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5–10

5.4.3

FM1 (Biphase Mark)

In FM1 encoding, also known as biphase mark, a transition is present on every bit cell

boundary, and an additional transition may be present in the middle of the bit cell. In FM1,

a ‘0’ is sent as no transition in the center of the bit cell and a ‘1’ is sent as a transition in

the center of the bit cell. FM1 encoded data contains sufficient information to recover a

clock from the data.

5.4.4

FM0 (Biphase Space)

In FM0 encoding, also known as biphase space, a transition is present on every bit cell

boundary and an additional transition may be present in the middle of the bit cell. In FM0

a ‘1’ is sent as no transition in the center of the bit cell and a ‘0’ is sent as a transition in

the center of the bit cell. FM0 encoded data contains sufficient information to recover a

clock from the data.

5.4.5

Manchester Decoding

In addition to these four methods, the SCC can be used to decode Manchester (biphase

level) data using the DPLL in the FM mode and programming the receiver for NRZ data.

Manchester encoding always produces a transition at the center of the bit cell. If the tran-

sition is High-to-Low, the bit is a ‘1’; if the transition is Low-to-High, the bit is a ‘0’.

5.4.6

Data Encoding Programming

The data encoding method to be used should be selected in the initialization procedure

before the transmitter and receiver are enabled but no other restrictions apply. Note, in

Figure 5–4, that in NRZ and NRZI the receiver samples the data only on one edge. How-

ever, in FM1 and FM0, the receiver samples the data on both edges. Also, as shown in

Figure 5–4, the transmitter defines bit cell boundaries by one edge in all cases and uses

the other edge in FM1 and FM0 to create the mid-bit transition.

5.5

DIGITAL PHASE-LOCKED LOOP (DPLL)

The SCC contains a DPLL that can be used to recover clock information from a data

stream with NRZI or FM coding. The DPLL is driven by a clock that is nominally 32

(NRZI) or 16 (FM) times the data rate. The DPLL uses this clock, along with the data

stream, to construct a receive clock for the data. This clock can then be used as the re-

ceive clock, the transmit clock, or both.

Figure 5–5 shows a block diagram of the DPLL. It consists of a 5-bit counter, an edge

detector, and a pair of output decoders. The clock for the DPLL comes from the output of

a two-input multiplexer, and the two outputs go to the transmitter and receive clock multi-

plexers. The DPLL is controlled by seven commands that are encoded in bits D7, D6, and

D5 of WR14.

Edge Detector

RxD

Receive

Clock

Decode

Count Modifier

5-Bit Counter

Transmit

Clock

Decode

Figure 5–5. DPLL

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