N/2
Bit Start
BRCLK
Counter
BITCLK
N/2-1 N/2-2
1
N/2
N/2-1
1
N/2
N/2-1
N/2-2
0
N/2
N/2-1
1
INT(N/2) + m(= 0)
INT(N/2) + m(= 1)
1
0
N/2
Bit Period
N
: INT(N/2)
EVEN
N
: INT(N/2) + R(= 1)
ODD
m: corresponding modulation bit
R: Remainder from N/2 division
Majority Vote:
(m= 0)
(m= 1)
eUSCI_A Operation – UART Mode
777
SLAU367P – October 2012 – Revised April 2020
Copyright © 2012–2020, Texas Instruments Incorporated
Enhanced Universal Serial Communication Interface (eUSCI) – UART Mode
30.3.9 UART Baud-Rate Generation
The eUSCI_A baud-rate generator is capable of producing standard baud rates from nonstandard source
frequencies. It provides two modes of operation selected by the UCOS16 bit.
A quick setup for finding the correct baud rate settings for the eUSCI_A can be found in
30.3.9.1 Low-Frequency Baud-Rate Generation
The low-frequency mode is selected when UCOS16 = 0. This mode allows generation of baud rates from
low-frequency clock sources (for example, 9600 baud from a 32768-Hz crystal). By using a lower input
frequency, the power consumption of the module is reduced. Using this mode with higher frequencies and
higher prescaler settings causes the majority votes to be taken in an increasingly smaller window and,
thus, decrease the benefit of the majority vote.
In low-frequency mode, the baud-rate generator uses one prescaler and one modulator to generate bit
clock timing. This combination supports fractional divisors for baud-rate generation. In this mode, the
maximum eUSCI_A baud rate is one-third the UART source clock frequency BRCLK.
Timing for each bit is shown in
. For each bit received, a majority vote is taken to determine
the bit value. These samples occur at the N/2 – 1/2, N/2, and N/2 + 1/2 BRCLK periods, where N is the
number of BRCLKs per BITCLK.
Figure 30-10. BITCLK Baud-Rate Timing With UCOS16 = 0
Modulation is based on the UCBRSx setting as shown in
. A 1 in the table indicates that m = 1
and the corresponding BITCLK period is one BRCLK period longer than a BITCLK period with m = 0. The
modulation wraps around after 8 bits but restarts with each new start bit.
Table 30-2. Modulation Pattern Examples
UCBRSx
Bit 0
(Start Bit)
Bit 1
Bit 2
Bit 3
Bit 4
Bit 5
Bit 6
Bit 7
0x00
0
0
0
0
0
0
0
0
0x01
0
0
0
0
0
0
0
1
⋮
0x35
0
0
1
1
0
1
0
1
0x36
0
0
1
1
0
1
1
0
0x37
0
0
1
1
0
1
1
1
⋮
0xff
1
1
1
1
1
1
1
1
The correct setting of UCBRSx can be found as described in
.