MC9S08RC8 Datasheet, PDF (114/234 Pages) Freescale Semiconductor, Inc – Microcontrollers

Carrier Modulator Transmitter (CMT) Block Description
CMTCLK ÷ 8
CARRIER OUT(f_CG)
MODULATOR GATE
MARK
SPACE
MARK
IRO PIN (TIME MODE)
IRO PIN
(BASEBAND MODE)
Figure 8-5. Example CMT Output in Time and Baseband Modes
8.5.2.2 Baseband Mode
Baseband mode (MCGEN bit is set and BASE bit is set) is a derivative of time mode, where the mark and
space period is based on (CMTCLK ÷ 8) counts. The mark and space calculations are the same as in time
mode. In this mode the modulator output will be at a logic 1 for the duration of the mark period and at a
logic 0 for the duration of a space period. See Figure 8-5 for an example of the output for both baseband
and time modes. In the example, the carrier out frequency (f_CG) is generated with a high count of $01 and
a low count of $02, which results in a divide of 3 of CMTCLK with a 33 percent duty cycle. The modulator
down-counter was loaded with the value $0003 and the space period register with $0002.
NOTE
The waveforms in Figure 8-5 and Figure 8-6 are for the purpose of
conceptual illustration and are not meant to represent precise timing
relationships between the signals shown.
8.5.2.3 FSK Mode
When the modulator operates in FSK mode (MCGEN bit is set, FSK bit is set, and BASE bit is clear), the
modulation mark and space periods consist of an integer number of carrier clocks (space period can be 0).
When the mark period expires, the space period is transparently started (as in time mode). The carrier
generator toggles between primary and secondary data register values whenever the modulator space
period expires.
MC9S08RC/RD/RE/RG Data Sheet, Rev. 1.11
114
Freescale Semiconductor