MC68LC302 Datasheet, PDF (76/169 Pages) Motorola, Inc – Low Power Integrated Multiprotocol Processor Reference Manual

Communications Processor (CP)
4.3.8.5 UART EVENT REGISTER. The SCC event register (SCCE) is called the UART
event register when the SCC is operating as a UART.
7
6
5
4
3
2
1
0
CTS CD IDL BRK CCR BSY TX RX
4.3.8.6 UART MASK REGISTER. The SCC mask register (SCCM) is referred to as the
UART mask register when the SCC is operating as a UART. If a bit in the UART mask reg-
ister is a one, the corresponding interrupt in the event register will be enabled. If the bit is
zero, the corresponding interrupt in the event register will be masked. This register is cleared
upon reset.
4.3.9 Autobaud Controller (New)
The autobaud function determines the baud rate and format of an asynchronous data
stream starting with a known character. This controller may be used to implement the stan-
dard AT command set or other characters.
In order to use the autobaud mode, the serial communication controller (SCC) is initially pro-
grammed to BISYNC mode. The SCC receiver then synchronizes on the falling edge of the
START bit. Once a START bit is detected, each bit received is processed by the autobaud
controller. The autobaud controller measures the length of the START bit to determine the
receive baud rate and compares the length to values in a user supplied lookup table. After
the baud rate is determined, the autobaud controller assembles the character and compares
it against two user-defined characters. If a match is detected, the autobaud controller inter-
rupts the host and returns the determined nominal start value from the lookup table. The
autobaud controller continues to assemble the characters and interrupt the host until the
host stops the reception process. The incoming message should contain a mixture of even
and odd characters so that the user has enough information to decide on the proper char-
acter format (length and parity). The host then uses the returned nominal start value from
the lookup table, modifies the SCC configuration register (SCON) to generate the correct
baud rate, and reprograms the SCC to UART mode.
Many rates are supported including: 150, 300, 600, 1200, 2400, 4800, 9600, 14.4K, 19.2K,
38.4K, 57.6K, 64K, 96K, 115.2K and 230K. To estimate the performance of the autobaud
mode, the performance table in Appendix A can be used. The maximum full-duplex rate for
a BISYNC channel is one-tenth of the system clock rate. So a 25 MHz IMP can support 230K
autobaud rate with another low-speed channel (<50 kbps) and a 20 MHz IMP can support
115.2K autobaud rate with 2 low-speed channels. The performance can vary depending on
system loading, configuration, and echoing mode.
It is important that the highest priority SCC be used for the autobaud function, since it is run-
ning at a very high rate. Any SCC that is guaranteed to be idle during the search operation
of the autobaud process will not impact the performance of autobaud in an application. Idle
is defined as not having any transmit or receive requests to/from the SCC FIFOs.
4.3.9.1 AUTOBAUD CHANNEL RECEPTION PROCESS. The interface between the auto-
baud controller and the host processor is implemented with shared data structures in the
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MC68LC302 REFERENCE MANUAL
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