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c++timerarduinopwmmotordriver

Using 4 16bit timers for 400hz PWM

I'm dealing with arduino mega based quadcopter and trying to make PWM frequency for 4 motors - 400hz each.
I've found an interesting solution where 4 ATmega2560 16bit timers are used to control 4 ESCs with PWM so it could reach 400hz frequency. 700 to 2000µs are normal pulse widths ESC are dealing with.
1sec/REFRESH_INTERVAL = 1/0.0025 = 400hz.

this is servo.h lib:
#define MIN_PULSE_WIDTH       700     // the shortest pulse sent to a servo  
#define MAX_PULSE_WIDTH      2000     // the longest pulse sent to a servo 
#define DEFAULT_PULSE_WIDTH  1000     // default pulse width when servo is attached
#define REFRESH_INTERVAL     2500     // minimum time to refresh servos in microseconds 

#define SERVOS_PER_TIMER       1     // the maximum number of servos controlled by one timer 
#define MAX_SERVOS   (_Nbr_16timers  * SERVOS_PER_TIMER)

The problem is to make it work each PWM should be controlled with 1 16bit timer. Otherwize, say, 2 escs on 1 timer would give 200hz. So all of 16bit timers are busy controlling 4 ESC but I still need to read input PPM from receiver. To do so I need at least one more 16bit timer which I don't have anymore.
It's still one 8bit timer free bit it can only read 0..255 numbers while normal number escs operate with are 1000..2000 and stuff.

So what would happen if I'll use same 16bit timer for both pwm and ppm reading? Would it work? Would it decrease speed drastically?
I have arduino working in pair with Raspberry Pi which controls data filtering, debugging, and stuff, is it better to move ppm reading to Raspberry?

Solution

  • To answer one of your questions:

    So what would happen if I'll use same 16bit timer for both pwm and ppm reading? Would it work?

    Yes. When your pin change interrupt fires you may just read the current TCNT value to find out how long it has been since the last one. This will not in any way interfere with the timer's hardware PWM operation.

    Would it decrease speed drastically?

    No. PWM is done by dedicated hardware, software operations running at the same time will not affect its speed and neither will any ISRs you may have activated for the corresponding timer. Hence, you can let the timer generate the PWM as desired and still use it to a) read the current counter value from it and b) have an output compare and/or overflow ISR hooked to it to create a software-extended timer.

    Edit in response to your comment:

    Note that the actual value in the TCNT register is the current timer (tick) count at any moment, irrespective of whether PWM is active or not. Also, the Timer OVerflow interrupt (TOV) can be used in any mode. These two properties allow to make a software-extended timer for arbitrary other time measurement tasks via the following steps:

    1. Install and activate a timer overflow interrupt for the timer/counter you want to use. In the ISR you basically just increment a (volatile!) global variable (timer1OvfCount for example), which effectively counts timer overflows and thus extends the actual timer range. The current absolute tick count can then be calculated as timer1OvfCount * topTimerValue + TCNTx.
    2. When an event occurs, e.g a rising edge on one pin, in the handling routine (e.g. pin-change ISR) you read the current timer/couter (TCNT) value and timer1OvfCount and store these values in another global variable (e.g. startTimestamp), effectively starting your time measurement.
    3. When the second event occurs, e.g. a falling edge on one pin, in the handling routine (e.g. pin-change ISR) you read the current timer/couter (TCNT) value and timer1OvfCount. Now you have the timestamp of the start of the signal in startTimestamp and the timestamp of the end of the signal in another variable. The difference between these two timestamps is exactly the duration of the pulse you're after.

    Two points to consider though:

    1. When using phase-correct PWM modes the timer will alternate between counting up and down successively. This makes finding the actual number of ticks passed since the last TOV interrupt a little more complicated.
    2. There may be a race condition between one piece of code first reading TCNT and then reading timer1OvfCount, and the TOV ISR. This can be countered by disabling interrupts, then reading TCNT, then reading timer1OvfCount, and then checking the TOV interrupt flag; if the flag is set, there's a pending, un-handled overflow interrupt -> enable interrupts and repeat.

    However, I'm pretty sure there are a couple of library functions around to maintain software-extended timer/counters that do all the timer-handling for you.