MC68LC302 Datasheet, PDF (80/169 Pages) Motorola, Inc – Low Power Integrated Multiprotocol Processor Reference Manual

Communications Processor (CP)
The RxPTR is a 2 word location that contains a 32-bit pointer to a buffer in external memory
used for assembling the received characters and must be initialized before the
Enter_Baud_Hunt command is issued.
NOTE
Since a length for this external buffer is not given, the user must
provide enough space in memory for characters to be assem-
bled and written until the autobaud process is to avoid overwrit-
ing other data in memory. This location is not used as the
CHARACTER7 value in the control character table until the
channel operates in normal UART mode. After reception begins
in normal UART mode (i.e. the “a” or “A” is found), this entry is
available again as a control character table entry.
The TxBD entry is used as the transmit character descriptor for smart echo or software
transmit. This location is not used as the CHARACTER8 value in the control character table
until the channel operates in normal UART mode. After reception begins in normal UART
mode (i.e. the “a” or “A” is found), this entry is available again as a control character table
entry.
4.3.9.4 AUTOBAUD PROGRAMMING MODEL. The following sections describe the details
of initializing the autobaud microcode, preparing for the autobaud process, and the memory
structures used.
4.3.9.4.1 Preparing for the Autobaud Process. The host begins preparation for the auto-
baud process with the following steps. Steps 1 and 2 are required if the SCC has been used
after reset or after UART mode in order to re-enable the process.
1. Disable the SCC by clearing the ENR and ENT bits. (The host may wish to precede
this action with the STOP_TRANSMIT commands to abort transmission in an orderly
way).
2. Issue the ENTER_HUNT_MODE command to the SCC (This ensures that an open
buffer descriptor is closed).
3. Set up all the autobaud parameters in the autobaud specific parameter RAM shown in
Table 4-3, the autobaud command descriptor shown in Table 4-4, and the lookup table
shown in Table 4-4. Of these three areas, the autobaud controller only modifies the
autobaud specific parameter RAM and the first word of the autobaud command de-
scriptor during its operation.
4. Write the SCON to configure the SCC to use the baud rate generator clock of 16x the
maximum supported baud rate. A typical value is $4000 assuming a 1.8432 MHz clock
rate on TIN1 and a maximum baud rate of 115.2K, but this can change depending on
the maximum baud rate and the EXTAL frequency.
5. Write the DSR of the SCC with the value $7FFF in order to detect the START bit.
6. The host initiates the autobaud search process by issuing the Enter_Baud_Hunt com-
mand
7. Write the SCM of the SCC with $1133 to configure it for BISYNC mode, with the REVD
MOTOROLA
MC68LC302 REFERENCE MANUAL
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