Motorola MPC8260 - Manual (Page 579)

MOTOROLA Chapter 19. Serial Communications Controllers (SCCs) 19-23
Part IV. Communications Processor Module
Figure 19-14. DPLL Transmitter Block Diagram
The DPLL can be driven by one of the baud rate generator outputs or an external clock,
CLKx. In the block diagrams, this clock is labeled HSRCLK/HSTCLK. The HSRCLK/
HSTCLK should be approximately 8x, 16x, or 32x the data rate, depending on the coding
chosen. The DPLL uses this clock, along with the data stream, to construct a data clock that
can be used as the SCC Rx and/or Tx clock. In all modes, the DPLL uses the input clock to
determine the nominal bit time. If the DPLL is bypassed, HSRCLK/HSTCLK is used
directly as RCLK/TCLK.
At the beginning of operation, the DPLL is in search mode, whereas the Þrst transition
resets the internal DPLL counter and begins DPLL operation. While the counter is
counting, the DPLL watches the incoming data stream for transitions; when one is detected,
the DPLL adjusts the count to produce an output clock that tracks incoming bits.
The DPLL has a carrier-sense signal that indicates when data transfers are on RXD. The
carrier-sense signal asserts as soon as a transition is detected on RXD; it negates after the
programmed number of clocks in GSMR_L[TSNC] when no transitions are detected.
To prevent itself from locking on the wrong edges and to provide fast synchronization, the
DPLL should receive a preamble pattern before it receives the data. In some protocols, the
preceding ßags or syncs can function as a preamble; others use the patterns in Table 19-8.
When transmission occurs, the SCC can generate preamble patterns, as programmed in
GSMR_L[TPP, TPL].
DPLL
HSTCLK
TEND
TENC
Transmitter
0
S
Recovered Clock
HSTCLK TCLK
1
1x Mode
0
S
1
TENC = NRZI
D
CLK
Q
SCCT Data
0
S
1
1x Mode
TDCR
D
CLK
Q
HSTCLK
TXEN
HSTCLK
TXD
TINV
Encoded Data