MC9S08RC8 Datasheet, PDF (115/234 Pages) Freescale Semiconductor, Inc – Microcontrollers

Carrier Modulator Transmitter (CMT) Block Description
The space period provides an interpulse gap (no carrier). If CMTCMD3:CMTCMD4 = $0000, then the
modulator and carrier generator will switch between carrier frequencies without a gap or any carrier
glitches (zero space).
Using timing data for carrier burst and interpulse gap length calculated by the CPU, FSK mode can
automatically generate a phase-coherent, dual-frequency FSK signal with programmable burst and
interburst gaps.
The mark and space time equations for FSK mode are:
t_mark = (CMTCMD1:CMTCMD2 + 1) ÷ f_CG
Eqn. 8-7
t_space = CMTCMD3:CMTCMD4 ÷ f_CG
Eqn. 8-8
Where f_CG is the frequency output from the carrier generator. The example in Figure 8-6 shows what the
IRO pin output looks like in FSK mode with the following values: CMTCMD1:CMTCMD2 = $0003,
CMTCMD3:CMTCMD4 = $0002, primary carrier high count = $01, primary carrier low count = $02,
secondary carrier high count = $03, and secondary carrier low count = $01.
CARRIER OUT (f_CG)
MODULATOR GATE
MARK1
SPACE1
MARK2
SPACE2
MARK1
SPACE1
MARK2
IRO PIN
Figure 8-6. Example CMT Output in FSK Mode
8.5.3 Extended Space Operation
In time, baseband, or FSK mode, the space period can be made longer than the maximum possible value
of the space period register. Setting the EXSPC bit in the CMTMSC register will force the modulator to
treat the next modulation period (beginning with the next load of the counter and space period register) as
a space period equal in length to the mark and space counts combined. Subsequent modulation periods will
consist entirely of these extended space periods with no mark periods. Clearing EXSPC will return the
modulator to standard operation at the beginning of the next modulation period.
8.5.3.1 EXSPC Operation in Time Mode
To calculate the length of an extended space in time or baseband modes, add the mark and space times and
multiply by the number of modulation periods that EXSPC is set.
MC9S08RC/RD/RE/RG Data Sheet, Rev. 1.11
Freescale Semiconductor
115