Texas Instruments MSP430x1xx - Manual (Page 221)

Asynchronous Operation
12-5
USART Peripheral Interface, UART Mode
12.3.2 Baud Rate Generation in Asynchronous Communication Format
Baud rate generation in the MSP430 differs from other standard
serial-communication interface implementations.
12.3.2.1 Typical Baud Rate Generation
Typical baud-rate generation uses a prescaler from any clock source and a
fixed, second-clock divider that is usually divide-by-16. Figure 12–5 shows a
typical baud-rate generation.
Figure 12–5. Typical Baud-Rate Generation Other Than MSP430
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1
16-Bit Prescaler/Divider
RC
:16
BRCLK
Start
8 8 15
BRSCLK
BITCLK
UBR0 UBR1
0 7 0 7
Select Clock Source
Clock1
Clockn
H
L
H
L
H
L
Start
BRSCLK
BITCLK
Take Majority Vote of Receive Bit
Baud rate =
BRCLK
n 16
Typical baud-rate schemes often require specific crystal frequencies or cannot
generate some baud rates required by some applications. For example,
division factors of 18 are not possible, nor are noninteger factors such as
13.67.
12.3.2.2 MSP430 Baud Rate Generation
The MSP430 baud rate generator uses one prescaler/divider and a modulator
as shown in Figure 12–6. This combination works with crystals whose
frequencies are not multiples of the standard baud rates, allowing the protocol
to run at maximum baud rate with a watch crystal (32,768 Hz). This technique
results in power advantages because sophisticated, MSP430 low-power
operations are possible.