Reading PPM-signal from a RC-Remote - Servo is dithering / shaking

Projects Programming
1

Because i need high resolution (0.5µs) i wrote a sketch using TIMER4/5 with extern trigger on ICP-pin 48/49 evaluating the signal. That works really great for itself on a MEGA.
This is the sketch:

// Arduino CookBook 18.0
// https://www.inkling.com/read/arduino-cookbook-michael-margolis-2nd/chapter-18/recipe-18-8
// ICP1-Pin for UNO & Timer1: Pin 8
// ICP5-Pin for Mega & Timer5: Pin 48

#define NUM_OF_CHL 8         // we are working with an 8-ch-Transmitter
#define NUM_OF_AVG 3         // 3 added values for average

const int    inputCapturePin = 48;    // input pin fixed to internal Timer
const int    prescale = 8;            // prescale factor (each tick 0.5 us @16MHz)
const byte   prescaleBits = B010;     // pecision scale factor 8
const long   precision = (1000000/(F_CPU/1000)) * prescale; // time per counter tick in ns
volatile int overflows = 0;

volatile long valuesInt[9] = {0};
volatile long valuesUse[9] = {0};
volatile byte counter = NUM_OF_CHL;
volatile byte average  = NUM_OF_AVG;
volatile boolean ready = false;
long timelastloop;

/* Overflow interrupt vector */
ISR(TIMER5_OVF_vect)                 // here if no input pulse detected
{
  overflows++;                       // increment overflow count
}

/* ICR interrupt vector */
ISR(TIMER5_CAPT_vect)
{
  TCNT5 = 0;                            // reset the counter
  if( bitRead(TCCR5B ,ICES5) == true)   // wait for rising edge
  {                                     
    long time = ICR5 + (overflows * 65536);  // save the input capture value
    overflows = 0;
    time = time * precision / 1000;          // time in ms 
    if (time > 2500)   // this is a gap between set of pulses
    {
      valuesInt[8] = valuesInt[8] + time;
      counter = 0; 
      if (average == NUM_OF_AVG)
      { for (int i = 0; i < NUM_OF_CHL + 1; i++)
        {
          valuesUse[i] = (valuesInt[i] + 0.5) / average;
          valuesInt[i] = 0;
        }
        average = 0;
        ready = true;
      }  
      average++;
    }
    else
    { if (counter < NUM_OF_CHL)
      {
        valuesInt[counter] = valuesInt[counter] + time;
        counter++;
      }
    }
  }
}

void setup()
{
  Serial.begin(115200);
  Serial.println(F("Start reading PPM-Signal from Remote-Control"));
  Serial.print( precision);     // report duration of each tick in nanoseconds
  Serial.println(F(" nanoseconds per tick"));

  pinMode(inputCapturePin, INPUT_PULLUP); // ICP pin (digital pin 8 on Arduino/48 on Mega) as input
  TCCR5A = 0 ;               // Normal counting mode
  TCCR5B = prescaleBits ;    // set prescale bits
  TCCR5B |= _BV(ICES1);      // enable input capture
  TIMSK5 =  _BV(ICIE1);      // enable input capture interrupt for timer 1
  TIMSK5 |= _BV(TOIE1);      // enable overflow interrupt

  timelastloop = micros();   // measure loop-time with "standard" micros()
}

void loop()
{
  if (ready)
  {
    long timenew = micros();
    Serial.print(timenew - timelastloop); Serial.print(F(" | "));
    for (int i = 0; i < NUM_OF_CHL + 1; i++)
    {
      Serial.print(valuesUse[i]);
      if (i < 8) Serial.print(F(" - "));
    }
    Serial.println();
    ready = false;
    timelastloop = timenew;
  }
}

These are the resulting timings:

Start reading PPM-Signal from Remote-Control
496 nanoseconds per tick
1356 | 0 - 0 - 0 - 0 - 0 - 0 - 0 - 0 - 11127
67464 | 1384 - 1389 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67412 | 1384 - 1388 - 1505 - 1402 - 940 - 1505 - 1500 - 1501 - 11120
67416 | 1384 - 1389 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67424 | 1384 - 1389 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67424 | 1384 - 1389 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67432 | 1384 - 1390 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11119
67416 | 1384 - 1388 - 1505 - 1402 - 940 - 1505 - 1501 - 1501 - 11120
67424 | 1384 - 1389 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67416 | 1384 - 1389 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67424 | 1384 - 1389 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67424 | 1384 - 1389 - 1505 - 1402 - 940 - 1505 - 1500 - 1501 - 11120
67424 | 1384 - 1389 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11118
67420 | 1384 - 1389 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67424 | 1384 - 1389 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67420 | 1384 - 1390 - 1505 - 1401 - 940 - 1505 - 1500 - 1501 - 11120
67424 | 1384 - 1389 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67412 | 1384 - 1388 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67432 | 1383 - 1389 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67424 | 1384 - 1390 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67424 | 1384 - 1388 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67416 | 1384 - 1386 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67416 | 1384 - 1389 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
67424 | 1384 - 1389 - 1505 - 1401 - 940 - 1505 - 1500 - 1501 - 11120
67424 | 1384 - 1389 - 1505 - 1401 - 940 - 1505 - 1501 - 1501 - 11120
and so forth

Afterwards i want to drive a servo with that readed values and used simple code like shown in servo-demo shown here:
http://arduino.cc/en/Tutorial/Knob or http://arduino.cc/en/Tutorial/Sweep
Stand alone it works as expected - when i combine the demo-code with my sketch, servo is also driven as wanted.

But some curious behaviour occurs:
The servo is dithering, shaking ! Looks like as it is supplied with AC with approx. 10-20 Hz.
This behaviour remains until i disconnect the PPM-signal from Pin 48/49.
Remember: Driving the servo alone with the Demo works fine.

I commented out serval lines in the servo-lib so that only timer5 or 4 used and that its not the same used by the reading-routine.
No change.
Also i can use the simple demo-code alone with none of my code above:
Soon as i connect the PPM-signal to pin 48 or 49, the servo is rambling over the desk ....

Doesn't matter what i try .... problem of dithering, shaking servo still remains.
Does anyone have a solution or idea why this happens ?

2

At a guess I would say it is caused by the fact that you have two interrupts going off one for measurement and the other for servo control. Both are time critical and when one ISR is being serviced the other has to wait. Therefore your jitteriness is the beating of the two rates.
Cure?
Well try and move one function into hardware like an external chip for the servo control.

3

Hmm ... may be that could be the prob.
But as you can see in the listing, TIMER4 (not as described Timer5 in code, i changed that afterwards) is triggered only every 800-2200µs (control-stick min./max., not shown).
Servo-lib is manual restricted to TIMER5. I don't know on wich clock-sequence it will work.

But why does this behaviour occur when i simply connect the PPM-signal to pin 49 and TIMER4 isn't masked for that ?
Say: I only have the rudimental demo-code for knob or sweep - TIMER4 is not activated.

From my point of view external reading will result in no really remedy. I also need something like an interrupt to be noticed when a signal is evaluated.
Constantly polling values would be not the right way. My poor MEGA has a lot of other things to do .... 8)

4

Because i need high resolution (0.5µs)[/quote

What kind of servo are you using that is capable of using that low of a signal resolution?

5

Constantly polling values would be not the right way.

Who said that?
Once you off load your servo drive to hardware there is no polling to do.