I'm finishing up a PCB that uses a FT232, with power supplied by it's ous regulator. In the middle of debugging, I've realised that if it was supplied by the USB, it would be easier to debug, as the serial converter would be always connected and I wouldn't miss serial communication right after the start of the program while the USB was enumerating.
Actually, at first I wasn't using a ferrite bead and it was working ok and I was still using C16 as 100 nF. It was working ok, but as the datasheet recommended a ferrite bead and I had an old motherboard laying around with lots of ferrite beads in it, I've put it in the PCB. After doing that, my weekend ended and everything was put on hold until the next one.
In the next weekend, I connected the board to the USB and got a device unknown error message from Windows (Win7 64bits). Somehow I didn't correlate the new ferrite bead with the failure, so I tested several things:
Vcc = USB voltage
- No shorts between Vcc, Gnd, USBDP and USBDM
- Vcc is at 5V
- Tested the current in Vcc by suppling it with a PS. It was ok at ~10 mA
- Used a new USB cable (the same USB cable was used to transfer the images from my camera to the PC)
- Used another USB port
- Used another computer
- Swaped the FT232 for a new one
- Swaped the USB connector for a new one (twice!)
- Swaped all capacitors
- Changed C16 from 100nF to 4.7uF (ceramic, from that old motherboard)
- Soldered the first FT232 to a new PCB, using the same circuitry. It is correctly enumerated and the serial port is created, fully functional. The first FT232 is working correctly.
Today I've remembered that everything was working without the ferrite bead, so I removed it. Bingo! It started working again. Then I tried another ferrite bead, again removed from that motherbord. No go.
Does anyone knows why the ferrite bead was screwing everything up in this PCB, when it is working correctly in the second? I just can't figure it out...
PS: Sorry for the long post, but I tried to make the most complete description of everything.