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MC68F375
CONFIGURABLE TIMER MODULE (CTM9)
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
13-60
Figure 13-19 CTM9 Example — Double Edge Output Compare
13.11.6 CTM9 Output Pulse Width Modulation With DASM
Output waveforms can be generated with any duty cycle without software involvement.
The software sets up a DASM with the compare times for the rising and falling edges,
and they are automatically repeated. The software does not need to respond to inter-
rupts to generate continuous pulses. The period may be selected as the period of a
free-running counter submodule time-base, times a binary multiplier selected in the
DASM. Multiple PWM outputs can be created from multiple DASMs and share one
counter submodule, provided that the periods of all of the output signals are a binary
multiple of the time-base, and that the counter submodule is operating in a free-run-
ning mode. Each DASM has a software selectable “don’t care” on high-order bits of
the time-base comparison so that the period of one output can be a binary multiple of
another signal. Masking the time-base serves to multiply the period of the time-base
by a binary number to form the period of the output waveform. The duty cycle can vary
from one cycle to 64K cycles. The frequency can range from 0.3 Hz to 62.5 KHz,
though the resolution decreases at the higher frequencies to as low as 7 bits. The gen-
eration of output square wave signals is of course the special case where the high and
low time are equal.
When an MCSM is used to drive the time-base, the modulus value is the period of the
output PWM signal. Figure 13-20 shows such an example. The polarity of the leading
edge of an output waveform is programmable for a rising or a falling edge. The soft-
ware selects the period of the output signal by programming the MCSM with a modulus
16-Bit Up Counter
Submodule Bus
Clock
Select
16-Bit Compare B
Output
Flip-Flop
Output
Pin
16-Bit Compare A
16-Bit Register B2
16-Bit Register A
Output
Compare
Double Action Submodule
Interrupt
on
From
Prescaler
or Pin
Trailing
Bus
Select
Bus
Select
Two
16-Bit
Time-
Base
Buses
Edge
Modulus Counter Submodule
Free Running or
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