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USART Operation: UART Mode
13-11
USART Peripheral Interface, UART Mode
13.2.6 UART Baud Rate Generation
The USART baud rate generator is capable of producing standard baud rates
from non-standard source frequencies. The baud rate generator uses one
prescaler/divider and a modulator as shown in Figure 13−7. This combination
supports fractional divisors for baud rate generation. The maximum USART
baud rate is one-third the UART source clock frequency BRCLK.
Figure 13−7. MSP430 Baud Rate Generator
Bit Start
mX
BRCLK
88
UCLKI
ACLK
SMCLK
SMCLK
11
BITCLK
10
01
00
2
0
2
7
2
8
2
15
Compare (0 or 1)
Modulation Data Shift Register
(LSB first)
16−Bit Counter
Q0
............
Q15
m0m7
...
...
8
UxBR1 UxBR0
Toggle
FF
N =
R
R
R
UxMCTL
+0 or 1
SSEL1 SSEL0
Timing for each bit is shown in Figure 13−8. For each bit received, a majority
vote is taken to determine the bit value. These samples occur at the N/2−1,
N/2, and N/2+1 BRCLK periods, where N is the number of BRCLKs per
BITCLK.
Figure 13−8. BITCLK Baud Rate Timing
N/2
Bit Start
BRCLK
Counter
BITCLK
N/2−1 N/2−2
1 N/2 N/2−1 1 N/2 N/2−1N/2−2
0 N/2 N/2−11
INT(N/2) + m(= 0)
INT(N/2) + m(= 1)
1 0 N/2
Bit Period
N
EVEN
: INT(N/2)
N
ODD
: INT(N/2) + R(= 1)
m: corresponding modulation bit
R: Remainder from N/2 division
Majority Vote:
(m= 0)
(m= 1)