Zilog Z16C35 - Manual (Page 41)

ISCC
User Manual
UM011002-0808
35
the receiver samples the data in the middle of the bit cell. In NRZI mode, the DPLL does
not require a transition in every bit cell, so this mode is useful for recovering the clocking
information from NRZ and NRZI data streams.
In the FM mode, the DPLL clock must be 16 times the data rate. In this mode the transmit
clock output of the DPLL lags the receive clock outputs by 90 degrees to make the trans-
mit and receive bit cell boundaries the same, because the receiver must sample FM data at
one-quarter and three-quarters bit time.
3.5.1 DPLL Operation in the NRZI Mode
To operate in NRZI mode, the DPLL must be supplied with a clock that is 32 times the
data rate. The DPLL uses this clock, along with the receive data, to construct receive and
transmit clock outputs that are phased to properly receive and transmit data.
To do this, the DPLL divides each bit cell into four regions, and makes an adjustment to
the count cycle of the 5-bit counter dependent upon in which region a transition on the
receive data input occurred. This is shown in Figure 3-5.
Figure 3–8. DPLL in NRZI Mode
Ordinarily, a bit cell boundary will occur between count 15 and count 16, and the DPLL
output will cause the data to be sampled in the middle of the bit cell. However, four differ-
ent situations may happen:
The DPLL actually allows the transition marking a bit cell boundary to occur anywhere
during the second half of count 15 or the first half of count 16 without making a correction
to its count cycle.
If the transition marking a bit cell boundary occurs between the middle of count 16 and
count 31, the DPLL is sampling the data too early in the bit cell. In response to this, the
DPLL extends its count by one during the next 0 to 31 counting cycle, which effectively
moves the edge of the clock that samples the receive data closer to the center of the bit
cell.
1 18 19 20 17 16 21 22 23 24 25 26 27 28 29 30 31 2 3 4 5 7 8 9 12 13 14 11 10 15 0
No Change No Change
Bit Cell
Count
Connection
DPLL Out
Add One Count Subtract One Count
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