Texas Instruments MSP430x1xx manual Uart Baud Rate Generation, 7. MSP430 Baud Rate Generator

USART Operation: 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

	SSEL1		SSEL0 N =
UCLKI
		00	BRCLK
ACLK		01
SMCLK		10
SMCLK		11

215	...	28		27	...	20
	UxBR1				UxBR0
	8				8
		16−Bit Counter			R
	Q15		............		Q0
	+0 or 1 Compare (0 or 1)					Toggle
						FF
						R
	Modulation Data Shift Register					R
		(LSB first)
mX		m7	8	m0
			UxMCTL			Bit Start

BITCLK

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

	Majority Vote: (m= 0)
Bit Start	(m= 1)

BRCLK
Counter	1	N/2	N/2−1	N/2−2	1	N/2	N/2−1
	N/2 N/2−1 N/2−2	0	N/2	N/2−1	1	0	N/2
	1
BITCLK
	INT(N/2) + m(= 0)			NEVEN: INT(N/2)
	INT(N/2) + m(= 1)				NODD : INT(N/2) + R(= 1)

Bit Period

m:corresponding modulation bit R: Remainder from N/2 division

USART Peripheral Interface, UART Mode 13-11