Future Technology Devices
International Ltd.

AN232B-05 Configuring FT232R,FT2232 and FT232B Baud Rates

Copyright © 2006 Future Technology Devices International Ltd.

I

AN232B-05 Configuring FT232R, FT2232 and FT232B Baud Rates

Table of Contents
Part I Setting Baud Rates for FT232R,
FT2232 and FT232BM Devices

21 Introduction................................................................................................................................... 22 Setting the...................................................................................................................................
Baud Rate
33 Baud Rate...................................................................................................................................
Calculation
45

Part II Aliasing Baud Rates

1 Aliasing ................................................................................................................................... 52 Aliasing Using...................................................................................................................................
the Original Sub-Integer Divisors
63 Aliasing Using...................................................................................................................................
the Additional FT232B Sub-Integer Divisors
8Part III Revision History, Disclaimer,Contact Information

101 Document...................................................................................................................................
Revision History
102 Disclaimer................................................................................................................................... 113 Contact Information................................................................................................................................... 1213Index

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Setting Baud Rates for FT232R, FT2232 and FT232BM Devices

1

Setting Baud Rates for FT232R, FT2232 and FT232BM
Devices

1.1

Introduction

2

One of the most basic operations with the FT232R, FT2232 (UART mode) and FT232B devices issetting a Baud rate. This application note describes the various ways that this can be done. It also
describes how to use non-standard baud rates with FTDI's USB-Serial IC devices, including atechnique called 'aliasing' - where one Baud rate is substituted by the driver in place of anotherbaud rate.
The second generation FT232B USB UART IC added additional granularity to the divisor used togenerate the device Baud rate. This allows a much larger number of baud rates to be used withthe FT232B when compared to its predecessor, the FT8U232AM.

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31.2

AN232B-05 Configuring FT232R, FT2232 and FT232B Baud Rates

Setting the Baud Rate
When using FTDI Virtual COM Port (VCP) drivers, the Baud rate can be set in the Device Manager
or from an application. When the Baud rate is set from an application program it is done in exactly
the same way as with a standard Windows COM Port - by simply passing the required Baud rate tothe port using the Windows VCOMM API calls or a COM Port library. Both standard and nonstandard baud rates can be set in this manner.
When using FTDI's D2XX direct driver the function FT_SetBaudRate can be used to set bothstandard and non-standard baud rates.
If a non-standard Baud rate is required the formulae in this application note can be used todetermine if it is possible. If the required non-standard Baud rate is possible simply pass it to thedriver as normal and the required divisor will be calculated by the driver. For D2XX there is nolonger any need to use the function FT_SetDivisor, but it has been left in for backward
compatibility.

Copyright © 2006 Future Technology Devices International Ltd.

Setting Baud Rates for FT232R, FT2232 and FT232BM Devices

1.3

4Baud Rate Calculation
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 16and 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 subinteger prescalers - 0.125, 0.25 or 0.5. The FT232R, FT2232 (UART mode) and FT232B supporta 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 and16384 (214).
Note: Divisor = 1 and Divisor = 0 are special cases. A divisor of 0 will give 3 MBaud, and a divisorof 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 therequired Baud rate.
The exact Baud rate may not be achievable - however as long as the actual Baud rate used iswithin +/-3% of the required Baud rate then the link should function without errors. When a Baudrate 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.
For example:
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.

Copyright © 2006 Future Technology Devices International Ltd.

5AN232B-05 Configuring FT232R, FT2232 and FT232B Baud Rates

2Aliasing Baud Rates

2.1

Aliasing
The file FTDIPORT.INF contains entries that are used as the divisors for standard Baud rates. Bychanging these it is possible to alias standard Baud rates with non-standard values - for instancereplacing 115kBaud with 512kBaud. Users would then set up the FT232R, FT2232 (UART mode)or FT232B to operate at 512kBaud by selecting 115kBaud for the appropriate serial port.

Copyright © 2006 Future Technology Devices International Ltd.

