Motorola DSP56301 - Manual (Page 258)

SCI Programming Model
8 -22 DSP56301 User’s Manual
8.6.4 SCI Data Registers
The SCI data registers are divided into two groups: receive and transmit, as shown in Figure
8-7. There are two receive registers: a Receive Data Register (SRX) and a serial-to-parallel
Receive Shift Register. There are also two transmit registers: a Transmit Data Register (called
either STX or STXA) and a parallel-to-serial Transmit Shift Register.
8.6.4.1 SCI Receive Register (SRX)
Data bits received on the RXD signal are shifted into the SCI receive shift register. When a
complete word is received, the data portion of the word is transferred to the byte-wide SRX.
This process converts serial data to parallel data and provides double buffering. Double
buffering promotes flexibility 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
Figure 8-7. SCI Programming Model—Data Registers
SRX
SRX
SRX
RXD SCI Receive Data Shift Register
Note: SRX is the same register decoded at three different addresses.
STX
STX
STX
TXD SCI Transmit Data Shift Register
Note: Bytes are masked on the fly.
1. STX is the same register decoded at four different addresses.
STXA
(a) Receive Data Register
(b) Transmit Data Register
SCI Receive Data Register High (Read Only)
SCI Receive Data Register Middle (Read Only)
SCI Receive Data Register Low (Read Only)
0 7 8 15 16 23
0 7 8 15 16 23
0 7 8 15 16 23
SCI Transmit Data Register High (Write Only)
SCI Transmit Data Register Middle (Write Only)
SCI Transmit Data Register Low (Write Only)
SCI Transmit Data Address Register (Write Only)