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I designed a PCB that incorporates FT234XD (Datasheet) USB-UART converter but no matter what I tried, I couldn't get it working. Host computers does not even react let alone give an error so it's probably not a driver issue. The IC is connected to STM32F030 MCU.

What I tried so far, removing DM,DP 47pF caps, shorting 22Ω resistors, shorting filter inductor, reversing DP and DM, testing several Type-B cables as wells as direct header connection.

I see 3.3V and 2.5V on RX/TX pins, DP is pulled up to 3.3V. So IC regulator is working. I'm using the IC in bus powered configuration. My filter inductor is Sunlord GZ2012D601TF

Schematic PCB Datasheet Reference

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    \$\begingroup\$ Have you tried removing the zener diodes? It may be the case, that your datalines are to slow because of them. \$\endgroup\$
    – EeEmDee
    Commented Nov 27, 2023 at 20:10
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    \$\begingroup\$ It is not even clear what diodes they are. Remove them and try. \$\endgroup\$
    – Justme
    Commented Nov 27, 2023 at 20:18
  • \$\begingroup\$ Diodes are Liown SMAJ5.0A. Apparently diode capacitance can effect USB enumeration. I'll try removing them. Should the diodes be selected to have breakdown voltage higher than the one I chose? \$\endgroup\$
    – Berkays
    Commented Nov 27, 2023 at 20:20
  • \$\begingroup\$ Thank you. Removing the diodes solved the problem. \$\endgroup\$
    – Berkays
    Commented Nov 28, 2023 at 0:53

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The diodes are completely unsuitable for high speed data lines such as 12Mbps USB. They are also unidirectional so they don't protect from negative ESD.

The diodes seem to have capacitance beyond 1000pF. If it was only 1000pF, it would form an RC filter with the 27 ohm resistors that has a cut-off below 6 MHz, so a 12 Mbps signal has no chance of working.

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