I understand the the best way to approach this problem is by using interrupts, and I understand that the overall idea is that the loop is interrupted when a pulse is detected, and I also understand how to measure then length of the pulse.
What I do not understand is why the results are quite so random, and I don't understand how to change the raw data so that I can get a result which is more like that returned by pulseIn().
Most of the code that I've looked at has something like this to measure the length of the pulse;
thanks to vinceherman for this code
void Chan1_begin() // enter Chan1_begin when interrupt pin goes HIGH.
{
Chan1_startPulse = micros(); // record microseconds() value as Chan1_startPulse
detachInterrupt(Chan1Interrupt); // after recording the value, detach the interrupt from Chan1_begin
attachInterrupt(Chan1Interrupt, Chan1_end, FALLING); // re-attach the interrupt as Chan1_end, so we can record the value when it goes low
}
void Chan1_end()
{
Chan1_val = micros() - Chan1_startPulse; // when interrupt pin goes LOW, record the total pulse length by subtracting previous start value from current micros() vlaue.
detachInterrupt(Chan1Interrupt); // detach and get ready to go HIGH again
attachInterrupt(Chan1Interrupt, Chan1_begin, RISING);
if (Chan1_val < 1000 || Chan1_val > 2000) { Chan1_val = Chan1_val_last;}
else {Chan1_val_last = Chan1_val;}
}
/*
Copyright 2011 Lex Talionis (Lex.V.Talionis at gmail)
This program is free software: you can redistribute it
and/or modify it under the terms of the GNU General Public
License as published by the Free Software Foundation,
either version 3 of the License, or (at your option) any
later version.
This code uses pin change interrupts and timer 1 to mesure the
time between the rise and fall of 3 channels of PPM
(Though often called PWM, see http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1253149521/all)
on a typical RC car reciver. It could be extended to as
many channels as you like. It uses the PinChangeInt library
to notice when the signal pin goes high and low, and the
Timer1 library to record the time between.
*/
#include <PinChangeInt.h> // http://playground.arduino.cc/Main/PinChangeInt
#include <PinChangeIntConfig.h>
#include <TimerOne.h> // http://playground.arduino.cc/Code/Timer1
#define NO_PORTB_PINCHANGES //PinChangeInt setup
#define NO_PORTC_PINCHANGES //only port D pinchanges (see: http://playground.arduino.cc/Learning/Pins)
#define PIN_COUNT 3 //number of channels attached to the reciver
#define MAX_PIN_CHANGE_PINS PIN_COUNT
#define RC_TURN 3 //arduino pins attached to the reciver
#define RC_FWD 2
#define RC_FIRE 4
byte pin[] = {RC_FWD, RC_TURN, RC_FIRE}; //for maximum efficency thise pins should be attached
unsigned int time[] = {0,0,0}; // to the reciver's channels in the order listed here
byte state=0;
byte burp=0; // a counter to see how many times the int has executed
byte cmd=0; // a place to put our serial data
byte i=0; // global counter for tracking what pin we are on
void setup() {
Serial.begin(115200);
Serial.print("PinChangeInt ReciverReading test");
Serial.println(); //warm up the serial port
Timer1.initialize(2200); //longest pulse in PPM is usally 2.1 milliseconds,
//pick a period that gives you a little headroom.
Timer1.stop(); //stop the counter
Timer1.restart(); //set the clock to zero
for (byte i=0; i<3; i++)
{
pinMode(pin[i], INPUT); //set the pin to input
digitalWrite(pin[i], HIGH); //use the internal pullup resistor
}
PCintPort::attachInterrupt(pin[i], rise,RISING); // attach a PinChange Interrupt to our first pin
}
void loop() {
cmd=Serial.read(); //while you got some time gimme a systems report
if (cmd=='p')
{
Serial.print("time:\t");
for (byte i=0; i<PIN_COUNT;i++)
{
Serial.print(i,DEC);
Serial.print(":");
Serial.print(time[i],DEC);
Serial.print("\t");
}
Serial.print(burp, DEC);
Serial.println();
/* Serial.print("\t");
Serial.print(clockCyclesToMicroseconds(Timer1.pwmPeriod), DEC);
Serial.print("\t");
Serial.print(Timer1.clockSelectBits, BIN);
Serial.print("\t");
Serial.println(ICR1, DEC);*/
}
cmd=0;
switch (state)
{
case RISING: //we have just seen a rising edge
PCintPort::detachInterrupt(pin[i]);
PCintPort::attachInterrupt(pin[i], fall, FALLING); //attach the falling end
state=255;
break;
case FALLING: //we just saw a falling edge
PCintPort::detachInterrupt(pin[i]);
i++; //move to the next pin
i = i % PIN_COUNT; //i ranges from 0 to PIN_COUNT
PCintPort::attachInterrupt(pin[i], rise,RISING);
state=255;
break;
/*default:
//do nothing
break;*/
}
}
void rise() //on the rising edge of the currently intresting pin
{
Timer1.restart(); //set our stopwatch to 0
Timer1.start(); //and start it up
state=RISING;
// Serial.print('r');
burp++;
}
void fall() //on the falling edge of the signal
{
state=FALLING;
time[i]=readTimer1(); // read the time since timer1 was restarted
// time[i]=Timer1.read(); // The function below has been ported into the
// the latest TimerOne class, if you have the
// new Timer1 lib you can use this line instead
Timer1.stop();
// Serial.print('f');
}
unsigned long readTimer1() //returns the value of the timer in microseconds
{ //rember! phase and freq correct mode counts
//up to ICR1 then down again
unsigned int tmp=TCNT1;
char scale=0;
switch (Timer1.clockSelectBits)
{
case 1:// no prescalse
scale=0;
break;
case 2:// x8 prescale
scale=3;
break;
case 3:// x64
scale=6;
break;
case 4:// x256
scale=8;
break;
case 5:// x1024
scale=10;
break;
}
while (TCNT1==tmp) //if the timer has not ticked yet
{
//do nothing -- max delay here is ~1023 cycles
}
tmp = ( (TCNT1>tmp) ? (tmp) : (ICR1-TCNT1)+ICR1 );//if we are counting down add the top value
//to how far we have counted down
return ((tmp*1000L)/(F_CPU /1000L))<<scale;
}
but the code seems to jam with the Timer1 at the first call to Timer1.restart(); on line 47
is there something in the TimerOne library that I have to change to use it on the mega?
