DmxSimple Interframe Delay

Have an issue where a 30-chan DMX board will not respond to my DMX controller, which uses a Nano running DmxSimple. Other DMX devices work fine. Someone in a controller forum suggested that the interframe delay with DmxSimple is 2msec and the 30-chan controller expects closer to 1msec so ignores the DMX frames. Suggested to change the DmxSimple code to lower the delay. DMX spec says >1.2 msec.

Looked at the DmxSimple code and changing interframe delay isn't obvious. Anyone have ideas how to change it?

Anyone have ideas how to change it?

With a text editor, of course. If you need help understand what to change, posting a link to the library would be useful.

Duh, of course, with a text editor!

So, looking to find what in the code affects “Break to Break” time (DMX2512 packet length) or “interframe delay” which is supposedly set to 2msec, and yes, seeing the code would be helpful…


 * DmxSimple - A simple interface to DMX.
 * Copyright (c) 2008-2009 Peter Knight,! All rights reserved.

#ifndef DmxSimple_h
#define DmxSimple_h

#include <inttypes.h>

#if RAMEND <= 0x4FF
#define DMX_SIZE 128
#define DMX_SIZE 512

class DmxSimpleClass
    void maxChannel(int);
    void write(int, uint8_t);
    void usePin(uint8_t);
extern DmxSimpleClass DmxSimple;



 * DmxSimple - A simple interface to DMX.
 * Copyright (c) 2008-2009 Peter Knight,! All rights reserved.
#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>
#include "pins_arduino.h"

#if defined(ARDUINO) && ARDUINO >= 100
  #include <Arduino.h>
  #include <wiring.h>

#include "DmxSimple.h"

/** dmxBuffer contains a software copy of all the DMX channels.
volatile uint8_t dmxBuffer[DMX_SIZE];
static uint16_t dmxMax = 16; /* Default to sending the first 16 channels */
static uint8_t dmxStarted = 0;
static uint16_t dmxState = 0;

static volatile uint8_t *dmxPort;
static uint8_t dmxBit = 0;
static uint8_t dmxPin = 3; // Defaults to output on pin 3 to support! DMX shield

void dmxBegin();
void dmxEnd();
void dmxSendByte(volatile uint8_t);
void dmxWrite(int,uint8_t);
void dmxMaxChannel(int);

/* TIMER2 has a different register mapping on the ATmega8.
 * The modern chips (168, 328P, 1280) use identical mappings.
#if defined(__AVR_ATmega168__) || defined(__AVR_ATmega168P__) || defined(__AVR_ATmega328P__) || defined(__AVR_ATmega1280__)
#elif defined(__AVR_ATmega8__)
/* Produce an appropriate message to aid error reporting on nonstandard
 * platforms such as Teensy.
#warning "DmxSimple does not support this CPU"

/** Initialise the DMX engine
void dmxBegin()
  dmxStarted = 1;

  // Set up port pointers for interrupt routine
  dmxPort = portOutputRegister(digitalPinToPort(dmxPin));
  dmxBit = digitalPinToBitMask(dmxPin);

  // Set DMX pin to output

  // Initialise DMX frame interrupt
  // Presume Arduino has already set Timer2 to 64 prescaler,
  // Phase correct PWM mode
  // So the overflow triggers every 64*510 clock cycles
  // Which is 510 DMX bit periods at 16MHz,
  //          255 DMX bit periods at 8MHz,
  //          637 DMX bit periods at 20MHz

/** Stop the DMX engine
 * Turns off the DMX interrupt routine
void dmxEnd()
  dmxStarted = 0;
  dmxMax = 0;

