PulsePosition not working

[schlank]

Hello!

I am trying the LoopBack example on Teensy 3.2 and Arduino 1.8.5 OSX. I downloaded the newest version from GitHub but still it won't work. There is nothing popping up in the serial console.
I want to READ from a RC receiver and with another, easy interrupt based example like this:

// Define which pin we’re using to accept the signal
#define RCPIN 1

// We have to declare as volatile as this will be modified
// by the interrupt routine, and read outside of it
volatile uint16_t channel;
uint16_t channel_start;

void setup() {
Serial.begin(115200);
Serial.println("Starting…");

// Mark the pin as an INPUT
pinMode(RCPIN, INPUT);

// Attach an interrupt handler to be called whenever
// the pin changes from LOW to HIGH or vice versa
attachInterrupt(RCPIN, RCchannel, CHANGE);
}

void loop() {
// Output the channel value
Serial.print("RC channel:\t");
Serial.println(channel);
delay(50);
}

void RCchannel() {
// If the pin is HIGH, start a timer
if (digitalRead(RCPIN) == HIGH) {
channel_start = micros();
} else {
// The pin is now LOW so output the difference
// between when the timer was started and now
channel = (uint16_t) (micros() – channel_start);
}
}

It works! So no electrical issue. Any idea where to start debugging?

[manitou]

You might try updating to the latest IDE. On my T3.2 I ran PulsePosition lib loopback example with pin 9 jumpered to pin 10. I also added while(!Serial); as the first thing in setup(). Sketch works for me on 1.8.13/1.53

...
239 :  600.02  1500.00  759.23  1500.00  1500.00  1234.54  
240 :  600.02  1500.00  759.23  1500.00  1500.00  1234.54  
241 :  600.02  1500.00  759.23  1500.00  1500.00  1234.54  
242 :  600.02  1500.00  759.23  1500.00  1500.00  1234.54
...

https://github.com/PaulStoffregen/PulsePosition

[schlank]

Hi!

I realised that PPM means that several servo channels are somewhat encoded in one wire? My receiver has 3 outputs with a servo channel each.. so I guess it is a different technology? PWM vs. PPM?

[KurtE]

In many cases like this, it might help to know more information, like what receiver you are using... Also how you have it hooked up and the like.

[schlank]

Hello!

Connection:
GND<-->GND
CH1 <-> PIN10
5V <-> 5V

[KurtE]

I could be wrong, but I think you will only receive one channel pin pin, with a simple pulse that centers around 1500us
You can probably get the value using: pulseIn (https://www.arduino.cc/reference/en/...ed-io/pulsein/)

There are other ways as well, like setting up interrupts, but may not be very accurate.
As PulsePositionInput allows pin 10, I am pretty sure one could create a version of the library that maybe captures the time of when pin goes high and goes low to return the width...

[schlank]

PulseIn looks sufficient for reading one channel. Is the version that is used for Teensy interrupt based?

[KurtE]

Nope, just a hard loop.

If you look at the sources(pins_teensy.c)

#if defined(KINETISK)
uint32_t pulseIn_high(volatile uint8_t *reg, uint32_t timeout)
{
	uint32_t timeout_count = timeout * PULSEIN_LOOPS_PER_USEC;
	uint32_t usec_start, usec_stop;

	// wait for any previous pulse to end
	while (*reg) {
		if (--timeout_count == 0) return 0;
	}
	// wait for the pulse to start
	while (!*reg) {
		if (--timeout_count == 0) return 0;
	}
	usec_start = micros();
	// wait for the pulse to stop
	while (*reg) {
		if (--timeout_count == 0) return 0;
	}
	usec_stop = micros();
	return usec_stop - usec_start;
}

uint32_t pulseIn_low(volatile uint8_t *reg, uint32_t timeout)
{
	uint32_t timeout_count = timeout * PULSEIN_LOOPS_PER_USEC;
	uint32_t usec_start, usec_stop;
	
	// wait for any previous pulse to end
	while (!*reg) {
		if (--timeout_count == 0) return 0;
	}
	// wait for the pulse to start
	while (*reg) {
		if (--timeout_count == 0) return 0;
	}
	usec_start = micros();
	// wait for the pulse to stop
	while (!*reg) {
		if (--timeout_count == 0) return 0;
	}
	usec_stop = micros();
	return usec_stop - usec_start;
}

// TODO: an inline version should handle the common case where state is const
uint32_t pulseIn(uint8_t pin, uint8_t state, uint32_t timeout)
{
	if (pin >= CORE_NUM_DIGITAL) return 0;
	if (state) return pulseIn_high(portInputRegister(pin), timeout);
	return pulseIn_low(portInputRegister(pin), timeout);;
}

[schlank]

Well OK than maybe even my quick and dirty solution from the internet I posted at the beginning is better.

[KurtE]

In the past I often started with a quick and dirty, which at times is sufficient depending on how accurate do you need it.

Something like: (Note typing in on fly so probably issue)...

volatile uint32_t last_pulse_width = 0;
uint32_t pulse_start_time = 0;  
void pulse_width_isr() {
    uint32_t isr_time = micros();
    if (digitalReadFast(10) {
        pulse_start_time = isr_time;
    } else {
        last_pulse_width = isr_time - pulse_start_time;
    }
}

void setup() {
    attachInterrupt(10, &pulse_width_isr);
    ...
}

void loop() {
    ...
    if (last_pulse_width ...
}

This is pretty basic, I grab the micros at start of ISR so both paths should hopefully have about same interrupt delay time.