Texas Instruments TMS320C3x - Manual (Page 412)

Serial Ports
12-33 Peripherals
12.2.10.1 Continuous Transmit and Receive Modes
When you choose continuous mode, consecutive writes do not generate or
expect new sync pulse signaling. Only the first word of a block begins with
an active synchronization. Thereafter, data is transmitted as long as new
data is loaded into DXR before the last word has been transmitted. As soon
as TXRDY is active and all of the data has been transmitted out of the shift
register, the DX pin is placed in a high-impedance state, and a subsequent
write to DXR initiates a new block and a new FSX.
Similarly with FSR, the receiver continues shifting in new data and loading
DRR. If the data-receive buffer is not read before the next word is shifted in,
you will lose subsequent incoming data. You can use the RFSM bit to terminate
the receive-continuous mode.
12.2.10.2 Handshake Mode
The handshake mode (HS = 1) allows for direct connection between processors.
In this mode, all data words are transmitted with a leading 1 (see Figure 12–23).
For example, in order to transmit an 8-bit word, the first bit sent is a 1, followed
by the 8-bit data word.
Once the serial port transmits a word in this mode, it does not transmit another
word until it receives a separately transmitted 0 bit. Therefore, the 1 bit that
precedes every data word is a request bit.
Figure 12–23. Data Word Format in Handshake Mode
Data word (8 bits)
DX
Leading 1
1
After a serial port receives a word that has been read from the DRR (with the
leading 1), the receiving serial port sends a single 0 to the transmitting serial
port. The single 0 bit acts as an acknowledge bit (see Figure 12–24). This
single acknowledge bit is sent every time the DRR is read, even if the DRR
does not contain new data.
Figure 12–24. Single 0 Sent as an Acknowledge Bit
Single 0
0 DX