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I've made a simple PCB to interface an RF transceiver with a PC. I can't say it's perfect, I completely disregarded impedances and similar stuff when designing it. However, it seems to work, but only when the FTDI chip is... warm. Literally, no matter what value of resistors and capacitors I put on the board, everything is fine if I heat up the FTDI chip before connecting to the PC. When I make a "cold start" (pun intended), Windows fails to read device descriptor. What is happening? Tested on multiple FTDI FT232RL chips - same thing happens. However, when I connect a off-the-shelf FTDI USB-UART dongle, everything works perfectly, so not a driver issue I guess.

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  • \$\begingroup\$ How warm is warm and how cold is cold? If warm is actually hot, redo your solder joints. Show us a photo of the board. \$\endgroup\$
    – DKNguyen
    May 27 at 13:11
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    \$\begingroup\$ The 3V3OUT bypass cap has wrong value. It should be 100nF. \$\endgroup\$
    – Justme
    May 27 at 14:34
  • \$\begingroup\$ @DKNguyen By warm i mean like 40-50 degrees Celsius, sometimes it's enough to warm it up using your finger, sometimes a hairdryer. Definitely not the solder-melting kind of warm. By cold I mean room temp. \$\endgroup\$
    – PineLel
    May 27 at 14:34
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    \$\begingroup\$ It might be an issue with the reset pin, in the data sheet it says to either leave it floating or at VCC if not in use, but right now it's receiving 3.3V while VCC is 5V (maybe there is some undefined behavior happening). Table 5.11 suggest this won't be a problem, but check the voltage on the reset pin just to be safe. It also might be worth trying to lift the RESET pin to see if leaving it floating fixes the problem \$\endgroup\$
    – soup
    May 27 at 17:05
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    \$\begingroup\$ It's also worth looking at section 6.2 of the datahsheet, it mentions that some 'USB hosts or hub controllers will power up erratically' if current is allowed to flow from RESET# to USBDP through an internal 1.5k resistor (which it will if power is provided through JP1 pin 3 before attaching the USB) \$\endgroup\$
    – soup
    May 27 at 17:14

2 Answers 2

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You forgot to connect the TEST pin to GND.

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It's always a good start to compare your schematics to the configuration specified by the datasheet.

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Source: Datasheet


Also, the USB ID pin has to be left unconnected (or pulled high) to enable the USB DEVICE role.

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  • \$\begingroup\$ I cut the RESET trace and bridged TEST to AGND, no success either :( \$\endgroup\$
    – PineLel
    May 27 at 21:42
  • \$\begingroup\$ For time being (until we know more), remove C3 and C4, and replace C1 with 100nF. Also, add 4.7uF in parallel to C14. And maybe it's just your layouting software, but I can't see any GND connections. I would at least expect to see traces to the vias. \$\endgroup\$
    – Seir
    May 27 at 22:00
  • \$\begingroup\$ Replaced C1 with 100nF, no success. In the morning (it's midnight here) I'll try removing C3 and C4 as well as adding 4.7uF to C14. As for the ground connections, the screenshot, for readibility, is without GND polygon visible. \$\endgroup\$
    – PineLel
    May 27 at 22:43
  • \$\begingroup\$ I thought as much. Just make sure that you haven't missed any of the 4 GNDs. Also, check that VCCIO is jumpered to VCC (JP1: 2-3). \$\endgroup\$
    – Seir
    May 27 at 22:55
  • \$\begingroup\$ VCCIO is currently hard-soldered to VCC on JP1 to avoid any jumper-related issues. Will check the solder connections once more tomorrow under a microscope \$\endgroup\$
    – PineLel
    May 27 at 23:00
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In addition to pull downs and pullups (always check and use them, don't let the device configure itself in a state you won't know) Make sure you also check the soldering on the chip. Thermal expansion can make intermittent connections change over temperature. For example in the picture below pins that are not fully soldered can make or break connection over temperature.

enter image description here Source: https://www.seeedstudio.com/blog/2021/06/18/13-common-pcb-soldering-problems-to-avoid/

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