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M0A21/M0A23 Series
May 06, 2022
Page
489
of 746
Rev 1.02
M0
A21
/M
0
A
2
3
SE
RIES
TEC
H
NICAL
RE
FEREN
C
E
M
ANUAL
Data Write Access
(USCI_TXDAT)
Data Read Access
(USCI_RXDAT)
TX_BUF
P
e
ri
p
h
e
ra
l
D
e
v
ic
e
U
s
e
r
in
te
rf
a
c
e
RX_BUF0
RX_BUF1
Figure 6.12-5 Data Access Structure
Transmit Data Path
The transmit data path is based on 16-bit wide transmit shift register (TX_SFTR) and transmit buffer
TX_BUF. The data transfer parameters like data word length is controlled commonly for transmission
and reception by the line control register USCI_LINECTL.
Transmit Buffering
The transmit shift register cannot be directly accessed by user. It is updated automatically with the value
stored in the transmit buffer (TX_BUF) if a currently transmitted data is finished and new data is valid
for transmission.
Shift Control
& Status
Control
TX_SFTR
Data
Serial Bus
Clock Input
Control Input
Shift Data
Output
TX_BUF
16
Transmit Buffer
Status
USCI_LINECTL
USCI_BUFSTS
USCI_TXDAT
Figure 6.12-6 Transmit Data Path
Transmit Data Validation
The status of TXEMPTY (USCI_BUFSTS[8]) indicates the transmission data is valid or not in the
transmit buffer (TX_BUF) and the TXSTIF (USCI_PROTSTS[1]) labels the start conditions for each data.
If the USCI controller is a Master, the data transfer can only be started with valid data in
the transmit buffer (TX_BUF). In this case, the transmit shift register is loaded with the
content of transmit buffer.
Note:
Master defines the start of data transfer.
If the USCI controller is a Slave, a data transfer requested by Master and it has to be
started independently of the status in transmit buffer (TX_BUF). If a data transfer is
requested and started by the Master, the transmit shift register is loaded from specific
protocol control signal if it is valid for transmission.