Figure 8-7 Sci Programming Model Data Registers - Motorola DSP56309 User Manual

23 16
SRX
RXD
Note: 1. SRX is the same register decoded at three different addresses.
23 16
STX
SCI Transmit Data Shift Register
23 16
Note: 1. Bytes are masked on the fly.
2. STX is the same register decoded at four different addresses.

Figure 8-7 SCI Programming Model Data Registers

8.3.4.1 SCI Receive Registers (SRX)
Data bits received on the RXD signal are shifted into the SCI receive shift register. When
a complete word has been received, the data portion of the word is transferred to the
byte-wide SRX. This process converts the serial data to parallel data and provides
double-buffering. Double-buffering provides flexibility to the programmer and
increased throughput since the programmer can save (and process) the previous word
while the current word is being received.
The SRX can be read at three locations as SRXL, SRXM, and SRXH. When SRXL is read,
the contents of the SRX are placed in the lower byte of the data bus and the remaining
bits on the data bus are read as 0s. Similarly, when SRXM is read, the contents of SRX are
placed in the middle byte of the bus, and when SRXH is read, the contents of SRX are
placed in the high byte with the remaining bits read as 0s. Mapping SRX as described
allows three bytes to be efficiently packed into one 24-bit word by ORing three data
bytes read from the three addresses.
MOTOROLA
15 8 7
SRX
SCI Receive Data Shift Register
(a) Receive Data Register
15 8 7
STX
15 8 7
(b) Transmit Data Register
DSP56309UM/D
Serial Communication Interface (SCI)
0
SCI Receive Data Register High (Read Only)
SCI Receive Data Register Middle (Read Only)
SRX
SCI Receive Data Register Low (Read Only)
0
SCI Transmit Data Register High (Write Only)
SCI Transmit Data Register Middle (Write Only)
STX
SCI Transmit Data Register Low (Write Only)
TXD
0
SCI Transmit Data Address Register (Write Only)
STXA
SCI Programming Model
AA0694
8-19