I'm a little confused.

The STM32F2 datasheet says only this, vaguely stated in the features:

HNP/SNP/IP inside (no need for any external resistor)

Is that referring to the D+/D- termination resistors?? Ie. I don't use external termination resistors? That'd be a first for me.

While I'm on the subject, stupid question time... how do I figure power dissipation for these resistors? I attempted to search for that answer and found nothing, which suggests people likely aren't putting 1W (expensive, big) on every cheap-o USB device and they certainly aren't blowing up or I'd find a million hits. So it must not simply be 5^2V / 27 Ohm = 1W. What is it?

  • \$\begingroup\$ These are series termination resistors, they pass very little current (ideally zero) \$\endgroup\$ – Mike Jun 24 '15 at 5:24

The resistors they are referring to are for the USB protocol. There are assorted resistor values/locations required depending on the device type. For example, if you want to design a dual-role OTG device you are supposed to put 15k-Ohm pulldowns on both the DP and DM lines. The STM32 takes care of these considerations internally so you don't have to.

Regarding the series termination resistors, your power calculation is assuming that the resistor is the only thing limiting the current. But USB transceiver lines don't carry any significant current; they are high-impedance when receiving. I have used 0.063-Watt (0402 package) resistors successfully. They don't even get warm.

When I built a USB Full Speed device with a STM32F1xx I added 2.2 Ohm resistors on the DP and DM lines. I'm not sure that they were strictly necessary, since I wasn't going to High Speed data rates. I found out later that 22 Ohm was more appropriate in the general case. But the circuit worked just fine.

Also, please see this related question.

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  • \$\begingroup\$ I saw that question, but as it is a different family, I thought it may not apply (shows a trend with STM documentation thought!). Currently the person that did the schematic for this part of our design spec'd 51 ohm. I'm waiting to learn why but that seems very high considering the documentation appears to suggest zero. FYI: this is a full speed device only. \$\endgroup\$ – bcsteeve Jun 24 '15 at 9:26

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