/** Transmit a complete DMX byte
 * We have no serial port for DMX, so everything is timed using an exact
 * number of instruction cycles.
 * Really suggest you don't touch this function.
void dmxSendByte(volatile uint8_t value)
  uint8_t bitCount, delCount;
  __asm__ volatile (
    "ld __tmp_reg__,%a[dmxPort]\n"
    "and __tmp_reg__,%[outMask]\n"
    "st %a[dmxPort],__tmp_reg__\n"
    "ldi %[bitCount],11\n" // 11 bit intervals per transmitted byte
    "rjmp bitLoop%=\n"     // Delay 2 clock cycles. 
    "ldi %[delCount],%[delCountVal]\n"
    "dec %[delCount]\n"
    "brne delLoop%=\n"
    "ld __tmp_reg__,%a[dmxPort]\n"
    "and __tmp_reg__,%[outMask]\n"
    "ror %[value]\n"
    "brcc sendzero%=\n"
    "or __tmp_reg__,%[outBit]\n"
    "st %a[dmxPort],__tmp_reg__\n"
    "dec %[bitCount]\n"
    "brne bitLoop%=\n"
      [bitCount] "=&d" (bitCount),
      [delCount] "=&d" (delCount)
      [dmxPort] "e" (dmxPort),
      [outMask] "r" (~dmxBit),
      [outBit] "r" (dmxBit),
      [delCountVal] "M" (F_CPU/1000000-3),
      [value] "r" (value)

/** DmxSimple interrupt routine
 * Transmit a chunk of DMX signal every timer overflow event.
 * The full DMX transmission takes too long, but some aspects of DMX timing
 * are flexible. This routine chunks the DMX signal, only sending as much as
 * it's time budget will allow.
 * This interrupt routine runs with interrupts enabled most of the time.
 * With extremely heavy interrupt loads, it could conceivably interrupt its
 * own routine, so the TIMER2 interrupt is disabled for the duration of
 * the service routine.

  // Prevent this interrupt running recursively

  uint16_t bitsLeft = F_CPU / 31372; // DMX Bit periods per timer tick
  bitsLeft >>=2; // 25% CPU usage
  while (1) {
    if (dmxState == 0) {
      // Next thing to send is reset pulse and start code
      // which takes 35 bit periods
      uint8_t i;
      if (bitsLeft < 35) break;
      *dmxPort &= ~dmxBit;
      for (i=0; i<11; i++) _delay_us(8);
      *dmxPort |= dmxBit;
    } else {
      // Now send a channel which takes 11 bit periods
      if (bitsLeft < 11) break;
    // Successfully completed that stage - move state machine forward
    if (dmxState > dmxMax) {
      dmxState = 0; // Send next frame
  // Enable interrupts for the next transmission chunk

void dmxWrite(int channel, uint8_t value) {
  if (!dmxStarted) dmxBegin();
  if ((channel > 0) && (channel <= DMX_SIZE)) {
    if (value<0) value=0;
    if (value>255) value=255;
    dmxMax = max((unsigned)channel, dmxMax);
    dmxBuffer[channel-1] = value;

void dmxMaxChannel(int channel) {
  if (channel <=0) {
    // End DMX transmission
    dmxMax = 0;
  } else {
    dmxMax = min(channel, DMX_SIZE);
    if (!dmxStarted) dmxBegin();

/* C++ wrapper */

/** Set output pin
 * @param pin Output digital pin to use
void DmxSimpleClass::usePin(uint8_t pin) {
  dmxPin = pin;
  if (dmxStarted && (pin != dmxPin)) {

/** Set DMX maximum channel
 * @param channel The highest DMX channel to use
void DmxSimpleClass::maxChannel(int channel) {

/** Write to a DMX channel
 * @param address DMX address in the range 1 - 512
void DmxSimpleClass::write(int address, uint8_t value)
	dmxWrite(address, value);
DmxSimpleClass DmxSimple;

Had thoughts of trying DmxSerial instead, but it uses serial port 0 which I’m told prohibits using that port for anything else like debug printing, and the way I code, really need debug printing!

Possibly, the timer interrupt frequency is what determines the "break to break" time. But, I haven't a clue what you need to change to get the desired time.