MC68LC302 Datasheet, PDF (77/169 Pages) Motorola, Inc – Low Power Integrated Multiprotocol Processor Reference Manual

Communications Processor (CP)
SCC parameter RAM and in external memory and through the use of a special command to
the SCC.
The autobaud controller uses receive buffer descriptor number 7 (Rx BD7) for the autobaud
command descriptor. This Rx BD is initialized by the host to contain a pointer to a lookup
table residing in the external RAM (contains the maximum and nominal START bit length for
each baud rate). The host also prepares two characters against which the autobaud control-
ler will compare the received character (usually these characters are ‘a’ and ‘A’) and the host
initializes a pointer to a buffer in external memory where the assembled characters will be
stored until the host stops the autobaud process. Finally, the host initializes the SCC data
synchronization register (DSR) to $7FFF in order to synchronize on the falling edge of the
START bit.
Once the data structures are initialized, the host programs the SCON register to provide a
sampling clock that is 16X the maximum supported baud rate. The host then issues the
Enter_Baud_Hunt command and enables the SCC in the BISYNC mode.
The autobaud controller reception process begins when the START bit arrives. The auto-
baud controller then begins to measure the START bit length. With each byte received from
the SCC that “belongs” to the START bit, the autobaud controller increments the start length
counter and compares it to the current lookup table entry. If the start length counter passed
the maximum bit length defined by the current table entry, the autobaud controller switches
to the next lookup table entry (the next slower baud rate). This process goes on until the
autobaud controller recognizes the end of the START bit. Then, the autobaud controller
starts the character assembly process.
The character assembly process uses the nominal bit length, taken from the current lookup
table entry, to sample each incoming bit in it’s center. Each bit received is stored to form an
8-bit character. When the assembly process is completed (a STOP bit is received), the char-
acter is compared against two user-defined characters.
If the received character does not match any of the two user defined characters, the auto-
baud controller re-enters the Enter_Baud_Hunt process. The host is not notified until a
match is encountered.
If a match is found, the character is written to the received control character register (RCCR)
with the corresponding status bit set in Rx BD7. The channel will generate the control char-
acter received (CCR) interrupt (bit 3 in the SCCE), if enabled. If the character matched, but
a framing error was detected on the STOP bit, the autobaud controller will also set the fram-
ing error status bit in Rx BD7.
The autobaud controller then continues to assemble the incoming characters and to store
them in the external data buffer. The host receives a CCR interrupt after each character is
received.The host is responsible for determining the end of the incoming message (for
example, a carriage return), stopping the autobaud process, and reprogramming the SCC
to UART mode. The autobaud controller returns the nominal START bit length value for the
detected baud rate from the lookup table and a pointer to the last character received that
was written to the external data buffer. The host must be able to handle each character inter-
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MC68LC302 REFERENCE MANUAL
MOTOROLA