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• The modulation mark period consists of an integer number of (CMTCLK ÷ 8) clock
periods.
• The modulation space period consists of 0 or an integer number of (CMTCLK ÷ 8)
clock periods.
With an 8 MHz IF and MSC[CMTDIV] = 00, the modulator resolution is 1 μs and has a
maximum mark and space period of about 65.535 ms each . See Figure 40-17 for an
example of the Time and Baseband mode outputs.
The mark and space time equations for Time and Baseband mode are:
t
mark
= (CMD1:CMD2 + 1) ÷ (f
CMTCLK
÷ 8)
t
space
= CMD3:CMD4 ÷ (f
CMTCLK
÷ 8)
where CMD1:CMD2 and CMD3:CMD4 are the decimal values of the concatenated
registers.
Carrier out
(fcg)
Modulator gate
IRO signal
(Time mode)
IRO signal
(Baseband mode)
Mark
Space
Mark
CMTCLK 8
Figure 40-17. Example: CMT output in Time and Baseband modes with OC[CMTPOL]=0
40.7.3.2 Baseband mode
Baseband mode, that is, when MSC[MCGEN] and MSC[BASE] are 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.
Chapter 40 Carrier Modulator Transmitter (CMT)
K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012
Freescale Semiconductor, Inc.
Preliminary
1005
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