Information

Table 38-20. Mode functions (continued)
Mode Function
FSK The modulator can count carrier periods and instruct the
carrier generator to alternate between two carrier frequencies
whenever a modulation period consisting of mark and space
counts, expires.
The modulator provides a simple method to control protocol timing. The modulator has a
minimum resolution of 1.0 μs with an 8 MHz. It can count bus clocks to provide real-
time control, or carrier clocks for self-clocked protocols.
The modulator includes a 17-bit down counter with underflow detection. The counter is
loaded from the 16-bit modulation mark period buffer registers, CMD1 and CMD2. The
most significant bit is loaded with a logic 0 and serves as a sign bit.
When Then
The counter holds a positive value The modulator gate is open and the carrier signal is driven to
the transmitter block.
The counter underflows The modulator gate is closed and a 16-bit comparator is
enabled which compares the logical complement of the value
of the down counter with the contents of the modulation
space period register which has been loaded from the
registers, CMD3 and CMD4.
When a match is obtained, the cycle repeats by opening the modulator gate, reloading the
counter with the contents of CMD1 and CMD2, and reloading the modulation space
period register with the contents of CMD3 and CMD4.
The modulation space period is activated when the carrier signal is low to prohibit cutting
off the high pulse of a carrier signal. If the carrier signal is high, the modulator extends
the mark period until the carrier signal becomes low. To deassert the space period and
assert the mark period, the carrier signal must have gone low to ensure that a space period
is not erroneously shortened.
If the contents of the modulation space period register are all zeroes, the match will be
immediate and no space period will be generated, for instance, for FSK protocols that
require successive bursts of different frequencies).
MSC[MCGEN] must be set to enable the modulator timer.
The following figure presents the block diagram of the modulator.
Functional description
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
858 Freescale Semiconductor, Inc.