Timer Counter w/ Atmel Studio 7 & UNO Serie (Episode#06)

Let’s start with a new peripheral: Timer Counter!

But now let’s toggle LED at every 5 milliseconds using Timer Counter Zero or One.

We are going to start off where we were in previous episode#05 (we have an interruption generated when we push the button attached on Arduino Pin 7:)

Step#01→ Timer Zero. On page 101 of the Atmega328p data sheet we encounter ( 12.7.2 Clear Timer on Compare Match (CTC) Mode).

The waveform frequency (f) is defined by the following equation:

foc = foc_CPU/2*N[1+OCRnx]

The N variable represents the prescale factor (1, 8, 64, 256, or 1024);

foc is the frequency output on compare match;

foc_CPU is the frequency of our Arduino(16 MHz) cpu;

OCRnx is the , the OC0A output that can be set to toggle its logical level on each compare match.

Good!

Step#02→ If we calculate what is the highest frequency we can get with 8-bits counter (Timer0) we have…

foc = 16000000/2*1024*[1+255]
foc = 30,517578125

2⁸= 256–1=255 (zero base count)

note: The counter will pop up 30 times per second; It is great for operations with communication, but very fast for interactions with humans:/

Step#03→ Let’s try out Timer One! (16-bit log:) on page 131 of the Atmega328p data sheet;

Let’s scale down a bit our prescale to 256:

foc = 16000000/2*256*[1+65535]
foc = 0,476837158203125

2¹⁶ = 65536–1= 65535 (zero base too)

note: This will works quite nicely for us; It will overflow pretty much twice a second:)

Step#04→ Now coding! First thing to do is comment the sleep mode; so leave it so that we can have a sequence in our code.

The counting sequence is determined by the setting of the Waveform Generation mode bits (WGM13:0) located in the Timer/Counter Control Registers A and B (TCCR1A and TCCR1B).

There is nothing for us on TCCR1A (we want normal mode operation) so we’re going to TCCR1B register straight away (page 136):

TCCR1B |= (1 << CS12);

note: this set the Clock Select Bit Description is equal to 256 prescaler (0b100 )

Step#05→ Go to this part of data sheet (page 136 ):

13.11.8 TIMSK1 — Timer/Counter1 Interrupt Mask Register

And set this code:

TIMSK1 |= (1 << TOIE1);

note: Bit 0 — TOIE1: Timer/Counter1, Overflow Interrupt Enable:)

Step#06→ Go to the top of our code (already there!):

#include <avr/interrupt.h>

Step#07→ Enable interrupts (already there!):

sei();
while() {...}

Step#08→ Implement our Interrupt Service Routine (ISR):

Head over 12.1 Interrupt Vectors (page 137) in ATmega48A and ATmega48PA of ATMega238p data sheet and find the vector of interest out.

Write it down:

ISR(TIMER1_OVF_vect)
  {
     LedToggle;
  }

note: Now the LED is toggle automatically at roughtly each 5 secs :) and there you have it!

In the next post we’re gonna control the timer that toggle the LED a little bit precisely using the output compare mode.

Stay tuned! Bye!

Here is the code till here:

https://github.com/giljr/avr/tree/master/atmel

Look for Episode #06 ;)

References & Credits

Atmel Code base Github

Follow along with the entire ‘Getting Started with AVR’ series: http://bit.ly/GettingStartedwithAVR

→Goto Episode#07

Complete Serie:

#00 HowTo-Load-Into-Arduino-AS7

#01_Hello-World

#02_Presents-AVR-Freaks

#03_Pressing-Button

#04_Pin-Change-Interrup

#05_Battery-Driven-App

#06_Timer Counter

#07_Compare Mode

#08

#09

#10