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Rabbit 4000 Microprocessor User’s Manual
Several types of data encoding are available in the HDLC mode. In addition to the normal
NRZ, they are NRZI, biphase-level (Manchester), biphase-space (FM0), and biphase-
mark (FM1). Examples of these encodings are shown below. Note that the signal level
does not convey information in NRZI, biphase-space, and biphase-mark. Instead it is the
placement of the transitions that determine the data. In biphase-level it is the polarity of
the transition that determines the data.
Figure 17-1. Examples of Data Encoding In the HDLC Mode
In the HDLC mode the internal clock comes from the output of Timer A2/Timer A3 or the
dedicated divider. The timer/divider output is divided by 16 to form the transmit clock,
and is fed to the digital phase-locked loop (DPLL) to form the receive clock. The DPLL is
basically just a divide-by-16 counter that uses the timing of the transitions on the receive
data stream to adjust its count. The DPLL adjusts the count so that the DPLL output will
be properly placed in the bit cells to sample the receive data. To work properly, then, tran-
sitions are required in the receive data stream. NRZ data encoding does not guarantee tran-
sitions in all cases (a long string of zeros, for example), but the other data encodings do.
NRZI guarantees transitions because of the inserted zeros, and the biphase encodings all
have at least one transition per bit cell.
The DPLL counter normally counts by 16, but if a transition occurs earlier or later than
expected, the count will be modified during the next count cycle. If the transition occurs
earlier than expected, it means that the bit cell boundaries are early with respect to the
DPLL-tracked bit-cell boundaries, so the count is shortened by either one or two counts. If
the transition occurs later than expected, it means that the bit-cell boundaries are late with
SERIAL CLOCK
NRZ DATA
NRZI
NRZI
BIPHASE LEVEL
BIPHASE SPACE
BIPHASE SPACE
BIPHASE MARK
BIPHASE MARK
DATA
1
0
1
1
0
0
1
0
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