Information

• 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
38-18 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
• Secondary carrier low count = 0x01
IRO signal
Modulator gate
Carrier out (fcg)
Mark1
Space1
Mark2
Space2
Mark1
Mark2
Space1
Figure 38-18. Example: CMT output in FSK mode
Chapter 38 Carrier Modulator Transmitter (CMT)
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
Freescale Semiconductor, Inc. 861