Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 807

29.3.7 Sending a Break
A break is an all-zeros character without stop bits. It is sent by issuing a
sending any outstanding data, the SMC sends a character of consecutive zeros, the number of which is the
sum of the character length, plus the number of start, parity, and stop bits. The SMC sends a programmable
number of break characters according to BRKCR and then reverts to idle or sends data if a
is issued before completion. When the break completes, the transmitter sends at least one idle
TRANSMIT
character before sending any data to guarantee recognition of a valid start bit.
29.3.8 Sending a Preamble
A preamble sequence provides a way to ensure that the line is idle before a new message transfer begins.
The length of the preamble sequence is constructed of consecutive ones that are one character long. If the
preamble bit in a BD is set, the SMC sends a preamble sequence before sending that buffer. For 8 data bits,
no parity, 1 stop bit, and 1 start bit, a preamble of 10 ones would be sent before the first character in the
buffer. If no preamble sequence is sent, data from two ready transmit buffers can be sent on the transmit
pin with no delay between them.
29.3.9 Handling Errors in the SMC UART Controller
The SMC UART controller reports character reception errors via the channel RxBD status fields and the
SMC event register (SMCE).
controller has no transmission errors.
Error
Overrun The SMC maintains a two-character length FIFO for receiving data. Data is moved to the buffer after
the first character is received into the FIFO; if a receiver FIFO overrun occurs, the channel writes
the received character into the internal FIFO. It then writes the character to the buffer, closes it, sets
RxBD[OV], and generates the RX interrupt if it is enabled. Reception then resumes as normal.
Overrun errors that occasionally occur when the line is idle can be ignored.
Parity The channel writes the received character to the buffer, closes it, sets the PR bit in the BD, and
generates the RX interrupt if it is enabled. Reception then resumes as normal.
Idle Sequence An idle is found when a character of all ones is received, at which point the channel counts
Receive consecutive idle characters. If the count reaches MAX_IDL, the buffer is closed and an RX interrupt
is generated. If no receive buffer is open, this does not generate an interrupt or any status
information. The idle counter is reset each time a character is received.
Framing The SMC received a character with no stop bit. When it occurs, the channel writes the received
character to the buffer, closes the buffer, sets FR in the BD, and generates the RX interrupt if it is
enabled. When this error occurs, parity is not checked for the character.
Break Sequence The SMC receiver received an all-zero character with a framing error. The channel increments
BRKEC, generates a maskable BRK interrupt in SMCE, measures the length of the break sequence,
and stores this value in BRKLN. If the channel was processing a buffer when the break was
received, the buffer is closed with the BR bit in the RxBD set. The RX interrupt is generated if it is
enabled.
Freescale Semiconductor
Table 29-7 shows the possible UART receiving errors. The SMC UART
Table 29-7. SMC UART Errors
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Serial Management Controllers (SMCs)
STOP TRANSMIT
Description
command. After
RESTART
29-13