Page 314 - Programming Microcontrollers in C

A Pulse Width Modulation Program 299

A Pulse Width Modulation Program

While the MC68HC16 has a built-in pulse width modulation
(PWM) system, it is sometimes desired to achieve more flexible reso­
lution than can be obtained with the built-in system. Therefore, it is
not unreasonable that one would want to use the general-purpose
timer to create a PWM. Prior to writing the code for a PWM in this
manner, we should look at the basic time period of the processor.
Unless otherwise directed, all of the timers in the GPT are driven by
the system clock. This signal is passed through a prescaler controlled
by the bits CPR2 through CPR0 in the register TMSK2. The default
prescaler value will cause a clock rate to the GPT of the system clock
divided by four.
The question now is the clock rate. Recall that the clock control
is a portion of the SIM. Within the SIM there is a register named
SYNCR. The clock frequency is controlled by the three bit fields
named W, X, and Y in this register. When operating at a low crystal
frequency—between 25 and 50 kHz—the formula for the system
frequency is given by

(
F= F 4 y +1 )(2 2w+x ) 
s r  
where y has a value between 0 and 63, and both w and x can have
values of 0 or 1. With a frequency f of 32767 and the W, X, and Y
default values of 0, 0, and 63, respectively, the device will come out
of reset with a system frequency of 8.388 MHz. To be able to get the
finest resolution for our timing functions, let us plan to operate the
system clock frequency at its maximum value by changing the value
of X from its default value of 0 to 1. This change will cause the
system frequency to be 16.776704 MHz. The frequency of the input
into the GPT is one-fourth this value or 4.194176 MHz. The time
period for this frequency is 238 nanoseconds.
The code for a PWM on the MC68HC11 was shown in Chapter 5. A
program that implements a PWM on the MC68HC16 is shown below.

/* This program provides a pwm output to OC3. The period
will be the integer value found in pwm_period, and the
on time will be the integer value found in pwm_count.
Keep pwm_count less than pwm_period. */