12.4.5 Baud Rate Registers (DLR, DMR, and DFR)

Table 12-8. DLR
Bit Number Name R/W Reset Value Description
[7:0] DLR R/W 0x01 This divisor latch LSB register (Table 12-8) holds the LSB of the integer divisor value used to calculate the baud rate. The baud rate can be calculated using EQ 1, 2, 3, or 4.
Table 12-9. DMR
Bit Number Name R/W Reset Value Description
[7:0] DMR R/W 0x00 This divisor latch MSB register (Table 12-9) holds the MSB of the integer divisor value used to calculate the baud rate. The baud rate can be calculated using EQ 1, 2, 3, or 4.
Table 12-10. DFR
Bit Number Name R/W Reset Value Description
[5:0] DFR R/W 0x00 The fractional divisor register (Table 12-10) is used to store the fractional divisor used to calculate the fractional baud rate value in 1/64th.

0x0: 0/64

0x1: 1/64

….

0x3F: 63/64

As explained earlier, the divisor value has an integer part and a fractional part. Calculate the 6-bit number (k) which is the fractional divisor in Table 12-10 register by taking the fractional part of the required baud rate divisor and multiplying it by 64 (that is, 2n, where n is the fractional part which is 6) and adding 0.5 to account for rounding errors:

k = integer (fractional part of divisor value × 2n + 0.5)

For example, to generate the baud rate of 134.5 the reference clock is 18.432 MHz.

FAPBCLK = 18.432 MHz, Fractional BR = 134.5,
Hence, calculate the divisor value = (18.432 × 106) / (16 × 134.5) = 8,565.05

The integer part of divisor = 8565 and fractional part of divisor = 0.05

Therefore, the fractional part, k = integer ((0.05 × 64) + 0.5) = 3.7 ~ = 4

The following table contains the list of baud rates and corresponding values of DFR and DMR+DLR registers.

Table 12-11. Baud Rates and Divisor Values for the 18.432 MHz Reference Clock
Baud Rate Divisor DLR + DMR Integer Divisor DFR Fractional Divisor in 64th Percent Error
50 23,040 23,040 0 0.00000%
75 15,360 15,360 0 0.00000%
110 10,472.72 10,472 47 0.00007%
134.5 8,565.05 8,565 4 0.00008%
150 7,680 7,680 0 0.00000%
2,000 576 576 0 0.00000%
38,400 30 30 0 0.00000%
56,000 21.57 21 37 0.03255%