Texas Instruments TMS320C3x - Manual (Page 411)

Serial Ports
12-32
The transmit ready (XRDY) signal specifies that the data-transmit register
(DXR) is available to be loaded with new data. XRDY goes active as soon as
the data is loaded into the transmit-shift register (XSR). The last word may still
be shifting out when XRDY goes active. If DXR is loaded before the last word
has completed transmission, the data bits transmitted are consecutive; that is,
the LSB of the first word immediately precedes the MSB of the second, with
all signaling valid as in two separate transmits. XRDY goes inactive when DXR
is loaded and remains inactive until the data is loaded into the shifter.
The receive ready (RRDY) signal is active as long as a new word of data is
loaded into the data-receive register and has not been read. As soon as the
data is read, the RRDY bit is turned off.
When FSX is specified as an output, the activity of the signal is determined by
the internal state of the serial port. If a fixed data rate is specified, FSX goes
active when DXR is loaded into XSR. One serial-clock cycle later FSX turns
inactive and data transmission begins. If a variable data rate is specified, the
FSX pin is activated when the data transmission begins and remains active
during the entire transmission of the word. Again, the data is transmitted one
clock cycle after it is loaded into the data-transmit register.
An input FSX in the fixed data-rate mode must go active for at least one serial-
clock cycle and then inactive to initiate the data transfer. The transmitter then
sends the number of bits specified by the XLEN (bit field 19, serial-port global-
control register) bits. In the variable data-rate mode, the transmitter begins
sending from the time FSX goes active until the number of specified bits have
been shifted out. In the variable data-rate mode, when the FSX status changes
prior to all the data bits being shifted out, the transmission completes, and the
DX pin is placed in a high-impedance state. An FSR input is exactly comple-
mentary to the FSX.
When using an external FSX, if DXR and XSR are empty, a write to DXR results
in a DXR-to-XSR transfer. This data is held in the XSR until an FSX occurs.
When the external FSX is received, the XSR begins shifting the data. If XSR is
waiting for the external FSX, a write to DXR changes DXR, but a DXR-to-XSR
transfer does not occur. XSR begins shifting when the external FSX is received,
or when it is reset using XRESET.