Enhanced Serial Communications Interface (ESCI) Module
Data Sheet
MC68HC908GR60A MC68HC908GR48A MC68HC908GR32A
174
Enhanced Serial Communications Interface (ESCI) Module
MOTOROLA
13.4.2.3 Break Characters
Writing a 1 to the send break bit, SBK, in SCC2 loads the transmit shift register with
a break character. For TXINV = 0 (output not inverted), a transmitted break
character contains all 0s and has no start, stop, or parity bit. Break character length
depends on the M bit in SCC1 and the LINR bits in SCBR. As long as SBK is at 1,
transmitter logic continuously loads break characters into the transmit shift register.
After software clears the SBK bit, the shift register finishes transmitting the last
break character and then transmits at least one 1. The automatic 1 at the end of a
break character guarantees the recognition of the start bit of the next character.
When LINR is cleared in SCBR, the ESCI recognizes a break character when a
start bit is followed by eight or nine 0 data bits and a 0 where the stop bit should
be, resulting in a total of 10 or 11 consecutive 0 data bits. When LINR is set in
SCBR, the ESCI recognizes a break character when a start bit is followed by
9 or 10 0 data bits and a 0 where the stop bit should be, resulting in a total of 11 or
12 consecutive 0 data bits.
Receiving a break character has these effects on ESCI registers:
Sets the framing error bit (FE) in SCS1
Sets the ESCI receiver full bit (SCRF) in SCS1
Clears the ESCI data register (SCDR)
Clears the R8 bit in SCC3
Sets the break flag bit (BKF) in SCS2
May set the overrun (OR), noise flag (NF), parity error (PE),
or reception in progress flag (RPF) bits
13.4.2.4 Idle Characters
For TXINV = 0 (output not inverted), a transmitted idle character contains all 1s and
has no start, stop, or parity bit. Idle character length depends on the M bit in SCC1.
The preamble is a synchronizing idle character that begins every transmission.
If the TE bit is cleared during a transmission, the TxD pin becomes idle after
completion of the transmission in progress. Clearing and then setting the TE bit
during a transmission queues an idle character to be sent after the character
currently being transmitted.
NOTE:
When a break sequence is followed immediately by an idle character, this SCI
design exhibits a condition in which the break character length is reduced by one
half bit time. In this instance, the break sequence will consist of a valid start bit,
eight or nine data bits (as defined by the M bit in SCC1) of 0 and one half data bit
length of 0 in the stop bit position followed immediately by the idle character. To
ensure a break character of the proper length is transmitted, always queue up a
byte of data to be transmitted while the final break sequence is in progress.