Aliasing Baud Rates

2.2

6

Aliasing Using the Original Sub-Integer Divisors
There are two sections of the file FTDIPORT.INF which can be changed - one for Windows 98 &Windows Millennium Edition and the other for Windows 2000 and XP. The entry for Windows XP
is shown below with the relevant fields highlighted in bold. Note the entry for Windows 98/ME isvery similar and can be changed by following the same steps. It is found under the header
FtdiPort232.HW.AddReg.
[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,00,0
D,00,06,40,03,80,00,00,d0,80
Each field consists of a pair of bytes, ordered as follows: Byte0,Byte1. Bits 13 through 0 denote theinteger divisor while bits 16, 15 and 14 denote the sub-integer divisor, as follows
15,14 = 00 - sub-integer divisor = 015,14 = 01 - sub-integer divisor = 0.5
15,14 = 10 - sub-integer divisor = 0.25
15,14 = 11 - sub-integer divisor = 0.125
The divisor can be extracted for each entry in a few simple steps, as shown here for the entry
9c,80
Step 1 - re-order the bytes: 9c,80 => 809c Hex
Step 2 - extract the sub-integer divisor; 15 = 1, 14 = 0 => sub-integer = 0.25
Step 3 - extract the integer divisor: 13:0 = 009c Hex = 156 Dec
Step 4 - combine the integer and sub-integer divisors: 156.25 Dec
Step 5 - divide 3000000 by the divisor=> 3000000/156.25 = 19,200 baud
The following lists the standard values and their respective baud rates.
10,27 => divisor = 10000, rate = 300
88,13 => divisor = 5000, rate = 600
C4,09 => divisor = 2500, rate = 1200
E2,04 => divisor = 1250, rate = 2,400
71,02 => divisor = 625, rate = 4,800
38,41 => divisor = 312.5, rate = 9,600
9C,80 => divisor = 156, rate = 19,230
4E,C0 => divisor = 78, rate = 38,461
34,00 => divisor = 52, rate = 57,692
1A,00 => divisor = 26, rate = 115,384
0D,00 => divisor = 13, rate = 230,769
06,40 => divisor = 6.5, rate = 461,538
03,80 => divisor = 3.25, rate = 923,076
00,00 => RESERVED
D0,80 => divisor = 208.25, rate = 14406
To alias a baud rate of 920k baud to the standard 56k baud menu option under windows:- change the 34,00 entry to 03,80 (simple as both of the values are available from the previous list)
To alias a rate of 197k baud to the standard 4,800 baud menu option under windows :- find the best divisor, check that it is sufficiently accurate and then modify the appropriate INF fileentry.

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7

AN232B-05 Configuring FT232R, FT2232 and FT232B Baud Rates
3,000,000/197,000 = 15.228. The best divisor is 15.25.
3,000,000/15.25 = 196,721. 196,721/197,000 = 99.8%. This is well within the required 3%
tolerance.
Set bit 15:14 to 10 for a sub-integer divisor of 0.25, and the lower bits to 0F.
- replace the 71,02 entry with 0F,80.
Incorporating these two changes the FTDIPORT.INF entry becomes[FtdiPort232.NT.HW.AddReg]
HKR,,ConfigData,1,01,00,3F,3F,10,27,88,13,C4,09,E2,04,0F,80,38,41,9C,80,4E,C0,03,80,1A,00,0
D,00,06,40,03,80,00,00,00,00
This is for Windows 2000 and XP, the changes should be repeated in the Windows 98/ME entry
also.

Copyright © 2006 Future Technology Devices International Ltd.

Aliasing Baud Rates

2.3

8Aliasing Using the Additional FT232B Sub-Integer Divisors
The standard FTDIPORT.INF ConfigData Baud rate table only supports the original FT8U232AM
sub-integer prescalers and not the new ones added for the FT232B. To allow the addition subinteger prescalers to be used when aliasing a Baud rate the following ConfigData entry is needed:[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(Note - The developer should cut and paste the above entry into FTDIPORT.INF, replacing theoriginal one)
In this case each field consists of 4 bytes, ordered as follows: Byte0,Byte1,Byte2,Byte3. Bits 13
through 0 denote the integer divisor while bits 16, 15 and 14 denote the sub-integer divisor, as
follows
16,15,14 = 000 - sub-integer divisor = 0
16,15,14 = 001 - sub-integer divisor = 0.5
16,15,14 = 010 - sub-integer divisor = 0.25
16,15,14 = 011 - sub-integer divisor = 0.125
16,15,14 = 100 - sub-integer divisor = 0.375
16,15,14 = 101 - sub-integer divisor = 0.625
16,15,14 = 110 - sub-integer divisor = 0.75
16,15,14 = 111 - sub-integer divisor = 0.875
Note that this ConfigData entry is the same as before except that each field consists of 4 bytes
instead of 2 bytes. The first highlighted byte in ConfigData is the Flags byte and format of the fields
is determined by Bit4 of the Flags byte. In the example above, setting Flags byte to 11 means that
Bit4 is set and 4 byte fields are being used.
The divisor can be extracted for each entry in a few simple steps, as shown here for the entry
35,40,01,00
Step 1 - re-order the bytes: 35,40,01,00 => 00014035 Hex
Step 2 - extract the sub-integer divisor; 16 = 1, 15 = 0, 14 = 1 => sub-integer = 0.625
Step 3 - extract the integer divisor: 13:0 = 0035 Hex = 53 Dec
Step 4 - combine the integer and sub-integer divisors: 53.625 Dec
Step 5 - divide 3000000 by the divisor => 3000000/53.625 = 55944 baud
The following lists the standard values and their respective baud rates.
10,27,00,00 => divisor = 10000, rate = 300
88,13,00,00 => divisor = 5000, rate = 600
C4,09,00,00 => divisor = 2500, rate = 1200
E2,04,00,00 => divisor = 1250, rate = 2,400
71,02,00,00 => divisor = 625, rate = 4,800
38,41,00,00 => divisor = 312.5, rate = 9,600
9C,80,00,00 => divisor = 156, rate = 19,230
4E,C0,00,00 => divisor = 78, rate = 38,461
34,00,00,00 => divisor = 52, rate = 57,692
1A,00,00,00 => divisor = 26, rate = 115,384
0D,00,00,00 => divisor = 13, rate = 230,769
06,40,00,00 => divisor = 6.5, rate = 461,538
03,80,00,00 => divisor = 3.25, rate = 923,076
00,00,00,00 => RESERVED

