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In this mode, the modulator output will be at a logic 1 for the duration of the mark period
and at a logic 0 for the duration of a space period. See Figure 40-17 for an example of the
output for both Baseband and Time modes. In the example, the carrier out frequency (f
cg
)
is generated with a high count of 0x01 and a low count of 0x02 that results in a divide of
3 of CMTCLK with a 33% duty cycle. The modulator down counter was loaded with the
value 0x0003 and the space period register with 0x0002.
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, that is, when MSC[MCGEN] and
MSC[FSK] are set, and MSC[BASE] 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
40-18 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
Functional description
K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012
1006
Preliminary
Freescale Semiconductor, Inc.
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