Carrier Modulator Transmitter (CMT) Block Description
MC9S08RC/RD/RE/RG Data Sheet, Rev. 1.11
Freescale Semiconductor
109
8.5
Functional Description
The CMT module consists of a carrier generator, a modulator, a transmitter output, and control registers.
The block diagram is shown in
Figure 8-2. When operating in time mode, the user independently denes
the high and low times of the carrier signal to determine both period and duty cycle. The carrier generator
resolution is 125 ns when operating with an 8 MHz internal bus frequency and the CMTDIV1 and
CMTDIV0 bits in the CMTMSC register are both equal to 0. The carrier generator can generate signals
with periods between 250 ns (4 MHz) and 127.5
The possible duty cycle options will depend upon the number of counts required to complete the carrier
period. For example, a 1.6 MHz signal has a period of 625 ns and will therefore require 5
× 125 ns counts
to generate. These counts may be split between high and low times, so the duty cycles available will be
20 percent (one high, four low), 40 percent (two high, three low), 60 percent (three high, two low) and
80 percent (four high, one low).
For lower frequency signals with larger periods, higher resolution (as a percentage of the total period) duty
cycles are possible.
When the BASE bit in the CMT modulator status and control register (CMTMSC) is set, the carrier output
(fCG) to the modulator is held high continuously to allow for the generation of baseband protocols.
A third mode allows the carrier generator to alternate between two sets of high and low times. When
operating in FSK mode, the generator will toggle between the two sets when instructed by the modulator,
allowing the user to dynamically switch between two carrier frequencies without CPU intervention.
The modulator provides a simple method to control protocol timing. The modulator has a minimum
resolution of 1.0
s with an 8 MHz internal bus clock. It can count bus clocks (to provide real-time control)
The transmitter output block controls the state of the infrared out pin (IRO). The modulator output is gated
on to the IRO pin when the modulator/carrier generator is enabled.
A summary of the possible modes is shown in
Table 8-2.
Table 8-1. Clock Divide
Bus
Clock
(MHz)
CMTDIV1:CMTDIV0
Carrier
Generator
Resolution
(
s)
Min Carrier
Generator
Period
(
s)
Min
Modulator
Period
(
s)
8
0:0
0.125
0.25
1.0
8
0:1
0.25
0.5
2.0
8
1:0
0.5
1.0
4.0
8
1:1
1.0
2.0
8.0