Pulse Width Modulation

Thepulse width modulation (PWM) mode is a special case of the output compare. In it, the timer generates a periodic digital output signal with configurable high-time and period. Two registers form the main interface to the PWM, one for the period (also calledduty cycle) and one for the high-time (or the low-time). Some timers only allow the user to configure the high-time, and either use the full timer range as the period or offer a restricted choice of possible periods. In addition to these registers, the timer module provides bits to enable PWM and possibly for mode control.

PWM signals are useful for a lot of things. Apart from their uses in simple d/a converters they can be used to implement ABS in cars, to dim LEDs or numeric displays, or for motor control (servos, stepper motors, speed control of dc motors).

The internal realization of PWM is actually quite simple and just uses the counter and two com- pares. There are two possible implementations, one using an up-counter (or down-counter) and one using an up-down counter. In the following explanations, we will assume that the user specifies the high time of the signal, which we will call thecompare value, and that the period is given by thetop value.

In theup-counterversion, see Figure2.31, the output is set to high when the counter reaches zero, and it is set to low when the counter reaches the compare value. As soon as the top value is reached, the counter is reset to zero. The advantage of this method is its resource-efficiency. However, if you can update the compare and top values anytime within the duty cycle, you can produceglitchesin the PWM signal, which are invalid interim cycles. For example, if you set the top value below the current count value, the timer will count through its full range once before switching to the correct duty cycle.

PWM output

update

cmp updatetop maximum counter value

glitch glitch cmp

top

t t

Figure 2.31: PWM signal generated by an up-counter and the results of unbuffered updates. Controllers which use this method may hence only take over new top and compare values when the counter reaches zero. If you set the compare value above the top value, the output will be constantly high.

In theup-down-counterversion, see Figure2.32, the counter first counts up from zero to the top value and then switches direction and counts down back to zero. The counter starts by setting the output to high and begins counting at zero. Whenever the compare value is reached on the upcount, the output is set to low. When the compare value is reached again on the downcount, the output is set back to high. As you can see, this results in a nice symmetrical signal with a period that can be twice as long as that of a pure up-counter. Again, asynchronous updates of the compare or top value can result in glitches, so the controller must buffer the values until zero is reached.

PWM output

maximum counter value

cmp top

t t Figure 2.32: PWM signal generated by an up-down-counter.

In both versions, the attainable period is determined by the resolution of the timer. If the high time is set to zero or to (or above) the top value, this will generally result in a constant low or high signal.

2.6.5

Exercises

Exercise 2.6.1You only have two 8-bit timers on your 8-bit microcontroller but want to have a 16-bit timer for your application. Can you solve this problem in software? How does your solution work? What functions do you have to provide as an API (application program interface) to your timer? Do you have to think about asynchronous updates?

Exercise 2.6.2 Assume that your microcontroller has an operating frequency of 1 MHz and two timers, an 8- and a 16-bit timer. It is your task to select useful prescaler modes for the timers. Each

2.6. TIMER 67

timer can have four such modes between 2 and 1024 (and the values must be powers of 2). Which prescaler modes would you assign and why?

Exercise 2.6.3 Assume that your microcontroller is clocked with 4 MHz and that it offers an 8-bit timer operating with this frequency. You want to use this timer to measure the duration between two external events. What bounds does this impose on the duration (minimum and maximum interval). How large is your measurement error? How does a prescale value of 256 affect your answers?

Exercise 2.6.4If compare and top value updates are not buffered, how many different ways are there to produce a glitch when using an up-down-counter to generate a PWM signal? Give an example for each way you find. How would you solve the update problem? What if the controller can raise an interrupt whenever the PWM signal reaches zero?

Exercise 2.6.5 You want to measure the period of a periodic digital signal and decide to use the external event counter (pulse accumulator) for this purpose. Sketch how you can measure the period this way. How accurately can you measure the period? Compare this method to a solution with input capture.