Motorola MPC8260 - Manual (Page 649)

MOTOROLA Chapter 22. SCC BISYNC Mode 22-17
Part IV. Communications Processor Module
Table 22-14 describes SCCS Þelds.
22.16 Programming the SCC BISYNC Controller
Software has two ways to handle data received by the BISYNC controller. The simplest is
to allocate single-byte receive buffers, request an interrupt on reception of each buffer, and
implement BISYNC protocol entirely in software on a byte-by-byte basis. This ßexible
approach can be adapted to any BISYNC implementation. The obvious penalty is the
overhead caused by interrupts on each received character.
A more efÞcient method is to prepare and link multi-byte buffers in the RxBD table and use
software to analyze the Þrst two to three bytes of the buffer to determine the type of block
received. When this is determined, reception continues without further software
intervention until it encounters a control character, which signiÞes the end of the block and
causes software to revert to byte-by-byte reception.
To accomplish this, set SCCM[RCH] to enable an interrupt on every received byte so
software can analyze each byte. After analyzing the initial characters of a block, either set
PSMR[RTR] or issue a
RESET BCS CALCULATION command. For example, if a DLE-STX is
received, enter transparent mode. By setting the appropriate PSMR bit, the controller strips
the leading DLE from DLE-character sequences. Thus, control characters are recognized
only when they follow a DLE character. PSMR[RTR] should be cleared after a DLE-ETX
is received.
Alternatively, after an SOH is received, a RESET BCS CALCULATION should be issued to
exclude SOH from BCS accumulation and reset the BCS. Notice that PSMR[RBCS] is not
needed because the controller automatically excludes SYNCs and leading DLEs.
After the type of block is recognized, SCCE[RCH] should be masked. The core does not
interrupt data reception until the end of the current block, which is indicated by the
reception of a control character matching the one in the receive control character table.
Using Table 22-15, the control character table should be set to recognize the end of the
block.
Table 22-14. SCCS Field Descriptions
Bit Name Description
0Р5 С Reserved, should be cleared.
6 CS Carrier sense (DPLL). Shows the real-time carrier sense of the line as determined by the DPLL.
0 The DPLL does not sense a carrier.
1 The DPLL senses a carrier.
7 Ñ Reserved, should be cleared.