A Baud rate for the FT232R, FT2232 (UART mode) or FT232B is generated using the chips internal 48MHz clock. This is input to Baud rate generator circuitry where it is then divided by 16 and fed into a prescaler as a 3MHz reference clock. This 3MHz reference clock is then divided down to provide the required Baud rate for the device's on chip UART. The value of the Baud rate divisor is an integer plus a sub-integer prescaler. The original FT8U232AM only allowed 3 sub-integer prescalers - 0.125, 0.25 or 0.5. The FT232R, FT2232 (UART mode) and FT232B support a further 4 additional sub-integer prescalers - 0.375, 0.625, 0.75, and 0.875. Thus, allowed values for the Baud rate divisor are:
Divisor = n + 0, 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.875; where n is an integer between 2 and 16384 (214).
Note: Divisor = 1 and Divisor = 0 are special cases. A divisor of 0 will give 3 MBaud, and a divisor of 1 will give 2 MBaud. Sub-integer divisors between 0 and 2 are not allowed.
Therefore the value of the divisor needed for a given Baud rate is found by dividing 3000000 by the required Baud rate.
The exact Baud rate may not be achievable - however as long as the actual Baud rate used is within +/-3% of the required Baud rate then the link should function without errors. When a Baud rate is passed to the driver where the exact divisor required is not achievable the closest possible Baud rate divisor will be used as long as that divisor gives a Baud rate which is within +/- 3% of the Baud rate originally set.
A non-standard Baud rate of 490000 Baud is required.
Required divisor = 3000000 / 490000 = 6.122
The closest achievable divisor is 6.125, which gives a baud rate of 489795.9, which is well within the allowed +/- 3% margin of error. Therefore 490000 can be passed to the driver and the device will communicate without errors.