Decoding Radio Control signal pulses

I posted a library for decoding signals from a radio control transmitter (with a 'trainer port') or from an RC receiver that provides access to the pulse stream. The original code and discussion was in a huge thread titled '3 axis auto stabilized platform'

I've heard that people have trouble compiling the ServoDecode library under 0012. I have created a new thread here to make it easier to find. The following change is needed in ServoDecode.h file to get it to compile under Arduino release 0012.

Remove the line:


and replace it with:

typedef uint8_t byte;

This fixes a conflict with a pesky macro in wiring.h that I hope will be removed in future versions.

But in the mean time, you can make the above change to the previously posted ServoDecode file or use the code in the following posts. Note that there is no change needed to the ServoDecode.cpp file


// ServoDecodeClass.h 
//This library decodes typical RC composite servo inputs into individual channel pulse widths

#ifndef ServoDecode_H
#define ServoDecode_H

//#include  //removed for 0012
typedef uint8_t byte;

#define icpPin            8         // this interrupt handler must use pin 8
#define TICKS_PER_uS      2          // number of timer ticks per microsecond

typedef enum {
    NULL_state=-1, NOT_SYNCHED_state, ACQUIRING_state, READY_state, FAILSAFE_state    
 } decodeState_t;

#define MAX_CHANNELS    8         // maximum number of channels we can store, don't increase this above 8
#define MIN_IN_PULSE_WIDTH (750 * TICKS_PER_uS) //a valid pulse must be at least 750us (note clock counts in 0.5 us ticks)
#define MAX_IN_PULSE_WIDTH (2250 * TICKS_PER_uS) //a valid pulse must be less than  2250us 
#define SYNC_GAP_LEN      (3000 * TICKS_PER_uS) // we assume a space at least 3000us is sync (note clock counts in 0.5 us ticks)
#define FAILSAFE_PIN   13  // if defined, this will set the given pin high when invalid data is received

class ServoDecodeClass
      ServoDecodeClass(); //Constructor
      void begin();
      decodeState_t getState(); //State Function
      int GetChannelPulseWidth(byte chan);  // this is the access function for channel data
      void setFailsafe(byte chan, int value); // pulse width to use if invalid data, value of 0 uses last valid data
      void setFailsafe();// setFailsafe with no arguments sets failsafe for all channels to their current values
                       // useful to capture current tx settings as failsafe values 
      byte getChanCount();


extern ServoDecodeClass ServoDecode;  // make an instance for the user



#include "ServoDecode.h"

            State = READY_state;           // this is the second sync and all channel data is ok so flag that channel data is valid     
                NbrChannels = Channel; // save the number of channels detected
       else if( State == FAILSAFE_state)      {  
             if(Channel == NbrChannels){  // did we get good pulses on all channels
                  State = READY_state;
   Channel = 0;       // reset the channel counter 

  if(State == READY_state){
    State = FAILSAFE_state;  // use fail safe values if signal lost  
    Channel = 0; // reset the channel count

  // we want to measure the time to the end of the pulse 
  if( (_SFR_BYTE(TCCR1B) & (1<= SYNC_GAP_LEN){   // is the space between pulses big enough to be the SYNC
    else if(Channel < MAX_CHANNELS) {  // check if its a valid channel pulse and save it
      if( (ICR1 >= MIN_IN_PULSE_WIDTH)  && (ICR1 <= MAX_IN_PULSE_WIDTH) ){ // check for valid channel data                  
        Pulses[++Channel] = ICR1 / TICKS_PER_uS;  // store pulse length as microsoeconds
      else if(State == READY_state){
        State = FAILSAFE_state;  // use fail safe values if input data invalid  
        Channel = 0; // reset the channel count


