STM32F411 USB interface.

[phil from seattle]

I am in the process of designing a board around an STM32F411. I will only use the Full Speed interface and no Host Mode (at least initially). I've looked at a number of designs to see how they handle USB. I am seeing things that make for more questions than answers.

In some cases I am seeing low value resistors (10 ohm, 12 ohm) in series with D+/D- and in other cases none. I've also seen at least one design that calls out 0R resistors - maybe the designers weren't sure and added them to be safe? I assume the resistors are an attempt at impedance matching.  The STM appnote AN4879 says that FS Phys have the impedance matching resistors.  So why then the resistors on the boards?

In a similar vein, a number of designs I've looked at use a 1.5K pull up resistor on D+ (usually controlled via a transistor) and yet the data sheet and the app note say the 411 has a PU built in.

Were the circuit designers unaware of the features of the chip?  Early chip errata? Maybe clones don't support this? Maybe it is just a case of the first designer not being sure/aware and everyone just copied from him/her?

[thm_w]

Earlier STM32 parts needed that pullup, earlier than that probably needed the series resistors. Its just following old protocols. That or the board is designed to accept another ST part with the same pinout.

Of course check errata for anything relevant to USB. But if nothing there its not needed.
There are no F411 true clones to my knowledge.

[bugnate]

Earlier STM32 parts needed that pullup, earlier than that probably needed the series resistors. Its just following old protocols. That or the board is designed to accept another ST part with the same pinout.

This. Specifically I would bet that the vestigial bits or whatever are inherited from a STM32F103 or similar design.

[GromBeestje]

The Pull Up resistors were required for the F103. But as far as I am aware, the F103 didn't require series resistors. This was a requirement by earlier USB designs. Therefore, I'd guess such board was designed by someone who had experience with USB but not specifically with STM microcontrollers.

[phil from seattle]

Thanks guys.  That helps to confirm my suspicions.