FTDI C232HD-DDHSP-0 cable: BaudRate altered when Enable Selective Suspend ticked

[brumbarchris]

Hello everyone,

I am using a recently purchased FTDI C232HD-DDHSP-0 cable. I need to use it with on a Win10 based PC, at a custom baudrate of 1843200 Bps, 8 data bits, 1 Stop bit, with the Enabled Selective Suspend option activated:


Nevertheless, it looks like the baud rate is affected by setting this option in the Settings for the COM port of the Windows 10. You may see below an oscilloscope screenshot depicting some bytes sent via the FTDI cable over the UART to the device we have connected to the cable. The screenshot depicts the same message being sent over the UART from the FTDI cable with the Enable Selective suspend option being ticked and the same message sent with this option being un-ticked. In both cases, the displayed message is sent 5 seconds after opening the COM port with the PC software, so even in the case with Enable Selective suspend being ticked there is enough time for the FTDI cable to go out of suspend before sending the message.



-   On channel C1 (yellow):       Enabled Selective Suspend option is un-ticked (which is the default) and the FTDI cable works correctly and as expected (our device connected to the cable receives and correctly interprets the message sent from the PC via the FTDI cable). In this case, the duration of a byte transmitted (including the start and stop bits) is 5.425us, which is exactly the expected value corresponding to the configured baudrate of 1843200 Bps
-   On channel R1 (grey):            Enabled Selective Suspend option is ticked and the FTDI cable does not work correctly (our device connected to the cable does not understand the message sent from the PC via the FTDI cable). In this case, the duration of a byte transmitted (including the start and stop bits) decreased to 5.013us, which is quite far away from the expected value of 5.425us; it is then understandable that the message is not understood by our device connected to the cable.

The only difference in the two cases is the Enable Selective suspend option being ticked or un-ticked.

Maybe one other relevant point is that the default configuration of the FTDI cable was initially changed via the FT_Prog so that USB Remote Wakeup is disabled and Pull Down IO Pins in USB Suspend is enabled (see attached EEPROM configuration.txt). These changes are intended and they are valid throughout both the working and the unworking situations described above.

Other relevant information:
-   Operating system: Windows 10 Enterprise, 10.01.19045 Build 19045
-   VCP driver version: 2.12.36.4

Is there is something you suspect I am doing wrong, or if this a known problem of the FT232H on which this cable is based on?

Best regards,
Cristian