USB Circuit Analysis – Purpose of Capacitors and Resistors in USB Bus

circuit analysisftdiusb

Trying to understand the purpose of the capacitors and resistors in this bus powered configuration for the FTDI chip, I think the capacitors are for filtering high frequency noise, not sure what the resistors for though, maybe line termination ?
enter image description here

Best Answer

The series resistors are for line termination. From

The Universal Serial Bus (USB) line termintation is specified in the USB 1.1 specification to insure proper terminations so that signal integrity is maintained. The termination requirement varies depending on what driver chipset is used, whether the port is upstream or downstream, and if the transceiver operates in full or low speed.

When the USB 1.1 specification was written, a series resistor between the range of 28Ω to 44Ω was required. Most USB drivers will require a termination resistor of 16 - 33Ω.

This is in addition to the pullup/pulldowns used for identification.

The caps are used to ensure the line meets the required rise/fall times as well.