Information

Note
The waveforms in Figure 40-17 and Figure 40-18 are for the
purpose of conceptual illustration and are not meant to
represent precise timing relationships between the signals
shown.
40.7.3.3 FSK Mode
When the modulator operates in FSK mode (MSC[MCGEN] and MSC[FSK] bits are set,
and MSC[BASE] bit is cleared), the modulation mark and space periods consist of an
integer number of carrier clocks (space period can be zero). 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.
The space period provides an interpulse gap (no carrier). If CMD3:CMD4 = 0x0000, 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
= (CMD1:CMD2 + 1) ÷ f
cg
t
space
= CMD3:CMD4 ÷ f
cg
Where f
cg
is the frequency output from the carrier generator. The example in figure
below shows what the IRO signal looks like in FSK mode with the following values:
CMD1:CMD2 = 0x0003, CMD3:CMD4 = 0x0002, primary carrier high count = 0x01,
primary carrier low count = 0x02, secondary carrier high count = 0x03, and secondary
carrier low count = 0x01.
Chapter 40 Carrier Modulator Transmitter (CMT)
K10 Sub-Family Reference Manual, Rev. 6, Nov 2011
Freescale Semiconductor, Inc. 1055