MAX14830 datasheet(21/68 Pages) MAXIM

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Fractional Baud-Rate Generators

The internal fractional baud-rate generator provides a

high degree of flexibility and high resolution in baud-

rate programming. The baud-rate generator has a 16-bit

integer divisor and a 4-bit word for the fractional divisor.

The fractional baud-rate generator can be used with the

external crystal or clock source.

The integer and fractional divisors are calculated through

the divisor, D:

REF

16 BaudRate

where fREF is the reference frequency input to the baud-

rate generator and D is the ideal divisor. In 2x and 4x rate

modes, replace the divisor 16 by 8 or 4, respectively.

The integer divisor portion, DIV, of the divisor, D, is

obtained by truncating D:

DIV = TRUNC(D)

DIV can be a maximum of 16 bits wide and is programmed

into the 2-byte-wide registers DIVMSB and DIVLSB. The

minimum allowed value for DIVLSB is 1.

The fractional portion of the divisor, FRACT, is a 4-bit nib-

ble, which is programmed into BRGConfig[3:0]. The maxi-

mum value is 15, allowing the divisor to be programmed

with a resolution of 0.0625. FRACT is calculated as:

FRACT = ROUND(16 x (D-DIV)).

The following is an example of calculating the divisor. It is

based on a required baud rate of 190kbaud and a refer-

ence input frequency of 28.23MHz and default rate mode.

The ideal divisor is calculated as:

D = 28,230,000 / (16 x 190,000) = 9.286

hence DIV = 9.

FRACT = ROUND(4.579) = 0x05

so that DIVMSB = 0x00, DIVLSB = 0x09, and

BRGConfig[3:0] = 0x05.

The resulting actual baud rate can be calculated as:

REF

ACTUAL

16 D

For this example: DACTUAL = 9 + 5/16 = 9.313,

where DACTUAL = DIV + (FRACT/16) and

BRACTUAL= 28,230,000/(16 x 9.3125) = 189,463.087

baud

Thus, the baud rate is within 0.28% of the ideal rate.

2x and 4x Rate Modes

To support higher baud rates than possible with standard

(16x sampling) operation, the MAX14830 offers 2x and

4x rate modes. In this case, the reference clock rate only

needs to be either 8x or 4x of the baud rate, respectively.

In 4x mode only, the bits are only sampled once, at the

midbit instant, instead of the usual three samples to

determine the logic value of the bits. This reduces the

tolerance to line noise on the received data. The 2x and

4x modes are selectable through BRGConfig[5:4]. Note

that IrDA encoding and decoding does not operate in 2x

and 4x modes.

When 2x rate mode is selected, the actual baud rate is

twice the rate programmed into the baud-rate generator. If

4x rate mode is enabled, the actual baud rate on the line

is quadruple that of the programmed baud rate (Figure 8).

Figure 8. 2x and 4x Baud Rates

FRACTIONAL

RATE

GENERATOR

DIVLSB

DIVMSB

FRACT

NOTE: IrDA DOES NOT WORK IN 2x AND 4x MODES.

fREF

BRGConfig[5:4]

RATE MODE

SELECTION

1 x BAUD RATE,

2 x BAUD RATE,

4 x BAUD RATE

MAX14830

Quad Serial UART with 128-Word FIFOs

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