void ServoDecodeClass::begin(){
  Channel = 0;             
  State = NOT_SYNCHED_state;
  TCCR1A = 0x00;         // COM1A1=0, COM1A0=0 => Disconnect Pin OC1 from Timer/Counter 1 -- PWM11=0,PWM10=0 => PWM Operation disabled 
  TCCR1B = 0x02;         // 16MHz clock with prescaler means TCNT1 increments every .5 uS (cs11 bit set
  TIMSK1 = _BV(ICIE1)|_BV (TOIE1);   // enable input capture and overflow interrupts for timer 1
  for(byte chan = 1; chan <=  MAX_CHANNELS; chan++) 
        Failsafe[chan] = Pulses[chan]= 1500; // set midpoint as default values for pulse and failsafe

decodeState_t ServoDecodeClass::getState(){
  return State;

byte ServoDecodeClass::getChanCount(){
  return NbrChannels;

void  ServoDecodeClass::setFailsafe(byte chan, int value){ 
// pulse width to use if invalid data, value of 0 uses last valid data
  if( (chan > 0) && (chan <=  MAX_CHANNELS)  ) {
       Failsafe[chan] = value;
void  ServoDecodeClass::setFailsafe(){ 
// setFailsafe with no arguments sets failsafe for all channels to their current values
// usefull to capture current tx settings as failsafe values 
  if(State == READY_state) 
    for(byte chan = 1; chan <=  MAX_CHANNELS; chan++) {
        Failsafe[chan] = Pulses[chan];

int ServoDecodeClass::GetChannelPulseWidth( uint8_t channel)
  // this is the access function for channel data
  int result = 0; // default value
  if( channel <=  MAX_CHANNELS)  {
     if( (State == FAILSAFE_state)&& (Failsafe[channel] > 0 ) )
         result = Failsafe[channel]; // return the channels failsafe value if set and State is Failsafe
       else if( (State == READY_state) || (State == FAILSAFE_state) ){
         cli();       //disable interrupts
         result =  Pulses[channel] ;  // return the last valid pulse width for this channel
         sei(); // enable interrupts
  return result;
// make one instance for the user 
ServoDecodeClass ServoDecode = ServoDecodeClass() ;

ServoDecode.pde Test Sketch

// ServoDecodeTest


char * stateStrings[] = {
  "NOT_SYNCHED", "ACQUIRING", "READY", "in Failsafe"};

void setup()                    // run once, when the sketch starts
  ServoDecode.setFailsafe(3,1234); // set channel 3 failsafe pulse  width

void loop()                     // run over and over again
  int pulsewidth;

  // print the decoder state
  if( ServoDecode.getState()!= READY_state) {
    Serial.print("The decoder is ");
    for ( int i =0; i <=MAX_CHANNELS; i++ ){ // print the status of the first four channels
      Serial.print("Cx"); // if you see this, the decoder does not have a valid signal
      Serial.print("= ");
      pulsewidth = ServoDecode.GetChannelPulseWidth(i);
      Serial.print("  ");
  else {
    // decoder is ready, print the channel pulse widths
    for ( int i =1; i <=MAX_CHANNELS; i++ ){ // print the status of the first four channels
      Serial.print("= ");
      pulsewidth = ServoDecode.GetChannelPulseWidth(i);
      Serial.print("  ");
  delay(500); // update 2 times a second        

Thank you again for posting the servo decoder. I uploaded it and it worked like a charm! My only problem now is that when i try to use the analogWrite function whle running the decoder, it doesn't seem to work. Has anyone else run into this problem?

Hi dave, this code uses hardware support from timer1 so analogWrite() for digital pins 9 and 10 will not work. Try it on the other pins and report back here if it works.

I'm really new to arduino..... How do I use these 3 files. I have them each open in a tab and when I compile it says:

25: error: ServoDecode.h: No such file or directory In function 'void setup()': In function 'void loop()':

This code is intended to be used as an Arduino library. Create a new directory called ServoDecode in the directory with the other libraries ( ? hardware\libraries\ServoDecode)

Copy ServoDecode.h and ServoDecode.cpp into this directory.

The ServoDecode.pde code should be pasted into the IDE to create a new sketch. This example sketch includes but if you create your own sketch, you can use the 'Sketch/Import Library' function from the IDE menu.

