Supported Bit Rates on 16450 and Compatibles

The NS 16450 and other compatible UART devices (including the 8250- and 16550-Series UARTs) incorporate a Programmable Baud Generator feature that is driven as a function of the following constants:

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    CLOCK     = 1843200    ; crystal frequency
    CLOCK/16  = 115200     ; after divider

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Given these constants, the algorithm for determining which rates are supported is explained in the following examples:

• If 900 bps is specified, the bit rate is exactly 900 because it divides evenly into 115200 (115200/900 = 128). Bit rate, returned by IOCtl ASYNC_GETBAUDRATE, is 900.

• If 901 bps is specified, the bit rate does not change, and the IOCtl fails with an invalid parameter error because it cannot be supported within .01% (115200/901 = 128, 115200/128 = 900, which gives a .1111% error).

• If 907 bps is specified, the bit rate is 907.0866 because it can be supported within .01% (115200/907 = 127, 115200/127 = 907.0866, which gives a .0095% error). Bit rate, returned by ASYNC_GETBAUDRATE, is 907.

• If 110 bps is specified, the bit rate is 110.0287, even though the error is over .01% (115200/110 = 1047, 115200/1047 = 110.0287, which gives a .0260% error). Bit rate, returned by ASYNC_GETBAUDRATE, is 110.

Note: Where division is performed and the quotient is not a whole integer, an integer result is obtained by rounding off the fractional part of the quotient.