Texas Instruments MSP430x1xx - Manual (Page 238)

Utilizing Features of Low-Power Modes
12-22
Note: Writing to UTXBUF
Writing data to the transmit-data buffer must only be done if buffer UTXBUF
is empty; otherwise, an unpredictable character can be transmitted.
12.6 Utilizing Features of Low-Power Modes
There are several functions or features of the USART that support the ultralow
power architecture of the MSP430. These include:
- Support system start up from any processor mode by sensing of UART
frame-start condition
- Use the lowest input clock frequency for the required baud rate
- Support multiprocessor modes to reduce use of MSP430 resources
12.6.1 Receive - Start Operation From UART Frame
The most effective use of start detection in the receive path is achieved when
the baud-rate clock runs from SMCLK. In this configuration, the MSP430 can
be put into a low-power mode with SMCLK disabled. The receive-start
condition is the negative edge from the signal on pin URXD. Each time the
negative edge triggers the interrupt flag URXS, it requests a service when
enable bits URXIE and GIE are set. This wakes the MSP430 and the system
returns to active mode, supporting the USART transfer.
Figure 12–23. Receive - Start Conditions
Clear
URXS
Clear
τ
(S)
SYNC
Valid Start Bit
Receiver Collects Character
URXSE
From URXD
SYNC
PE
FE
BRK
URXEIE
URXWIE
RXWake
Erroneous Character
Will Not Set Flag URXIFG
Each Character or Address
Will Set Flag URXIFG
Character Received
or
Break Detected
URXIFG
URXIE
Request_
Interrupt_Service
SWRST
PUC
URXBUF Read
URXSE
IRQA
D Q
Three character streams do not set the interrupt flag (URXIFG):
- Erroneous characters (URXEIE = 0)
- Address characters (URXWIE = 1)
- Invalid start-bit detection
The interrupt software should handle these conditions.