Skip to main content

Attiny85 timer programming using Timer1

This Arduino sketch uses Timer1 to drive the LED blinker:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
/*
 * Program ATTiny85 to blink LED connected to PB1 at 1s interval.
 * Assumes ATTiny85 is running at 1MHz internal clock speed.
 */

#include <avr/io.h>
#include <avr/wdt.h>
#include <avr/sleep.h>
#include <avr/interrupt.h>

bool timer1 = false, led = true;

// Interrupt service routine for timer1
ISR(TIMER1_COMPA_vect)
{
  timer1 = true;
}

void setup() {

  // Setup output pins
  pinMode(1, OUTPUT);
  digitalWrite(1, led);
  set_sleep_mode(SLEEP_MODE_IDLE);

  // Setup timer1 to interrupt every second
  TCCR1 = 0; // Stop timer
  TCNT1 = 0; // Zero timer
  GTCCR = _BV(PSR1); // Reset prescaler
  OCR1A = 243; // T = prescaler / 1MHz = 0.004096s; OCR1A = (1s/T) - 1 = 243
  OCR1C = 243; // Set to same value to reset timer1 to 0 after a compare match
  TIMSK = _BV(OCIE1A); // Interrupt on compare match with OCR1A
  
  // Start timer in CTC mode; prescaler = 4096; 
  TCCR1 = _BV(CTC1) | _BV(CS13) | _BV(CS12) | _BV(CS10);
  sei();
}

void loop() {
  if (timer1) {
    timer1 = false;
    led = !led;
    digitalWrite(1, led);
  }
  sleep_enable();
  sleep_cpu(); // CPU goes to sleep here; will be woken up by timer1 interrupt
}

The internal RC oscillator can have a factory variance of up to 10%. I observe that my blinker pulsed slightly faster than 1Hz.

Thankfully, unlike the watchdog oscillator, the internal RC oscillator can be calibrated by using the OSCCAL register, or adjusting the value of OCR1A. I believe it should not be too difficult to automatically calibrate the oscillator between NTP calls by taking the difference in actual time and clock time (in seconds) and adjust OCR1A accordingly to get as close to 1Hz as possible.

So on initial startup, the ESPCLOCK may not be pulsing at 1Hz, but after 15 or 30 minutes, it should be fully calibrated.

The downside with this approach is we can only use SLEEP_MODE_IDLE to keep Timer1 running during sleep, which is not as power efficient  as using the Watchdog Timer.

Comments

  1. Hi .help me to change the function from arduino codes at attyni85 usb .please help with .thanks
    void setup() {
    // put your setup code here, to run once:

    //timer2
    OCR2A = 127; //50% duty cycle
    TCCR2A = _BV(COM2A0) | _BV(WGM21) | _BV(WGM20); //toggle output, fast PWM mode
    TCCR2B = _BV(WGM22) | _BV(CS21) | _BV(CS20); // fast PWM mode, prescale 1:64
    pinMode(11, OUTPUT);

    }

    void loop()
    {

    delay(200); //duration of each frequency
    OCR2A += 10; //step size of frequency change
    if(OCR2A >= 245){OCR2A = 10; } //range of sweep, 245 is max.

    }

    ReplyDelete
  2. there are only 2 timers on attiny85, timer0 and timer1

    ReplyDelete

Post a Comment

Popular posts from this blog

Adding "Stereo Mixer" to Windows 7 with Conexant sound card

This procedure worked for my laptop (Thinkpad E530) with a Conexant 20671 sound card, but I suspect it will work for other sound cards in the Conexant family. I was playing with CamStudio to do a video capture of a Flash-based cartoon so that I can put it on the WDTV media player and play it on the big screen in the living room for my kids. The video capture worked brilliantly, but to do a sound capture, I needed to do some hacking. Apparently, there was this recording device called "Stereo Mixer" that was pretty standard in the Windows XP days. This allowed you to capture whatever was played to the speaker in all its digital glory. Then under pressure from various organizations on the dark side of the force, Microsoft and soundcard makers starting disabling this wonderful feature from Windows Vista onwards. So after much Googling around, I found out that for most sound cards, the hardware feature is still there, just not enabled on the software side. Unfortunately, to

Hacking an analog clock to sync with NTP - Part 5

This is how it looks after I have put everything together. The Arduino sketch is available here . The 2 jumper wires soldered to the clock mechanism are connected to pins D0 and D1 on the ESP-12 (in any order). When the device first boots up, it presents an access point which can be connected to via the PC or smartphone. Once connected, the captive portal redirects the web browser to the configuration page:     A custom field has been added to the WiFi configuration page to enter the current clock time in HHMMSS format. Try to set the clock time to as close to the current time as possible using the radial dial at the back of the clock so the clock will have less work to do catching up. In the config page, the HTML5 Geolocation API is also used to obtain your current location (so if your web browser asks if you would like to share your location, answer "yes"). This is then passed to the Google Time Zone API to obtain the time and DST offset of your time z