Motorola DSP56309 User Manual page 215

Serial Communication Interface (SCI)
SCI Programming Model
8.2.1 Receive Data (RXD)
This input signal receives byte-oriented serial data and transfers the data to the SCI
receive shift register. Asynchronous input data is sampled on the positive edge of the
receive clock (1 ´ SCLK) if SCKP is cleared. RXD can be configured as a GPIO signal
(PE0) when the SCI RXD function is not being used.
8.2.2 Transmit Data (TXD)
This output signal transmits serial data from the SCI transmit shift register. Data
changes on the negative edge of the asynchronous transmit clock (SCLK) if SCKP is
cleared. This output is stable on the positive edge of the transmit clock. TXD can be
programmed as a GPIO signal (PE1) when the SCI TXD function is not being used.
8.2.3 SCI Serial Clock (SCLK)
This bidirectional signal provides an input or output clock from which the transmit
and/or receive baud rate is derived in asynchronous mode and from which data is
transferred in synchronous mode. SCLK can be programmed as a GPIO signal (PE2)
when the SCI SCLK function is not being used. This signal can be programmed as PE2
when data is being transmitted on TXD, since the clock does not need to be transmitted
in asynchronous mode. Because SCLK is independent of SCI data I/O, there is no
connection between programming the PE2 signal as SCLK and data coming out the TXD
signal.
8.3 SCI PROGRAMMING MODEL
The SCI programming model can be viewed as three types of registers:
¥ Control
Ð SCI control register (SCR) in Figure 8-1
Ð SCI clock control register (SCCR) in Figure 8-3
¥ Status
Ð SCI status register (SSR) in Figure 8-2
¥ Data transfer
8-4 DSP56309UM/D MOTOROLA