Datasheet

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
register bit assignments.
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
I
= 1 and BRD
F
= 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
6
) / (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 (fractional part of BAUDDIV x 64) + 0.5
These bits are cleared to 0 on reset.