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I/O Signals

DSP56303 User’s Manual, Rev. 2

Freescale Semiconductor

8-3

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. However, at least one

of the three signals must be selected as an SCI signal to release the SCI from reset.

To enable SCI interrupts, program the SCI control registers before any of the SCI signals are

programmed as SCI functions. In this case, only one transmit interrupt can be generated because

the Transmit Data Register is empty. The timer and timer interrupt operate regardless of how the

SCI pins are configured, either as SCI or GPIO.

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