Texas Instruments MSP430x1xx - Manual (Page 228)

Interrupt and Enable Functions
12-12
12.4.2 USART Transmit Enable Bit
The transmit enable bit UTXE, shown in Figure 12–13, enables or disables a
character transmission on the serial-data line. If this bit is reset, the transmitter
is disabled but any active transmission does not halt until the data in the
transmit shift register and the transmit buffer are transmitted. Data written to
the transmit buffer before UTXE has been reset may be modified or
overwritten—even after UTXE is reset—until it is shifted to the transmit shift
register. For example, if software writes a byte to the transmit buffer and then
resets UTXE, the byte written to the transmit buffer will be transmitted and may
be modified or overwritten until it is transferred into the transmit shift register.
However, after the byte is transferred to the transmit shift register, any
subsequent writes to UTXBUF while UTXE is reset will not result in
transmission, but UTXBUF will be updated with the new value.
Figure 12–13. State Diagram of Transmitter Enable
Idle State
(Transmitter
Enabled)
Transmit
Disable
Transmission
Active
UTXE = 0
No Data Written
to Transmit Buffer
Not Completed
UTXE = 1
UTXE = 0
UTXE = 1
Data Written to
Transmit Buffer Handle Interrupt
Conditions
Character
Transmitted
UTXE = 1
UTXE = 0 And Last Buffer
Entry Is Transmitted
When UTXE is reset and the current transmission is completed, new data
written to the transmit buffer will not be transmitted. Once the UTXE bit is set,
the data in the transmit buffer are immediately loaded into the transmit shift
register and character transmission is started.
Note: Writing to UTXBUF, UART Mode
Data should never be written to transmit buffer UTXBUF when the buffer is
not ready and when the transmitter is enabled (UTXE is set). Otherwise, the
transmission will have errors.
Note: Write to UTXBUF/Reset of Transmitter, UART Mode
Disabling the transmitter should be done only if all data to be transmitted has
been moved to the transmit shift register.
MOV.B #....,&UTXBUF
BIC.B #UTXE,&ME2 ; If BITCLK < MCLK then the
; transmitter might be stopped
; before the buffer is loaded
; into the transmitter shift
; register