Texas Instruments MSP430x1xx - Manual (Page 230)

Interrupt and Enable Functions
12-14
12.4.4 USART Transmit Interrupt Operation
In the transmit interrupt operation, shown in Figure 12–15, the transmit
interrupt flag UTXIFG is set by the transmitter to indicate that the transmitter
buffer UTXBUF is ready to accept another character. This bit is automatically
reset if the interrupt request service is started or a character is written into the
UTXBUF. This flag asserts a transmitter interrupt if the local (UTXIE) and
general interrupt enable (GIE) bits are set. The UTXIFG is set after a system
reset PUC signal, or removal of a SWRST.
Figure 12–15. Transmit Interrupt Operation
Clear
UTXIE
Clear
D
Character Moved From
Buffer to Shift Register
Request_
Interrupt_Service
SWRST
URXBUF Written Into Transmit Shift Register
Q
UTXIFG
IRQA
VCC
PUC or SWRST
Q
Set
The transmit interrupt enable UTXIE bit controls the ability of the UTXIFG to
request an interrupt, but does not prevent the flag UTXIFG from being set. The
UTXIE is reset with a PUC signal or a software reset (SWRST) bit. The
UTXIFG bit is set after a system reset PUC signal or software reset (SWRST),
but the UTXIE bit is reset to ensure full interrupt-control capability.