USB impedance matching for STM32F103C6

[pgregson1]

Posted on January 24, 2014 at 15:45

Hello,

The USB specification for Full Speed requires 1.5Kohms from D+ to 3.6 volts, 15Kohms from D+ to ground, and 90 ohms between D+ and D-.  What matching resistors are required to achieve this with the STM32F103C6?  The data sheet does not suggest values, and it does not provide the input impedance of each of D+ and D- nor does it provide the differential impedance between D+ and D-.Regards,Peter 

[ilmars]

Posted on January 26, 2014 at 17:22

What matching resistors are required to achieve this with the STM32F103C6?

Sometimes reference design or development board user manual, schematics & BOM can answer unanswered by datasheet, questions like this. Did you try?

[mregina]

Posted on February 05, 2014 at 23:34

Here's how I worked through it for STM32F205:

http://www.st.com/web/en/resource/technical/document/datasheet/CD00237pdf

hints that pull-ups and/or pull-downs on DM and DP are internal. No info on driver output impedance.

http://www.st.com/web/en/resource/technical/document/reference_manual/CD002257pdf

explicitly states that pull-ups and pull-downs are internal in section 3.2. No info on driver output impedance.

http://www.st.com/web/en/catalog/tools/FM116/SC959/SS1532/PF250374

schematic shows 22Ω for output impedance matching resistors; however, there is also an

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD000428pdf

which has 33Ω itself (ends up 22Ω in parallel with 33Ω, inspected and both populated on eval board) . . . doesn't make sense!

http://www.usb.org/developers/docs/usb20_docs/

states min. of 28Ω and max. of 44Ω for output impedance of

Full-Speed only driver in section 7.1.1.1.

Datasheets for some other STMicroelectronics products that are USB FS transceivers, such as

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00144pdf

, state that the driver internal output impedance is a min. of 8Ω and a max. of 24Ω and the application example shows an external20Ω impedance matching resistor in DP and DM . . . now this makes sense! My determination -- yours may differ -- is based on the assumption that the USB FS driver output impedance is similar to that of their other products and that the eval board has a bit of a snafu. For the STM32F205: 1. Internal pull-ups and/or pull-downs on DP and DM 2.Needs 20Ω for impedance matching on DP and DM.

https://my.st.com/public/STe2ecommunities/mcu/Lists/cortex_mx_stm32/Flat.aspx?RootFolder=https://my.st.com/public/STe2ecommunities/mcu/Lists/cortex_mx_stm32/STM32F4%20USB%20resistor%20confusion&FolderCTID=0x01200200770978C69A1141439FE559EB459D7580009C4E14902C3CDE46A77F0FFD06506F5B&currentviews=609

helpedto point the way. STMicro should rev the datasheet to explicitly state the USB FS PHY output impedance.