English
Language : 

MC68LC302 Datasheet, PDF (78/169 Pages) Motorola, Inc – Low Power Integrated Multiprotocol Processor Reference Manual
Communications Processor (CP)
rupt in order to determine parity and character length (this information may be overwritten
when the next character interrupt is presented to the host). The host uses the two received
characters to determine 1) whether a properly formed “at” or “AT” was received, and 2) the
proper character format (character length, parity).
Once this is decided, three possible actions can result. First, the host may decide that the
data received was not a proper “at” or “AT”, and issue the Enter_Baud_Hunt command to
cause the autobaud controller to resume the search process. Second, the host may decide
the “at” or “AT” is proper and simply continue to receive characters in BISYNC mode. Third,
the M68000 core may decide that the “at” or “AT” is proper, but a change in character length
or parity is required.
4.3.9.2 AUTOBAUD CHANNEL TRANSMIT PROCESS. The autobaud microcode pack-
age supports two methods for transmission. The first method is automatic echo which is sup-
ported directly in the SCC hardware, and the second method is a smart echo or software
transmit which is supported with an additional clock and software.
Automatic echo is enabled by setting the DIAG bits in the SCC mode register (SCM) to ‘10’
and asserting the CD pin (externally on SCC1 and on SCC2 and SCC3, either externally or
by leaving the pin as a general purpose input). The ENT bit of the SCC should remain
cleared. The transmitter is not used, so this echoing method does not impact performance.
The smart echo or software transmit requires use of an additional clock and the transmitter,
so the overall performance could be affected if other SCCs are running. This method
requires an additional clock for sampling the incoming bit stream since the baud rate gener-
ator (BRG) must be used to provide the correct frequency for transmission. The user needs
to provide the sampling clock that will be used for the autobaud function on the RCLK pin
(for example, a 1.8432 MHz clock for 115.2K). The clock that will be used for the SCC trans-
mission can be provided to the BRG from the system clock or on TIN1.The TIN1 and RCLK1
pins can be tied together externally. After the first two characters have been received and
character length and parity determined, the host programs the DSR to $FFFF, enables the
transmitter (by setting ENT), and programs the transmit character descriptor (overlays CON-
TROL Character 8). The host is interrupted after each character is transmitted.
For modem applications with the MC68LC302, SCC2 will be used as the DTE interface and
autobauding to the DTE baud rate will often be required. If use of the smart echo feature is
desired, the receive clock can be provided by the baud rate generator 2 (BRG2) internally
by resetting the RCS bit in the SCON2 register to zero. The separate transmit clock can be
provided externally to the TCLK2 pin through a hardwire connection. The TCS bit in the
SCON2 register should be set to one to enable the external clock source. After autobauding
is complete, both the transmit and receive clock sources can be derived internally from
BRG2 and the external pin connected to TCLK2 should be three stated to assure that it does
not contend with the TCLK2 pin.
4.3.9.3 AUTOBAUD PARAMETER RAM. When configured to operate in the autobaud
mode, the IMP overlays some entries of the UART-specific parameter RAM as illustrated in
Table 4-3.
MOTOROLA
MC68LC302 REFERENCE MANUAL
4-11