
MC68F375
CONFIGURABLE TIMER MODULE (CTM9)
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
13-61
value. The leading edge compare value is written into register A by software, and the
trailing edge time is written into register B1. When the leading edge value is reached,
the content of register B1 is transferred to register B2, to form the next trailing edge
value. Subsequent changes to the output pulse width are made by writing a new time
into register B1. Updates to the pulse width are always synchronized to the leading
edge of the waveform.
Figure 13-20 CTM9 Example — Pulse Width Modulation Output
It is typical to use the pulse width modulation mode of the DASM without interrupts,
though an interrupt can be enabled to occur on the leading edge. When the output is
an unchanging repetitive waveform, the DASM continuously generates the signal with-
out any software intervention. When the software needs to change the pulse width, a
new trailing edge time is written to the DASM. The output is changed on the next full
pulse. When the software needs to change the output at a regular rate, such as an
acceleration curve, the leading edge interrupt gives the software one period time to
update the new trailing edge time.
13.11.7 CTM9 Input Pulse Accumulation
Counting the number of pulses on an input signal is another capability of the CTM9.
Pulse accumulation uses either an FCSM or an MCSM. Since the counters in the
16-Bit Up Counter
Two
Submodule Bus
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
Bus
From
Prescaler
or Pin
16-Bit
Time-
Leading
16-Bit Register B1
Edge
Modulus Register
Modulus
Control
Clock
Select
Load
Base
Buses
Select
Bus
Select
Modulus Counter Submodule
Free Running or
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
..
.