Non-Standard Baud Rates in USB-Serial Bridge Controllers - KBA92442

Version: **

 

Question: Does USB-Serial support non-standard baud rates like 10325, 946578 bits/s and so on?

 

Answer:

Yes. Cypress™ USB-Serial Bridge Controllers support non-standard baud rates. The Cypress USB-Serial Configuration Utility allows you to set non-standard baud rates. Please note that if a third-party serial communication utility such as Tera Term is used to configure the serial port, then the settings provided by it will overwrite the settings provided in the Cypress USB-Serial Configuration Utility. Similarly, any other custom serial configuration utility can be used to set the baud rates for the USB-Serial Device port. Cypress USB-Serial Bridge Controllers support a maximum baud rate of 3 Mbps for UART.

Support for Non-Standard Baud Rates

Although the device supports non-standard baud rates, only certain values of non-standard baud rates are supported. Only those baudrates which lie within a margin of ±5% of the numerically closest supported non-standard baud rate are allowed. The device does not support baud-rates that do not meet the above criteria.

To calculate the exact baud rate that is supported by the device, you need to perform the following baud rate calculation:

Baud Rate Calculation

Call the required baud rate “BR_RD” and the supported non-standard baud rate “BD_SD”.

Constants used in the calculation:

  •   PCLK = 48 MHz
  •   Left Shift Factor = 32
  •   Oversampling Constant (OVS):

Table 1. OVS Values for Different Baud Rates


                                                                                
     Required Baud Rate (BD_RD)     Oversampling Constant (OVS)
     2 Mbps < BD_RD ≤ 3 Mbps     8
     1.5 Mbps < BD_RD ≤ 2 Mbps     12
     BD_RD ≤ 1.5 M     16

Here BR_RD is the baud rate required by the user.

Now you need to calculate a divisor. The divisor has two parts: the Integral Divisor and the Fractional Divisor.


             
    Integral Divisor = Integer (48000000/ (BD_RD*OVS))
   
    Fractional Divisor = Remainder ((Integer (48000000*32/ (BD_RD*OVS)))/ 32)
   
    Divisor = Integral Divisor + (Fractional Divisor/32)

Using the divisor obtained from the calculation above, the supported baud rate BD_SD can be calculated as follows:

             
    BD_SD= (PCLK)/ (Divisor *OVS)

             
    Difference = |BD_SD-BD_RD|
   
    %Difference = (Difference/BD_RD)*100%

If the %Difference calculated above is ≤5%, then the required baud rate BD_RD is supported by the device without any failures.

Example Calculation

Now let us take an example where 1764393 bits/s (~1.68 Mbps) is the required Baud Rate, BD_RD.

BD_RD 1764393 lies between 2 Mbps and 3 Mbps, hence OVS = 12 (from Table 1 above).

Calculating the Divisor:

             
    Integral Divisor = Integer (48000000/ (BD_RD*OVS))
    = (48000000/1764393*12)
    = 2
    Fractional Divisor = Remainder ((Integer (48000000*32/ (BD_RD*OVS)))/ 32)
    = Remainder((48000000*32/1764393*12)/32)
    = 8
    Divisor = Integral Divisor + (Fractional Divisor/32)
    = 2+(8/32) = 2.25

Now the supported baud rate BD_SD can be calculated as follows:

             
    BD_SD= (PCLK)/ (Divisor *OVS)
   
    BD_SD = (48000000)/ (2.25*12) = 1777777.7777 ~=1777777 bits/s

Hence the supported baud rate BD_SD is 1777777 bits/s.

The difference between the required baud rate, BD_RD, and the supported baud rate, BD_SD:

             
    |BD_SD-BD_RD| = |1777777-1764393| = 13384
   
    %Difference = (13384/1764393)*100 = 0.75%

The %Difference calculated above is well within the range of ±5%. Thus BD_RD = 1764393 bits/s is supported by the device.

Conclusion

The Cypress USB-Serial Bridge Controllers can support certain non-standard baud rates that satisfy the above specified criteria. Using the preceding method, you can easily find out if a particular baud rate will be supported by the Cypress USB-Serial Bridge Controller or not.

Attached with this article is an Excel file that you can use to find out if a particular baud rate is supported by the USB-Serial Bridge Controller Device. Enter the required baud rate against the “Enter Required Baud Rate (bits/s)” caption and the result will be displayed against the “Baud Rate Supported? (Yes/No)” caption.

Procedure for setting a Non-Standard Baud Rate through the Cypress USB-Serial Configuration Utility:

  1.   Please refer to the Cypress USB-Serial Configuration Utility User Guide for the steps to configure the USB-Serial Bridge Controller Device.
  2.   Under the SCB/UART Tab, click Configure and type in the required baud rate in the Baud Rate: box.
  3.   Figure 1 shows you how to set 1764393 bits/s as the baud rate for the USB-Serial device.

Figure 1. Setting a Non-Standard Baud Rate for a USB-Serial Device using the Cypress USB-Serial Configuration Utility