Motorola DSP56301 - Manual (Page 239)

I/O Signals
Serial Communication Interface (SCI) 8 -3
message and optionally transmit an acknowledgment to the sender. The particular message
format and protocol used are determined by the user’s software.
8.1.3.1 Transmitting Data and Address Characters
To send data, the 8-bit data character must be written to the STX register. Writing the data
character to the STX register sets the ninth bit in the frame to zero, which indicates that this
frame contains data. To send an 8-bit address, the address data is written to the STXA
register, and the ninth bit in the frame is set to one, indicating that this frame contains an
address.
8.1.3.2 Wired-OR Mode
Building a multidrop bus network requires connecting multiple transmitters to a common
wire. The Wired-OR mode allows this to be done without damaging the transmitters when the
transmitters are not in use. A protocol is still needed to prevent two transmitters from
simultaneously driving the bus. The SCI multidrop word format provides an address field to
support this protocol.
8.1.3.3 Idle Line Wakeup
A wakeup mode frees a DSP from reading messages intended for other processors. The usual
operational procedure is for each DSP to suspend SCI reception (the DSP can continue
processing) until the beginning of a message. Each DSP compares the address in the message
header with the DSP’s address. If the addresses do not match, the SCI again suspends
reception until the next address. If the address matches, the DSP reads and processes the
message and then suspends reception until the next address. The Idle Line Wakeup mode
wakes up the SCI to read a message before the first character arrives.
8.1.3.4 Address Mode Wakeup
The purpose and basic operational procedure for Address Mode Wakeup is the same as for
Idle Line Wakeup. The difference is that Address Mode Wakeup re-enables the SCI when the
ninth bit in a character is set to one (if cleared, this bit marks a character as data; if set, an
address). As a result, an idle line is not needed, which eliminates the dead time between
messages.
8.2 I/O Signals
Each of the three SCI signals ( RXD , TXD , and SCLK ) can be configured as either a GPIO
signal or as a specific SCI signal. Each signal is independent of the others. For example, if
only the
TXD signal is needed, the RXD and SCLK signals can be programmed for GPIO.