Carrier Modulator Transmitter (CMT)

9.5.1 Time Mode

When the modulator operates in time mode, the modulation mark and space periods consist of zero or an integer number of fosc ÷ 8 clocks (= 250 kHz @ 2 MHz osc). This provides a modulator resolution of 4 μs and a maximum mark and space periods of about 16 ms (each). However, to prevent carrier glitches which could affect carrier spectral purity, the modulator control gate and carrier clock are synchronized. The carrier signal is activated when the modulator gate opens. The modulator gate can only close when the carrier signal is low (the output logic level during space periods is low). If the carrier generator is in baseband mode (BASE bit in MCSR is high), the modulator output will be at a logic one for the duration of the mark period and at a logic zero for the duration of a space period. See Figure 9-8.

The mark and space time equations are:

tmark = (---------------------------------------------MBUFF + 1) × 8 secs

 

fosc

tspace =

SBUFF------------------------------×8 sec s

 

fosc

Setting the DIV2 bit in the MCSR will double mark and space times.

General Release Specification

 

MC68HC05RC16 — Rev. 3.0

 

 

 

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Carrier Modulator Transmitter (CMT)

MOTOROLA