Rabbit 6000 User’s Manual
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209
Figure 20-2 shows the adjustment ranges and output clock for the different modes of operation of
the DPLL. Each mode of operation will be described in turn.
Figure 20-2. Adjustment Ranges and Output Clock for Different DPLL Modes
With NRZ and NRZI encoding, all transitions occur on bit-cell boundaries and the data should be
sampled in the middle of the bit cell. If a transition occurs after the expected bit-cell boundary
(but before the midpoint), the DPLL needs to lengthen the count to line up the bit-cell boundaries.
This corresponds to the “add one” and “add two” regions shown. If a transition occurs before the
bit-cell boundary (but after the midpoint), the DPLL needs to shorten the count to line up the bit-
cell boundaries. This corresponds to the “subtract one” and “subtract two” regions shown. The
DPLL makes no adjustment if the bit-cell boundaries are lined up within one count of the divide-
by-16 counter. The regions that adjust the count by two allow the DPLL to synchronize faster to
the data stream when starting up.
With biphase-level encoding, there is a guaranteed “clock” transition at the center of every bit cell
and optional “data” transitions occur at the bit cell boundaries. The DPLL 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. This window is half a bit cell wide. Additionally, 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 the search mode. The search mode assumes that the next transition
seen is a clock transition and immediately synchronizes to this transition. No clock output is pro-
vided to the receiver during the search operation. Decoding biphase-level data requires that the
data be sampled at either the quarter or three-quarter point in the bit cell. The DPLL here uses the
quarter point to sample the data.
Summary of Contents for 6000
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