UCAxRXD
URXS
Majority Vote Taken
t
t
t
t
UCAxRXD
URXS
eUSCI_A Operation – UART Mode
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SLAU367P – October 2012 – Revised April 2020
Copyright © 2012–2020, Texas Instruments Incorporated
Enhanced Universal Serial Communication Interface (eUSCI) – UART Mode
30.3.7 eUSCI_A Receive Enable
The eUSCI_A module is enabled by clearing the UCSWRST bit and the receiver is ready and in an idle
state. The receive baud rate generator is in a ready state but is not clocked nor producing any clocks.
The falling edge of the start bit enables the baud rate generator and the UART state machine checks for a
valid start bit. If no valid start bit is detected the UART state machine returns to its idle state and the baud
rate generator is turned off again. If a valid start bit is detected, a character is received.
When the idle-line multiprocessor mode is selected with UCMODEx = 01, the UART state machine checks
for an idle line after receiving a character. If a start bit is detected, another character is received.
Otherwise, the UCIDLE flag is set after 10 ones are received, the UART state machine returns to its idle
state, and the baud rate generator is turned off.
30.3.7.1 Receive Data Glitch Suppression
Glitch suppression prevents the eUSCI_A from being accidentally started. Any glitch on UCAxRXD shorter
than the deglitch time t
t
is ignored by the eUSCI_A, and further action is initiated as shown in
(see the device-specific data sheet for parameters). The deglitch time t
t
can be set to four different values
using the UCGLITx bits.
Figure 30-8. Glitch Suppression, eUSCI_A Receive Not Started
When a glitch is longer than t
t,
or a valid start bit occurs on UCAxRXD, the eUSCI_A receive operation is
started and a majority vote is taken (see
). If the majority vote fails to detect a start bit, the
eUSCI_A halts character reception.
Figure 30-9. Glitch Suppression, eUSCI_A Activated
30.3.8 eUSCI_A Transmit Enable
The eUSCI_A module is enabled by clearing the UCSWRST bit and the transmitter is ready and in an idle
state. The transmit baud-rate generator is ready but is not clocked nor producing any clocks.
A transmission is initiated by writing data to UCAxTXBUF. When this occurs, the baud-rate generator is
enabled, and the data in UCAxTXBUF is moved to the transmit shift register on the next BITCLK after the
transmit shift register is empty. UCTXIFG is set when new data can be written into UCAxTXBUF.
Transmission continues as long as new data is available in UCAxTXBUF at the end of the previous byte
transmission. If new data is not in UCAxTXBUF when the previous byte has transmitted, the transmitter
returns to its idle state and the baud-rate generator is turned off.