Page 129 - Embedded Microprocessor Systems Real World Design

previous reading and subtract the new reading from it. When the counter rolls over
from FF to 00, the software must recognize that fact and correct the numbers; if it
doesn’t, negative values will result. Many microcontrollers that provide a capture-
type timer also provide a means for the counter to generate an interrupt when it
rolls over, which can simplify this software task.

PWM
Figure 3.5D shows a simple PWM scheme, similar to that implemented on the Atmel
AT9OS microcontrollers. An 8-bit up/down counter counts from 00 to FF, then back
down to 00. An &bit comparator compares the value in the 8-bit latch to the counter
value. When the two values are equal, the comparator clocks the “D” flipflop
(again, timing logic makes sure everything works correctly). If the counter is count-
ing up, a “1” is clocked into the “D” flip-flop. If the counter is counting down,
a “0” is loaded. The flip-flop output is connected to one of the microcontroller
output pins.
Say the microprocessor writes a value of WE into the latch. The counter counts
from 00 to FE, where the PWM output goes to “1” because the counter bits match
the latched value. The counter continues to FF, then back down through FE to zero.
When the counter passes through FE, the PWM output goes to zero. So in this case,
the PWM output is high for two counts (FE and FF) out of 256, or about .78 percent
duty cycle. If the microprocessor writes OxFO to the latch, the PWM output will
be high from FO to FF and back to FO, for a total of 30 counts or 11.7 percent
duty cycle.
A more sophisticated PWM timer would include a second latch and comparator
so the counter can reverse direction at values other than FF. In such a timer, this
comparator would set the frequency of the PWM signal while the other compara-
tor would set the duty cycle.
Some microprocessors provide other means to generate PWM. The 80188 does
not use an up/down counter but instead provides two comparators. After the first
count value is reached, the counter is reset and the second comparator is used to
indicate end-of-count. The output pin indicates which comparator is being used so
a PWM output can be generated by controlling the ratios of the comparator values.

Other Counter Features
Most microcontrollers provide a means to prescale the clock applied to the counter.
For instance, the 8-bit clock in the Atmel AT9OS8515 can be incremented with the
CPU clock, or the CPU clock can be divided by 8, 64, 256, or 1024. In addition,
the clock can be supplied by either the rising or falling edge of the external pin
on the device. All of these features are software-selectable by writing to registers.
The 1Gbit timer in the AT9OS8515 can be programmed to control an output pin
when it times out. The pin can be programmed to toggle each time the timer times

110 Embedded Microprocessor Systems