Baud rates are calculated using a Baud rate divisor. The file FTDIPORT.INF contains entries that are used as the divisors for standard baud rates. By changing these it is possible to alias standard baud rates with non-standard values - for instance replacing 115k Baud with 512k Baud. Users would then set up the device to operate at 512k Baud by selecting 115k Baud for the USB serial port.
The procedure for calculating Baud rate divisors is described fully in application note AN232B-05 Configuring FT232R, FT2232 and FT232B Baud Rates.
The FT8U232AM device supports sub-integer divisors of 0, 0.5, 0.25 and 0.125 only. These values can be configured through a Baud rate divisor table with two elements to define a Baud rate. The extract below from FTDIPORT.INF gives an example of the Baud rate table for standard Baud rates for the FT8U232AM device. Note that the Baud rate table is one line in the INF.
[FtdiPort232.NT.HW.AddReg]
| HKR,,"ConfigData",1,01,00,3F,3F,10,27,88,13,C4,09,E2,04,71,02,38,41,9C,80,4E,C0,34,00,1A,00,0D,00,06,40,03,80,00,00,D0,80
|
In the case of the FT232R, FT2232 and FT232B devices, sub-integer divisors of 0, 0.5, 0.25, 0.125, 0.375, 0.625, 0.75 and 0.875 are supported. This requires that the Baud rate table be extended to four elements per Baud rate as indicated below. This example extract shows the four-element Baud rate table for standard Baud rates. As with the original divisors, note that the Baud rate table is one line in the INF.
[FtdiPort232.NT.HW.AddReg]
| HKR,,"ConfigData",1,11,00,3F,3F,10,27,00,00,88,13,00,00,C4,09,00,00,E2,04,00,00,71,02,00,00,38,41,00,00,9C,80,00,00,4E,C0,00,00,34,00,00,00,1A,00,00,00,0D,00,00,00,06,40,00,00,03,80,00,00,00,00,00,00,D0,80,00,00
|
The Baud rate table used by each device is located in the registry under
| HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\FTDIBUS\{Device VID, PID and serial number}\0000\Device Parameters\ConfigData
|
Please note that the four-element table will not allow the use of the additional sub-integer divisors with FT8U232AM devices. The two-element table will work with all devices.
For a full explanation of calculating non-standard Baud rate divisors and how to arrange them in the Baud rate table, see application note AN232B-05 Configuring FT232R, FT2232 and FT232B Baud Rates.