There is some information in the playground on Arduino Libraries that goes into more detail.

Have fun!

what all needs to be changed in order for this to work with the 6-channel Vex

I don't think anything needs to be changed in the library, you may want to contact jamiemcshan about connecting it up - see this post:

When you get it going, please come back here and post details of how you did it.

Have fun!

It's every time in failsafe.

How I do the connections?

Where I connect the PWM input?

You connect the PPM stream to arduino pin 8.

Hello mem,

thanks for the great libraries. New life for the old receiver - I used your code with GWS R-4PII/H receiver with only 3 channels working - accidentally burned one channel long time ago. The beauty is I get all 6 channels on arduino. Now I'm wondering if the library could be used to decode Futaba PCM signal? Have you heard if anyone tried that? I found arduino helicopter autopilot thread (another great project) mentions PCM in several places, but looking at the code samples in the thread it seems its just PPM.

The technique used in this library is completely different from that needed to decode a signal encoded using the Futaba PCM protocol. That protocol uses proprietary serial encoding and would required much more processing than the PPM decoding used here.

In my opinion it would not be worth the effort to do this – of course you are welcome to take it on if you are up for the challenge. But I would think in most situations there would be no benefit and for those cases with strong interference you would be better of using 2.4ghz technology.

What was the reason you were interested in PCM?

I just thought it would be fun to get PCM out or PPM receiver. But PPM works great and I don't want to spend time working on PCM if will eat up too much arduino processing power, though your interrupt method could probably be adopted to read the bits of PCM transmission too?

By the way the library works with inverted signals too. For some reason my GWS receiver had it inverted and the only problem i noticed first channel had very high numbers (around 10,000 or so) but the rest of the channels were fine.

In my opinion, PCM would be a lot of work for no benefit. There are many more productive and fun things to do with your time.

BTW, if you want to use the library with a receiver with an inverted output, in ServorDecode.cpp change: volatile byte pulseEnd = PULSE_START_ON_RISING_EDGE ; // default value to volatile byte pulseEnd = PULSE_START_ON_FALLING_EDGE; // inverted pulses

then recompile the library (by deleting ServoDecode.o and rebuilding your sketch)

Bummer, should have asked earlier about the inverted. I already soldered a simple amplifier/inverter right on to GWS R4PII/H :( Well, it needed amplifier though - only had 2.5V impulses without it and was getting very noisy feedback from arduino controller back into the receiver. But with this code change i could have gotten away with just one transistor... Thanks for the info! ;)

I would have thought that the transistor and resistor would have been enough to invert the signal with the 2.5 volt pulses and drive the Arduino input. But no worries if you now have something that works.

Hi, I have a suggestion for the code, instead of using "TCNT1 = 0;", why not remember the previous value of ICR1 and keep track of overflows, this way you can get an unlimited value for the pulse width, also it's more accurate since it takes a few clock cycles to clear TCNT1

The only thing that can go wrong is if the capture event happens when TCNT1 is 0 and the capture interrupt executes before the overflow interrupt

Hi Frank, resetting TCNT to 0 takes 250 nanoseconds on a 16Mhz Arduino. Because the timer is counting in 500 nanosecond ticks I don't think this delay makes any difference. Indeed I would be surprised to hear if any of the radio control equipment that would be used with this kind of application would notice differences in timing of less than a microsecond.

I am not clear if the ability to detect unlimited values for the pulse width are something one would would use in radio control or if you have another application for the code.

Many thanks for taking the time to share your comments, its always interesting to see the different ways people approach things.

oh i totally agree that it's overkill, the RC radio would need a 16 bit ADC for the joystick if it wants to make the accuracy worthwhile, i'm not dividing my timer so i'm getting 0.05 microsecond resolution if i run my chip at 20 MHz

You connect the PPM stream to arduino pin 8.

Hi all, mem.... could you tell me what would be the designation for the pin 8, i´m using an Arduino Duemilinove board with an atmega 328 and i think pin 8 is not the indicated one for this board . ::)

thanks in advance