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SERIAL CONTROL MODULE: HDLC
Normal mode operation
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only uses the clock transitions to track the bit-cell boundaries, by ignoring all
transitions occurring outside a window around the center of the bit-cell. The window
is half a bit-cell wide.
Because the clock transitions are guaranteed, the DPLL requires that they always be
present. If no transition is found in the window around the center of the bit-cell for
two successive bit-cells, the DPLL is not in lock and immediately enters search
mode. Search mode presumes that the next transition seen is a clock transition and
immediately synchronizes to this transition. No clock output is provided to the
receiver during the search operation.
Biphase-Mark
and Biphase-
Space encoding
Biphase-mark and biphase-space encoding are identical per the DPLL and are similar
to biphase-level. The primary difference is the clock placement and data
transitions. With these encodings, the clock transitions are at the bit-cell boundary
and the data transitions are at the center of the bit-cell; the DPLL operation is
adjusted accordingly. Decoding biphase-mark or biphase-space encoding requires
that the data be sampled by both edges of the recovered receive clock.
IRDA-compliant
encode
There is an optional IRDA-compliant encode and decode function available. The
encoder sends an active-high pulse for a zero and no pulse for a one. The pulse is
1/4th of a bit-cell wide. The decoder watches for active-low pulses which are
stretched to one bit time wide to recreate the normal asynchronous waveform for the
receiver. enabling the IRDA-compliant encode/decode modifies the transmitter so
there are always two opening flags transmitted.
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Normal mode operation
The HDLC achieves normal mode operation by programming the HDLC and Wrapper
configuration registers.
Example
configuration
This example shows a normal mode operation configuration for a typical
application. Any field not specified in this table can be left at reset value.
Control register Field Value Comment
HDLC Control register CLK 0x3 Enable internal clock generation
HDLC Clock Divider High EN 0x1 Enable the internal clock divider; the
clock rate will be 1.8432 Mbps.