Motorola DSP56309 User Manual page 237

Serial Communication Interface (SCI)
Operating Modes
8.4.4 Preamble, Break, and Data Transmission Priority
Two or three transmission commands can be set simultaneously:
1. A preamble (TE is set.)
2. A break (SBK is set or is cleared.)
3. There is data for transmission (TDRE is cleared.)
After the current character transmission, if two or more of these commands are set, the
transmitter executes them in the following order:
1. Preamble
2. Break
3. Data
8.4.5 SCI Exceptions
The SCI can cause five different exceptions in the DSP. These exceptions are as follows
(ordered from the highest to the lowest priority):
1. SCI receive data with exception status is caused by receive data register full with
a receiver error (parity, framing, or overrun error). Clearing the pending interrupt
is done by reading the SCI status register, followed by a read of SRX. A long
interrupt service routine should be used to handle the error condition. This
interrupt is enabled by SCR bit 16 (REIE).
2. SCI receive data is caused by receive data register full. Reading SRX clears the
pending interrupt. This error-free interrupt can use a fast interrupt service routine
for minimum overhead. This interrupt is enabled by SCR bit 11 (RIE).
3. SCI transmit data is caused by transmit data register empty. Writing STX clears
the pending interrupt. This error-free interrupt can use a fast interrupt service
routine for minimum overhead. This interrupt is enabled by SCR bit 12 (TIE).
4. SCI idle line is caused by the receive line entering the idle state (10 or 11 bits of
1s). This interrupt is latched and then automatically reset when the interrupt is
accepted. This interrupt is enabled by SCR bit 10 (ILIE).
5. SCI timer is caused by the baud rate counter reaching zero. This interrupt is
automatically reset when the interrupt is accepted. This interrupt is enabled by
SCR bit 13 (TMIE).
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