MC68LC302 Datasheet, PDF (86/169 Pages) Motorola, Inc – Low Power Integrated Multiprotocol Processor Reference Manual

Communications Processor (CP)
4.3.9.8.1 Automatic Echo. This method uses the SCC hardware to automatically echo the
characters back on the TxD pin. The automatic echo is enabled by setting the DIAG bits in
the SCM to ‘10’. The transmitter should not be enabled. The hardware echo is done auto-
matically. The CD pin needs to be asserted in order for the characters to be transmitted
back. On SCC1, the external CD pin must be tied low. On SCC2 and SCC3, either the exter-
nal CD pin must be tied low or the CD pins should be left configured as general purpose
input pins (the CD signal to the SCC is then connected to ground internally).
Using the automatic echo, the receiver still autobauds correctly and performance is not
affected. The SCC echoes the received data with a few nanoseconds delay.
4.3.9.8.2 Smart Echo. This method requires addition hardware and software to implement.
The user must provide two clock sources. One clock source is the sample clock which is
input on RCLK and cannot be divided down. The BRG is used to divide the second clock
down to provide the clock used for transmit. The second clock can be either the system clock
or a clock connected to TIN1. The TIN1 and RCLK pins can be connected to each other
externally.
After the first character is received, the user must take the following steps:
1. Determine the baud rate from the returned NOM_START value and program SCON
to (input frequency/baud rate)-1, where the input frequency is either the system clock
or the clock on TIN1.
2. Program the DSR to $FFFF. The DSR will need to be programmed back to $7FFF be-
fore the Enter_Baud_Hunt command is issued again.
3. Set the ENT bit in the mode register.
4. Program the transmit character BD as show in Table 4-8.
15 14 13 12 11 10 9
8
76543210
R
CL
PE PM
CHAR
Table 4-8. Transmit Character BD
R (ready bit)
0 = Character is not ready
1 = Character is ready to transmit
CL (character len)
0 = 7 bits + parity or 8 bits with no parity
1 = 8 bits + parity
PE (parity enable)
0 = No parity
1 = Parity
MOTOROLA
MC68LC302 REFERENCE MANUAL
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