Arduino: Inaccurate timing with os_timer_t

Created on 24 Aug 2016 · 4Comments · Source: esp8266/Arduino

Description

I need my timer callback function to execute accurately but it periodically has a delay of a few hundred microseconds, usually around 200 microseconds. After that the timer tries to keep up and shortens one of the next cycles.

Why does that happen and is there any way to make the timer more accurate? I don't care about those 2-3 microseconds deviation.

Sketch

extern "C"
{
  #include "user_interface.h"
}

uint32_t last = 0;
uint32_t now = 0;
os_timer_t timer;

void setup() {
  Serial.begin(115200);
  system_timer_reinit();
  os_timer_setfn(&timer, myTimerFunc, NULL);
  ets_timer_arm_new(&timer, 1500, true, 0);
}

void myTimerFunc(void *pArg)
{
  now = micros();
  Serial.println(now - last);
  last = now;
}

void loop() {

}

Serial Output

Source

stnkl

All 4 comments

I found a way of getting more precise timing using timer0:

uint32_t last = 0;
uint32_t now = 0;
uint16_t interval_us = 1500;

void inline handler (void){
  timer0_write(ESP.getCycleCount() + interval_us * 80); // 160 when running at 160mhz

  now = micros();
  Serial.println(now - last);
  last = now;
}

void setup() {
  Serial.begin(115200);

  noInterrupts();
  timer0_isr_init();
  timer0_attachInterrupt(handler);
  timer0_write(ESP.getCycleCount() + interval_us * 80); // 160 when running at 160mhz
  interrupts();
}

void loop() {

}

It runs every 1500±2µs like this.

stnkl on 27 Aug 2016

👍1

I've created a timer library using timer 0 https://github.com/rsegecin/ESP8266Timer/tree/master. Thank you stnkl.

rsegecin on 1 Mar 2017

👍1

Hi. Just want to check, is timer 0 the hardware timer on ESP8266? I heard that using the hardware timer will possibly screw the other time-dependent functions, such as WIFI communication, etc. Is there anything I need to pay attention to when using it? I'll be grateful if anyone can share with me a link with some detailed doc or tutorial on how to use the hardware timer.

Cheers!

liuzheng1990 on 9 Nov 2018

Note that using this little modification provides highly better precision.
And think to never use time-consuming functions like "print" in the ISR...

uint32_t last = 0;
uint32_t now = 0;
uint16_t interval_us = 1500;
unsigned long next_event;

void inline handler (void){
  timer0_write(next_event + interval_us * 80); // 160 when running at 160mhz

  now = micros();
  Serial.println(now - last);
  last = now;
}

void setup() {
  Serial.begin(115200);

  noInterrupts();
  timer0_isr_init();
  timer0_attachInterrupt(handler);
  next_event = ESP.getCycleCount() + interval_us * 80; // 160 when running at 160mhz
  timer0_write(next_event );
  interrupts();
}