I am currently trying to make an FT231XS recognised by a Windows 10 computer, and it always says the device is unrecognised as soon as I plug it in (c.f. this other thread).

That board is always drawing 12 mA from the USB, in a USB powered configuration. I thought that the "2.5 mA max in suspend mode" rule was only for devices that were intended to keep running in USB suspend mode, but now that I'm desperate I'm wondering if the computer actually tests the compliance to this rule at the time of the enumeration.

Do we actually need to comply with this rule even when we are completely fine with just losing power when the device goes in suspend mode?

  • 2
    \$\begingroup\$ Do you have any reason to believe this is the problem? As opposed to bad wiring, poor signal integrity, or missing drivers, etc. \$\endgroup\$
    – Chris
    Commented Nov 16, 2022 at 21:12
  • \$\begingroup\$ It's the only one left, and still something I'd like to know regardless. I've now implemented the minimal circuit, nothing else, on a clean breadboard (from scratch), with clean wiring that I checked 3 times, and measured every single passive before putting them in. For the drivers I've already used the FT4222 without issue and many other FTDI-based cables so I'm investigating every other thing... \$\endgroup\$
    – user325962
    Commented Nov 16, 2022 at 21:36
  • \$\begingroup\$ From your schematic in the other question we can see that you have a ferrite bead between the USB connector and the rest of the circuit. Remove it and self-power your device with 5V and then plug it in. If this works then your guess about current consumption might be correct. If it doesn't then you're chasing a red herring here. \$\endgroup\$
    – brhans
    Commented Nov 16, 2022 at 22:28
  • \$\begingroup\$ In all likelihood your problem is that you're trying to run USB over a breadboard ... I'd be surprised if that works for anything other than a Low-Speed HID device. \$\endgroup\$
    – brhans
    Commented Nov 16, 2022 at 22:31
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    \$\begingroup\$ @user325962 Is it 1.5MHz? If so your device is being detected as a low-speed device. Check the impedances of your data lines. \$\endgroup\$
    – Chris
    Commented Nov 17, 2022 at 0:46


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