STM32F4 USB resistor confusion

[infoinfo991]

Posted on October 12, 2012 at 09:47

The datasheet of the STM32F405, appendix A.3 shows example application circuit for USB peripheral, it does not show any resistors.

But, the schematic for the STM32F4Discovery shows two 22 ohm resistors on the DP and DM signals, also it shows a EMIF02-USB03F2, which serves the same purpose.

These are terminating resistors, are they required for the STM32F405?

#usb #resistor

[Ixox]

Posted on October 12, 2012 at 10:29

EDITED : swaped ''High'' and ''Full'' after Clive1 remark ;)

From what i've anderstood, those 2 resistors are required for High Speed mode (USB 2 / 480Mbits/s).

http://www.usbmadesimple.co.uk/ums_6.htm

I have the core_hs successfully access a USB stick without the resistor in Full speed mode (12 Mbits/s).

Xavier

[Tesla DeLorean]

Posted on October 12, 2012 at 10:44

From what i've anderstood, those 2 resistors are required for Full Speed mode (USB 2 / 480Mbits/s).

 

I have the core_hs successfully access a USB stick without the resistor in High speed mode (12 Mbits/s).

Huh?

High Speed 480 Mbps

Full Speed 12 Mbps

http://en.wikipedia.org/wiki/Universal_Serial_Bus

[emalund]

Posted on October 12, 2012 at 15:57

there is, in some camps a misconception that since you could get by without termination in the days when the king of diamonds were still just a jack is still holds.

Reflections, reflections, ringing, ringing, ....

Erik

[tsuneo]

Posted on October 12, 2012 at 18:49

USB2.0 spec defines the equivalent output impedance of D+/D- buffer as 28 - 44 Ohm at the section, 7.1.1.1 Full-speed (12 Mb/s) Driver Characteristics. The value of the series damping resistor is determined with this range and the DC electrical characteristics of the buffer. However, ST data sheet doesn't refer to the buffer channel resistance. 22 Ohm is often chosen in such case, considering availability of the resistor.

Tsuneo

[gkpublic]

Posted on March 20, 2013 at 20:22

I found a discussion ...errr... ELSEWHERE talking about this. There was mentioned a white-paper that is applicable, and if I may, here is a link to it:

http://www.fairchildsemi.com/an/AN/AN-5052.pdf

[jackson ward]

I am also confused about these 22 ohm resistors. The EMIF02-USB03F2, according to the datasheet, has 33 Ohms on the data lines internally. This would bring the total termination resistance to 13.2 Ohms. So this would not be to USB spec right?

Just completed my design by copying the STM32F4 Discovery's USB layout as closely as I could. So fingers crossed that either the STM32F401RET6 has some internal data line resistance, bringing the total to what I understand is a proper amount -- or 13.2 Ohms is actually a good value.