RM0352 datasheet(124/138 Pages) STMICROELECTRONICS

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UART (universal asynchronous receive transmit)

RM0352

124/138

DocID024647 Rev 1

12.6.5

Integer baud rate register, UARTIBRD

The UARTIBRD register is the integer part of the baud rate divisor value.

Table 119 lists the

12.6.6

Fractional baud rate register, UARTFBRD

The UARTFBRD register is the fractional part of the baud rate divisor value.

Table 120 lists

the register bit assignments.

The baud rate divisor is calculated as follows:

•

Baud rate divisor BAUDDIV = (FUARTCLK / (16 × Baud rate))

where

FUARTCLK is the UART reference clock frequency.

The BAUDDIV is comprised of the integer value (BAUD DIVINT) and the fractional value

(BAUD DIVFRAC).

Note:

The contents of the UARTIBRD and UARTFBRD registers are not updated until

transmission or reception of the current character is complete.

The minimum divide ratio possible is 1 and the maximum is 65535(216 - 1). That is,

UARTIBRD = 0 is invalid and UARTFBRD is ignored when this is the case.

Similarly, when UARTIBRD = 65535 (that is 0xFFFF), then UARTFBRD must not be greater

than zero. If this is exceeded it results in an aborted transmission or reception.

Example 1 is an example of how to calculate the divisor value.

Example 1 Calculating the divisor value

If the required baud rate is 230400 and

UARTCLK = 4 MHz then:

baud rate divisor = (4 × 106) / (16 × 230400) = 1.085

This means BRD

= 1 and BRD

= 0.085.

Therefore, fractional part, m = integer [(0.085 × 64) + 0.5] = 5

Generated baud rate divider = 1 + 5/64 = 1.078

Generated baud rate = (4 × 10

) / (16 × 1.078) = 231911

Error = (231911 - 230400) / 230400 × 100 = 0.656%

Table 119. UARTIBRD register

Bits

Name

Function

15:0

BAUD DIVINT

The integer baud rate divisor.

These bits are cleared to 0 on reset.

Table 120. UARTFBRD register

Bits

Name

Function

5:0

BAUD DIVFRAC

The fractional baud rate divisor.

It results from formula: integer (fract

ional part of BAUDDIV x 64) + 0.5

These bits are cleared to 0 on reset.

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