Jameco Electronics 2000 manual Rabbit 3000 Microprocessor

clock rate must be very small, and depends on the longest possible run of zeros in the received frame. NRZI encoding guarantees at least one transition every six bits (with the inserted zeros). Since the DPLL can adjust by two counts every bit cell, the maximum dif- ference between the sending data rate and the DPLL output clock rate is 1/48 (~2%).

With Biphase data encoding (either -Level, -Mark or -Space), the DPLL runs only as long as transitions are present in the receive data stream. Two consecutive missed transitions causes the DPLL to halt operation and wait for the next available transition. This mode of operation is necessary because it is possible for the DPLL to lock onto the optional transi- tions in the receive data stream. Since they are optional, they will eventually not be present and the DPLL can attempt to lock onto the required transitions. Since the DPLL can adjust by one count every bit cell, the maximum difference between the sending data rate and the DPLL output clock rate is 1/16 (~6%).

With Biphase data encoding the DPLL is designed to work in multiple-access conditions where there may not be Flags on an idle line. The DPLL will properly generate an output clock based on the first transition in the leading zero of an opening Flag. Similarly, only the completion of the closing Flag is necessary for the DPLL to provide the extra two clocks to the receiver to properly assemble the data. In Biphase-Level mode, this means the transi- tion that defines the last zero of the closing Flag. In Biphase-Mark and Biphase-Space modes this means the transition that defines the end of the last zero of the closing Flag.

The figure below 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.

Bit cell
NRZI adj	none		add one	add two			subtract two		subtract one		none
NRZI Clock
Bi-L adj		ignore transitions		subtract one		none		add one		ignore transitions
Bi-L Clock
Bi-S adj	none		add one		ignore transitions				subtract one		none
Bi-S Clock
Bi-M adj	none		add one		ignore transitions				subtract one		none
Bi-M Clock

190 Rabbit 3000 Microprocessor