STM32L476 USB Termination

Rob.Riggs · ‎2016-04-12

Posted on April 13, 2016 at 04:20

Hi folks,

I am a little confused about USB termination requirements on the STM32L476 part. I read in the datasheet that termination resistors on D+/D- are not needed and that the impedance matching is done by the internal PHY (as well as pull-up).

However, I was reviewing the schematics for the STM32L4 Discovery board and it shows that termination resistors are used externally. As I would consider this a reference design, I am a bit confused by the discrepancy.

Are the 22 Ohm termination resistors needed on D+/D- with this part or not?

Thanks,

Rob

PS -- This forum software is absolutely horrible. It is impossible to tell if this question has already been asked and answered.

#usb #stm32l4-discovery-usb

Nesrine M_O · ‎2016-04-13

Posted on April 13, 2016 at 15:04

Hi Rob,

As it is mention in the STM32L476 related datasheet: no external termination series resistors are required on USB_DP (D+) and USB_DM (D-); the matching impedance is already included in the embedded driver.

Regarding the discovery schematic, the issue has been reported internally

-Syrine-

tsuneo · ‎2016-04-13

Posted on April 13, 2016 at 15:18

> I am a little confused about USB termination requirements on the STM32L476 part.

> Are the 22 Ohm termination resistors needed on D+/D- with this part or not?

Any external series resistors are not required on D+/D- lines.

In STM32L476xx datasheet (

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/DM00108832.pdf

),

''Table 89. USB electrical characteristics'' (p190) section, you'll see this note:

3. No external termination series resistors are required on USB_DP (D+) and USB_DM (D-);

the matching impedance is already included in the embedded driver.

For more confirmation, the driver impedance value on this table,

ZDRV (Output driver impedance): 28(min), 44(max) Ohm

is apparently aware of this requirement on the USB 2.0 spec,

7.1.1.1 Full-speed (12 Mb/s) Driver Characteristics (usb20_pdf)

the impedance of each of the drivers (ZDRV) must be between 28 Ohm and 44 Ohm

That is, any external resistor is supposed in the output driver design.

Tsuneo

PS.

Ah, while I'm writing my answer, the question has been answered..

Sorry for duplicated answer.

chsj · ‎2016-09-29

Posted on September 29, 2016 at 14:30

That answer really helps me cause i encountered some bad risetime in my new configuration.

However we use both FS and HS port but i cant find any about this in the HS section. Is the termination internal only for the FS.