Copyright © 2006 Future Technology Devices International Ltd.

9AN232B-05 Configuring FT232R, FT2232 and FT232B Baud Rates
D0,80,00,00 => divisor = 208.25, rate = 14406
To alias a rate of 920k baud to the standard 56k baud menu option under windows:- change the 34,00,00,00 entry to 03,80,00,00 (simple as both of the values are available from theprevious list)
To alias a rate of 197k baud to the standard 4,800 baud menu option under windows :- find the best divisor, check that it is sufficiently accurate and then modify the appropriate INF fileentry.
3000000/197,000 = 15.228. The best divisor is 15.25.
3000000/15.25 = 196,721. 196,721/197,000 = 99.8%. This is well within the required 3% tolerance.
Set bits 16:15:14 to 010 for a sub-integer divisor of 0.25, and the lower bits to 0F.
- replace the 71,02,00,00 entry with 0F,80,00,00.
Incorporating these two changes the FTDIPORT.INF entry becomes[FtdiPort232.NT.HW.AddReg]
HKR,,ConfigData,1,11,00,3F,3F,10,27,00,00,88,13,00,00,C4,09,00,00,E2,04,00,00,0F,80,00,00,38,41,00,00,9C,80,00,00,4E,C0,00,00,03,80,00,00,1A,00,00,00,0D,00,00,00,06,40,00,00,03,80,00,00,00,00,00,00,D0,80,00,00
This is for Windows 2000 and XP and the changes should be repeated in the Windows 98/ME
entry also.

Copyright © 2006 Future Technology Devices International Ltd.

Revision History, Disclaimer, Contact Information

3

Revision History, Disclaimer, Contact Information

3.1

Document Revision History
VersionRelease Date

Comments

1.0

March 2004

Initial release.

1.1February 2006

Format updated.
Various corrections throughout.

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10113.2

AN232B-05 Configuring FT232R, FT2232 and FT232B Baud Rates

Disclaimer
Copyright © 2006 Future Technology Devices International Ltd.
Neither the whole nor any part of the information contained in, or the product described in thismanual, may be adapted or reproduced in any material or electronic form without the prior writtenconsent of the copyright holder.
This product and its documentation are supplied on an as-is basis and no warranty as to theirsuitability for any particular purpose is either made or implied.
Future Technology Devices International Ltd. will not accept any claim for damages howsoever
arising as a result of use or failure of this product. Your statutory rights are not affected.
This product or any variant of it is not intended for use in any medical appliance, device or system
in which the failure of the product might reasonably be expected to result in personal injury.
This document provides preliminary information that may be subject to change without notice.

Copyright © 2006 Future Technology Devices International Ltd.

Revision History, Disclaimer, Contact Information

3.3

Contact Information
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12

13AN232B-05 Configuring FT232R, FT2232 and FT232B Baud Rates

Index

FT232BM 2, 4, 5
FT232R 2, 4, 5
FT8U232AM 2, 4
FTDIPORT.INF 5, 6, 8-2--M-

2 MBaud

4Margin of Error-33 MBaud-N4Non-Standard Baud Rate-4-2, 3, 5-R-48 MHz Clock4

Revision History-AAliasing

410-S2, 5Special Case Divisors 4Standard Baud Rate 5
Sub-Integer Divisor 6, 8
Sub-Integer Prescaler 4-BBaud Rate 2, 3, 6, 8
Baud Rate Divisor 4-C-

VCOMM APIVCP 3Clock 4
COM Port 3
COM Port Library 3
ConfigData 6, 8
Contact Information 12-DD2XX 3
Device Manager
Disclaimer 11Divisor 3, 6, 8-V3

-WWindows 2000 6, 8
Windows 98 6, 8
Windows ME 6, 8
Windows XP 6, 83-FFT_SetBaudRate 3
FT_SetDivisor 3
FT2232C 2, 4, 5

Copyright © 2006 Future Technology Devices International Ltd.