timer1 (1 << WGM12) What does this syntax do?

Projects Programming
1

I am trying to program timer one on an ATTINY84. I found the avr folder (C:\Program Files (x86)\Arduino\hardware\tools\avr\avr\include) and all sorts of useful functions inside it. One of the folders helps with interrupts which includes the timer interrupts.

To start out I want to blink the led using the timer one interrupt. What does the 1<<xxxxx syntax do? I see this in other peoples code, but I do not understand it.

Here is a code snippet I found online. I do not understand the bottom 5 lines with the << syntax. I know that << is a shift operator, however I do not understand why it is followed by a register or bit.

//set timer1 interrupt at 1Hz
  TCCR1A = 0;// set entire TCCR1A register to 0
  TCCR1B = 0;// same for TCCR1B
  TCNT1  = 0;//initialize counter value to 0
  // set compare match register for 1hz increments
  OCR1A = 15624;// = (16*10^6) / (1*1024) - 1 (must be <65536)
  // turn on CTC mode
  TCCR1B |= (1 << WGM12);
  // Set CS12 and CS10 bits for 1024 prescaler
  TCCR1B |= (1 << CS12) | (1 << CS10);  
  // enable timer compare interrupt
  TIMSK1 |= (1 << OCIE1A);

(where the snippet came from)

I would have used timerOne.h however it comes up with an error when I use an attiny84 instead of an Atmega328. At the moment I am using the internal 8MHz clock and p7 for my led. I am debugging using the ATMEGA328 since many of the registers are the same and it supports Serial.print().

2

bit math

Expains the shifting better than I can. When you see 1 << CS12 the CS12 will have been defined as a number (constant). So if

#define CS12 5

1 << CS12 is the same as 1 << 5.

3

In the datasheet, you see that each bit in a register has a name. The compiler defines those equal to the bit index, precisely for doing that sort of thing - that way you can see them set TCCR1B=(1<<WGM12) and know that they're setting WGM12, one of the waveform generation bits, instead of seeing TCCR1B=32 and being like... 32, where did the 32 come from?

The code is much easier to write and maintain like that.

4

Thank you for the help!