MC9S08RG60 Datasheet, PDF (98/232 Pages) Motorola, Inc – Microcontrollers

Carrier Modulator Transmitter (CMT) Module
The duty cycle of the carrier signal is controlled by varying the ratio of high time to low + high time. As
the input clock period is fixed, the duty cycle resolution will be proportional to the number of counts
required to generate the desired carrier period.
Duty Cycle = _{H-----i--g----h---c----oH---u--i-g-n---h-t---c+---o--L--u--o-n--w--t---c---o----u----n---t}
7.5.2 Modulator
The modulator has three main modes of operation:
• Gate the carrier onto the modulator output (time mode)
• Control the logic level of the modulator output (baseband mode)
• Count carrier periods and instruct the carrier generator to alternate between two carrier frequencies
whenever a modulation period (mark + space counts) expires (FSK mode)
The modulator includes a 17-bit down counter with underflow detection. The counter is loaded from the
16-bit modulation mark period buffer registers, CMTCMD1 and CMTCMD2. The most significant bit is
loaded with a logic zero and serves as a sign bit. When the counter holds a positive value, the modulator
gate is open and the carrier signal is driven to the transmitter block.
When the counter underflows, the modulator gate is closed and a 16-bit comparator is enabled that
compares the logical complement of the value of the down-counter with the contents of the modulation
space period register (which has been loaded from the registers CMTCMD3 and CMTCMD4).
When a match is obtained the cycle repeats by opening the modulator gate, reloading the counter with the
contents of CMTCMD1 and CMTCMD2, and reloading the modulation space period register with the
contents of CMTCMD3 and CMTCMD4.
If the contents of the modulation space period register are all zeroes, the match will be immediate and no
space period will be generated (for instance, for FSK protocols that require successive bursts of different
frequencies).
The MCGEN bit in the CMTMSC register must be set to enable the modulator timer.
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MC9S08RC/RD/RE/RG
Freescale Semiconductor