Arduino Timer1 - Cornell ECE

Using timer 1 ECE3400

The Arduino environment uses the Mega328 timer0, but does not touch timer1. Timer1 is a 16 bit counter that can be set to perfrom several different functions.

Timer1 functions

• Clock Sources
  • Timer1 can use a prescalar or increment based on input from an i/o pin (rising/falling edge). Prescalar divides the cpu clock by: off, 1, 8, 64, 256, 1024.
• Count to overflow.
  • Count to 2^16-1, then roll over to zero and keep counting.
  • Overflow may trigger an interrupt if enabled.
• Count to two preset values specified in Output Compare Register A or B (OCR1A, OCR1B).
  • Reaching any preset value can trigger interrupts if enabled.
  • Reaching the preset value in OCR1A can cause the timer to be reset. Counting to a preset value, folllowed by reset (and automatic continued counting) is the best way to generate a precision time base.
  • Reaching the preset value in OCR1A or OCR1B can cause an output pin to be set, cleared, or toggled. This feature can be used to generate square waves of aribtrary frequencies with no software overhead.
• Pulse-width modulator (PWM) mode.
  • Two PWM signals for each timer are output to pins based on preset value (OCR1A or OCR1B) which can be changed on every PWM cycle.
  • An interrupt can be generated on every PWM cycle if enabled.
  • In PWM mode, a timer cannot do anything else!
• Copy the value in the timer counter (current time) into an input capture register (ICR1) when an external event occurs.
  • The event can be a logic transition on one i/o pin.
  • The event can be a change in state of a built-in analog comparator (timer1 only!). This allows the cpu to determine when an analog voltage equals an arbitrary reference voltage.
  • This feature allows the cpu to determine the exact time of an external event.

There are various ways of timing intervals (and therefore frequency):

(1) Using input capture: Before the Arduino initialization code, write an interrupt service routine to copy timer 1 into a register and subtract to get a period. Note that the subtract is correct even if the counter overflows (once) between readings. Time resolution is 0.0625 microsecond. The logic-level input must be on Arduino pin 8. This uses the input-capture pin (ICP1) feature of timer1.

//********************************************************** // timer 1 capture variables for computing sq wave period volatile unsigned int T1capture, lastT1capture, period ; // timer 1 capture ISR ISR (TIMER1_CAPT_vect) begin // read timer1 input capture register T1capture = ICR1 ; // compute time between captures period = T1capture - lastT1capture; lastT1capture = T1capture ; end //**********************************************************

In the initialization section set up the timer (section 16 of datasheet, and especially 16.11)

//set up timer1 for full speed and //capture an edge on analog comparator pin B.3 // Set capture to positive edge, full counting rate TCCR1B = (1<<ICES1) + 1; // Turn on timer1 interrupt-on-capture TIMSK1 = (1<<ICIE1) ; // turn off other timer1 functions TCCR1A = 0; In the loop section, just read the period variable.

(2) Using external interrupt with timer1: Enable an external interrupt in the Arduino interface and set up timer1 to run continuously. TCCR1B = 1; // turn off other timer1 functions TCCR1A = 0;

Each time the external interrupt happens, read the timer register (TCNT1) and subtract. This method is less accurate than method 1 because of the time it takes to execute the interrupt.

(3) Using external interrupt with Arduino timer: Enable an external interrupt in the Arduino interface and read the Arduino microsecond function when the interrupt occurs. This method is the easiest, and much less accurate then either of the above methods. The time resolution cannot be better than 4 microcseconds.