Timers

Cornell University
Electrical Engineering 476
AVR microcontrollers
Hardware Timers

Introduction

All of the AVR MCUs have hardware support for measuring time. The Mega32 has three hardware timers, two 8-bit (timer0 and timer2) and one 16-bit timer (timer1). Each timer has a prescalar that divides the cpu clock by 1, 8, 64, 256, 1024. At each prescaled clock tick the timer counter register is automatically incremented by one. In addition, each timer can be stopped. Each timer can be used is several different ways. A short list follows for each timer.

Timer0 and timer2

Clock Sources
- Timer0 can use a prescalar or increment based on input from an i/o pin.
- Timer2 can use a prescalar or increment based on a separate crystal.
Count to overflow.
- Count to 255, then roll over to zero and keep counting.
- Overflow may trigger an interrupt if enabled.
Count to a preset value specified in the Output Compare Register (OCR0 or OCR2).
- Reaching the preset value can trigger an interrupt if enabled.
- Reaching the preset value 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 can cause an output pin to be set, cleared, or toggled. This feature can be used to generate a square wave of aribtrary frequency with no software overhead.
Pulse-width modulator (PWM) mode.
- A PWM signal is output to a pin based on a preset value which can be changed on every PWM cycle.
- An interrupt can be generated on every PWM cycle if enabled.

Timer1

Clock Sources
- Timer1 can use a prescalar or increment based on input from an i/o pin.
Count to overflow.
- Count to 2¹⁶-1, 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 1A or 1B (OCR1A, OCR1B).
- Reaching either 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.
- A PWM signal is output to a pin based on a preset value which can be changed on every PWM cycle.
- An interrupt can be generated on every PWM cycle if enabled.
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. 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.

Using Timers

Timing a short interval.
For timing very short intervals, you can get good resolution (1 cycle) by simply clearing the timer, starting it, then stopping and reading it. This is particularly useful for profiling execution time of code. It could also be useful for generating small delays by using a null-bodied while loop to test for the count value.
```
TCCR1B = 1 ;
TCNT1 = 0;
//C code to be profiled
TCCR1B = 0;
printf(" cycles=%d\n\r",TCNT1)  ;
```
Interrupt-based precision clock.
This example program generates a 1 millsecond time base using timer0 in compare-with-clear mode. It also uses the timer0 interrupt service routine (ISR) to update three virtual timers. In this program the three virtual timers are used to schedule three different C functions.

Three timers in three different modes.
This example uses timer2 to generate a square wave, uses timer1 and the analog comparator to measure the period of the square wave, and timer0 to generate a 1MSec time base. The timer2 OC2 output (pin D.7) is connected to the analog comparator input AIN1 (pin B.3). The AIN0 input of the comparator is connected to the internal 1.3 volt band gap reference voltage. There are two ISRs, one for the the timer0 time base and one to record the timer1 capture event. The period of the square wave is measured two different ways. The first way is to use the automatic input capture feature to get the time of 0-1 transition. The second is to poll the comparator ACO bit in a fast loop to determine when the transition occurs. The first method yields a period exactly equal to the period of the square wave programmed into OCR2. The polling method yields a period between 20 cycles too short and 15 too long. For accurate timing of external events, always use timer1 capture mode.

Minimizing jitter for entering an interrupt service routine (ISR).
There is a short delay between the time a hardware event occurs (e.g. timer event) and entering the corresponding ISR. At a minimum, the current instruction must finish executing, which may take one or two cycles, the program counter (PC) must be pushed on the stack, and the new PC loaded from the ISR vector table. There is thus a one or two cycle uncertainty between the time of the event and entering the ISR. For general timing this small uncertainty does not matter because it does not change the average period between ISR executions. But for video generation and other high speed, critically timed, programs the uncertainty can be troublesome. To overcome the uncertainty you can put the cpu to sleep before the timer interrupt event is expected. The cpu always takes the same number of cycles to wake up and enter the ISR. An example of this is given in video code. If you scroll down to main you will find the timer and sleep setup commands, followed by the usual endless loop with #asm ("sleep"); as its first statement. The program executes the loop once, then hangs waiting for the next timer interrupt.

Generating an audio square wave with no software overhead
If you set up a timer in clear-on-compare-match mode, turn off the match ISR, set the frequency using the appropriate output compare register, and turn on the waveform generation mode, then the timer will produce a square wave without software intevention. This is used in the video generation program where the cpu is fully occupied generating video, with very little time to generate audio. Timer2 is set up to generate the square wave based on a table of values corresponding to a musical scale.

Minimizing PWM noise when synthesizing an audio signal.
PWM analaog output depends upon the desired signal being slow enought that it does change too much during one PWM cycle. Lowpass filtering is essential at the output to form the average of the PWM cycle to convert to an analog value. The filtering is easier if the PWM frequency is higher, but often the computations necessary to update the PWM value take some time. One solution is to run the PWM at the highest speed (62,500 Hz with a 16 MHz clock), but only update the output compare register at a slow rate, perhaps 7812 Hz. This can be done by running timer0 in high-speed PWM mode at full rate, but not interrupting on the PWM cycle. Rather the update timing is derived from another timer. An example reads the ADC with ISR timing from timer1, but changes OCR0 to set the high-speed timer0 PWM in the timer1 ISR.