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.
Labels:

Comments

  1. Unknown20 October 2017 at 03:41

    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. ian1235 July 2021 at 15:02

    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
29 comments
Read more

Hacking a USB-C to slim tip adapter cable to charge the Thinkpad T450s

This hack is inspired by this post . A year ago, I bought an adapter cable for my wife's Thinkpad X1 Carbon (2nd Gen) that allows her to power her laptop with a 60W-capable portable battery (20V x 3A). A USB-C cable goes from the battery into the adapter, which converts it to the slim tip output required by the laptop. Everything works out of the box, so I didn't give much thought about it. Recently, I decided to buy a similar cable for my Thinkpad T450s. I know technically it should work because the T450s can go as low as 45W (20V x 2.25A) in terms of charging (though I have the 65W charger - 20V x 3.25A).  I went with another adapter cable because it was cheaper and also I prefer the single cable design. So imagine my surprise when the cable came and I plugged it into my laptop and it didn't work! The power manager just cycle in and out of charging mode before giving up with an error message saying there is not enough power. After much research and reading the Thinkwiki
Post a Comment
Read more

Using Google Dashboard to manage your Android device backup

I used to use AppBrain/Fast Web Install to keep track of which apps I have installed on my phone, and to make it easier to reinstall those apps when the phone gets wiped or replaced. But AppBrain had been going down the tubes, and Fast Web Install had always been a hit-and-miss affair. Android's own "backup to the cloud" system had previously been even more unusable. There isn't a place where you can see what has been backed up. And when you setup a new phone with your Google account, you just have to wait and pray that your favorite apps will be restored to the phone. Typically all the stars have to be aligned just right for this to happen. More often than not, after waiting for an hour or so and nothing happens, you just curse under your breath and proceed to install your favorites apps manually via the Play Store. But I just looked again recently and was pleasantly surprised that things are much more civilized now. Firstly there is a place now where you can loo
Post a Comment
Read more