Information

40.4 Modes of Operation
The CMT module operates in the following modes.
• Time—When operating in time mode, the user independently defines the high and
low times of the carrier signal to determine both period and duty cycle.
• Baseband—When MSC[BASE] bit is set, the carrier output (f
cg
) to the modulator is
held high continuously to allow for the generation of baseband protocols.
• Frequency shift key (FSK)—This 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 following table summarizes the CMT's modes.
Table 40-1. CMT Modes of Operation
Mode MSC[MCGEN]
1
MSC[BASE]
2
MSC[FSK]
2
MSC[EXSPC] Comment
Time 1 0 0 0
f
cg
controlled by primary high and
low registers.
f
cg
transmitted to CMT_IRO signal
when modulator gate is open.
Baseband 1 1 X 0
f
cg
is always high. CMT_IRO signal
high when modulator gate is open.
FSK 1 0 1 0
f
cg
control alternates between
primary high/low registers and
secondary high/low registers.
f
cg
transmitted to CMT_IRO signal
when modulator gate is open.
Extended
Space
1 X X 1
Setting MSC[EXSPC] bit causes
subsequent modulator cycles to be
spaces (modulator out not asserted)
for the duration of the modulator
period (mark and space times).
IRO Latch 0 X X X
OC[IROL] bit controls state of
CMT_IRO signal.
1. To prevent spurious operation, initialize all data and control registers before beginning a transmission (MSC[MCGEN]=1).
2. These bits are not double buffered and should not be changed during a transmission (while MSC[MCGEN]=1).
Chapter 40 Carrier Modulator Transmitter (CMT)
K20 Sub-Family Reference Manual, Rev. 6, Nov 2011
Freescale Semiconductor, Inc. 1007