DSP56303 datasheet(188/292 Pages) FREESCALE

188 / 292 page

DSP56303 User’s Manual, Rev. 2

8-20

Freescale Semiconductor

Serial Communication Interface (SCI)

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 the

data bus are read as zeros. Similarly, when SRXM is read, the contents of SRX are placed into the

middle byte of the bus, and when SRXH is read, the contents of SRX are placed into the high

byte with the remaining bits are read as 0s. This way of mapping SRX efficiently packs three

bytes into one 24-bit word by ORing three data bytes read from the three addresses.

The SCR WDS0, WDS1, and WDS2 control bits define the length and format of the serial word.

The SCR receive clock mode (RCM) defines the clock source.

Figure 8-7. SCI Programming Model—Data Registers

SRX

RXD

SCI Receive Data Shift Register

Note: SRX is the same register decoded at three different addresses.

STX

TXD

SCI Transmit Data Shift Register

Note: Bytes are masked on the fly.

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)

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)

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