Motorola MPC860 PowerQUICC User Manual page 803

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
before completion. When the break completes, the transmitter sends at least one idle
character before sending any data to guarantee recognition of a valid start bit.
30.3.8 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 Þrst 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.
30.3.9 Handling Errors in the SMC UART Controller
The SMC UART controller reports character reception errors via the channel RxBD status
Þelds and the SMC event register (SMCE). Table 30-7 shows the possible UART receiving
errors. The SMC UART 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
Þrst 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 An idle is found when a character of all ones is received, at which point the channel counts consecutive
Sequence idle characters. If the count reaches MAX_IDL, the buffer is closed and an RX interrupt is generated. If
Receive 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 The SMC receiver received an all-zero character with a framing error. The channel increments BRKEC,
Sequence 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.
MOTOROLA
Table 30-7. SMC UART Errors
Description
Chapter 30. Serial Management Controllers
Part V. The Communications Processor Module
RESTART TRANSMIT
is issued
30-13