LM3S3739 Luminary Micro, Inc, LM3S3739 Datasheet - Page 379

• Manufacturer Part Number: LM3S3739
• Description: Lm3s3739 Arm Microcontroller
• Manufacturer: Luminary Micro, Inc

14.2.2
14.2.3
June 02, 2008
Figure 14-2. UART Character Frame
Baud-Rate Generation
The baud-rate divisor is a 22-bit number consisting of a 16-bit integer and a 6-bit fractional part.
The number formed by these two values is used by the baud-rate generator to determine the bit
period. Having a fractional baud-rate divider allows the UART to generate all the standard baud
rates.
The 16-bit integer is loaded through the UART Integer Baud-Rate Divisor (UARTIBRD) register
(see page 393) and the 6-bit fractional part is loaded with the UART Fractional Baud-Rate Divisor
(UARTFBRD) register (see page 394). The baud-rate divisor (BRD) has the following relationship
to the system clock (where BRDI is the integer part of the BRD and BRDF is the fractional part,
separated by a decimal place.)
BRD = BRDI + BRDF = UARTSysClk / (16 * Baud Rate)
where UARTSysClk is the system clock connected to the UART.
The 6-bit fractional number (that is to be loaded into the DIVFRAC bit field in the UARTFBRD register)
can be calculated by taking the fractional part of the baud-rate divisor, multiplying it by 64, and
adding 0.5 to account for rounding errors:
UARTFBRD[DIVFRAC] = integer(BRDF * 64 + 0.5)
The UART generates an internal baud-rate reference clock at 16x the baud-rate (referred to as
Baud16). This reference clock is divided by 16 to generate the transmit clock, and is used for error
detection during receive operations.
Along with the UART Line Control, High Byte (UARTLCRH) register (see page 395), the UARTIBRD
and UARTFBRD registers form an internal 30-bit register. This internal register is only updated
when a write operation to UARTLCRH is performed, so any changes to the baud-rate divisor must
be followed by a write to the UARTLCRH register for the changes to take effect.
To update the baud-rate registers, there are four possible sequences:
■
■
■
■
Data Transmission
Data received or transmitted is stored in two 16-byte FIFOs, though the receive FIFO has an extra
four bits per character for status information. For transmission, data is written into the transmit FIFO.
If the UART is enabled, it causes a data frame to start transmitting with the parameters indicated
in the UARTLCRH register. Data continues to be transmitted until there is no data left in the transmit
UnTX
UARTIBRD write, UARTFBRD write, and UARTLCRH write
UARTFBRD write, UARTIBRD write, and UARTLCRH write
UARTIBRD write and UARTLCRH write
UARTFBRD write and UARTLCRH write
1
0
Start
n
LSB
5-8 data bits
MSB
if enabled
Parity bit
stop bits
Preliminary
1-2
LM3S3739 Microcontroller
379