Motorola DSP56301 user manual SCI Transmit Register STX

SCI Programming Model

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.

In Synchronous mode, the start bit, the eight data bits, the address/data indicator bit or the parity bit, and the stop bit are received, respectively. Data bits are sent LSB first if SSFTD is cleared, and MSB first if SSFTD is set. In Synchronous mode, a gated clock provides synchronization. In either Synchronous or Asynchronous mode, when a complete word is clocked in, the contents of the shift register can be transferred to the SRX and the flags; RDRF, FE, PE, and OR are changed appropriately. Because the operation of the receive shift register is transparent to the DSP, the contents of this register are not directly accessible to the programmer.

8.6.4.2 SCI Transmit Register (STX)

The transmit data register is a one-byte-wide register mapped into four addresses as STXL, STXM, STXH, and STXA. In Asynchronous mode, when data is to be transmitted, STXL, STXM, and STXH are used. When STXL is written, the low byte on the data bus is transferred to the STX. When STXM is written, the middle byte is transferred to the STX. When STXH is written, the high byte is transferred to the STX. This structure makes it easy for the programmer to unpack the bytes in a 24-bit word for transmission. TDXA should be written in 11-bit asynchronous multidrop mode when the data is an address and the programmer wants to set the ninth bit (the address bit). When STXA is written, the data from the low byte on the data bus is stored in it. The address data bit is cleared in 11-bit asynchronous multidrop mode when any of STXL, STXM, or STXH is written. When either STX (STXL, STXM, or STXH) or STXA is written, TDRE is cleared.

The transfer from either STX or STXA to the transmit shift register occurs automatically, but not immediately, after the last bit from the previous word is shifted out; that is, the transmit shift register is empty. Like the receiver, the transmitter is double-buffered. However, a delay of two to four serial clock cycles occurs between when the data is transferred from either STX or STXA to the transmit shift register and when the first bit appears on the TXD signal. (A serial clock cycle is the time required to transmit one data bit.)

The transmit shift register is not directly addressable, and there is no dedicated flag for this register. Because of this fact and the two- to four-cycle delay, two bytes cannot be written consecutively to STX or STXA without polling, because the second byte might overwrite the first byte. Thus, you should always poll the TDRE flag prior to writing STX or STXA to

Serial Communication Interface (SCI) 8-23