USB Large capacitance on data lines

Summary

This post details how the fitting of incorrect capacitors on USB lines can result in operational issues.

Hardware

After several years of use some of the surface mount capacitors on my FTDI branded debugger module showed signs of cracking, time to replace them.

FTDI VNC2 Debugger Module

Both capacitors (C8 & C9) associated with the USB D+ and D- lines were removed without any resulting communications issues. Schematic extract below was taken from the VNC2 Debugger datasheet.

FTDI VNC2 Debugger USB Input Section

As the USB line capacitors are there for multiple reasons, replacements were located. FTDI recommend capacitors in their Debugging Application Notes to prevent the reset of FTDI devices from spurious signals.

Wrongly Marked Components

The two USB line capacitors were replaced with what was presumed to be similar devices. Upon plugging the debugger into the computer an error message was displayed stating that the USB device could not be identified.

USB Signals

To diagnose further the oscilloscope was powered and the two USB signals probed.

USB Signals with Large Capacitors

The capture above shows the signals. Excessive rounding of the waveform usually indicates a high capacitance. 

Removing the new capacitors and verifying their capacitance confirmed that the value on the manufacturer packaging was incorrect.

USB Signals with Small Capacitors

With the capacitors removed the debugger began operating normally. Alternative replacement were fitted. The subsequent capture shown above is more indicative of USB signals.