restores registers, exits the isr, etc.

I have been able to minimize the redundant enters and exits of isrs by calling the rb0 isr function from within other isrs. This is possible for rb0 only because i think a dummy read of rb0 will automatically clear the pending rb0 interrupt flag.

But libor is right, you're better off using the quadrature encoder on some PICs, or you can do it also with flip flops and feeding the encoder bits into your external Timer1 pins rather than rb0. This way you'll latch the time without interrupt latency, and without the need of wasting cpu cycles in isrs. This is especially if your encoder rate is too high.

[Neutone]