USART Operation: UART Mode

14-11USART Peripheral Interface, UART Mode

14.2.6 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 147. This combination

supports fractional divisors for baud rate generation. The maximum USART

baud rate is one-third the UART source clock frequency BRCLK.

Figure 147. MSP430 Baud Rate Generator
Bit Start
mX
BRCLK 88
UCLKI
ACLK
SMCLK
SMCLK 11
BITCLK
10
01
00
20
27
28
215
Compare (0 or 1)
Modulation Data Shift Register
(LSB first)
16Bit 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 148. For each bit received, a majority

vote is taken to determine the bit value. These samples occur at the N/21,

N/2, and N/2+1 BRCLK periods, where N is the number of BRCLKs per

BITCLK.

Figure 148. BITCLK Baud Rate Timing
N/2
Bit Start
BRCLK
Counter
BITCLK
N/21 N/221 N/2 N/21 1 N/2 N/21N/22
0 N/2 N/211
INT(N/2) + m(= 0)
INT(N/2) + m(= 1)
1 0 N/2
Bit Period
NEVEN: INT(N/2)
NODD: INT(N/2) + R(= 1)
m: corresponding modulation bit
R: Remainder from N/2 division
Majority Vote: (m= 0)
(m= 1)