Why is it a voltage divider as configuration for the USB2514B in STM32MP157 devkit?

[DMårt]

Why is it a voltage divider as configuration for the USB2514B in STM32MP157 devkit?

I can see in the data sheet that there are two bus configurations abilities.

• I2C EEPROM
• SMBus

I don't know which one should be used. Let's look at the data sheet.

This explains that the pins CFG_SEL[0] and CFG_SEL[1] needs to be low or high before the pin RESET_N goes low.

And the reset can be done through the USB differential lines.

I don't understand this construction. I know that I2C should have a pull-up resistor of 4.7 kOhm, but here we have a 100 kOhm on both GND and 3V3, which makes this a voltage divider.

And the data sheet says that 10 kOhm should be the value for pull-up resistors.

Question:

What is the right configuration here if I want to connect my USB hub to my I2C1 ? I'm using I2C1 instead of I2C4 because the HDMI transmitter and PMIC are sharing the I2C4 bus. The reson is that I think this USB hub might use SMBus and PMIC must use I2C4 according to AN5031, so I2C1 must be unique for one I2C device.

Can you also explain the choice of 100 kOhm, and not 4.7 kOhm as the standard?

[PatrickF]

HI @DMårt 

did you look at right schematics ?

Regards.

[DMårt]

Hi @PatrickF

DNF means Does Not Fit. Does that mens that it could not fit the HW design, or it was removed? So in this case, it's only 100 kOhm pull-downs?

I'm looking at the en.mb1272-dk1-c03-schematic.pdf schematics.

STM32MP157D-DK1 - Discovery kit with STM32MP157D MPU - STMicroelectronics

[PatrickF]

Yes, DNF mean Do Not Fit, i.e. component is not populated during board assembly.

The Bill Of Material is the reference for the 'not fitted'

Regards.

[DMårt]

@PatrickF 

Were do you find that schematic? 

I don't find it here: STM32MP157D-DK1 - Discovery kit with STM32MP157D MPU - STMicroelectronics

So red cross (X) and DNF are the same?

Does it mean that they are removed completly, or just a track right over?

EDIT:

Ok! It was the DK2 version of the dev-kit. STM32MP157F-DK2 - Discovery kit with STM32MP157F MPU - STMicroelectronics

So if red cross means not assembled, that means I2C1 have a track instead of the solder bridge.

And this solder bridge is removed...or is it a track as well to GND?

Example:

If R73 is not fitted, but HSE_OUT must be connected to EN. Does that means C87, which is also not fitted, should be replaced as a track so EN will be grounded? That does not seems right. 

Hard to tell what is right here. If HSE_OUT should be pulled down with 1 kOhm, or connected to EN.

But I guessing that it going to look something like this

[PatrickF · Accepted solution]

Hi @DMårt 

Red cross (read it as 'cutout') and DNF is the same, mean the component placed on the PCB is not fitted, i.e. circuit is OPEN (This could explain the mistake you made around HSE interpretation, which is now OK on your latest proposal).

Same for Solder Bridges (SBxx), when not fitted or greyed, it mean circuit is OPEN.

Usually DNF components or solder bridges are options to allows some specific modes trials or different board behavior.

Regards,

[DMårt]

@PatrickF 

So what you're telling me, is that I don't need I2C connection from STM32MP151 to the USB Hub? 

All the communiction will be done through the USB_FS_HS ?

By the way, are the 100 kOhm resistor really needed? Can't I use 10 kOhm instead?

[PatrickF]

Hi, for resistor value and I2C needs, please check your requirements and the datasheet of the related component.

My understanding is that we could use the HUB without any SW configuration.

Regards.

[DMårt]

@PatrickF 

Yes!

My understanding is the same. I2C is not needed here because if SDA and SCL is pulled low with a 47 to 100 kOhm resistor. I think a suitable resistor value for I2C is 4.7 kOhm, low as high.

By turning SDA and SCL low, the configuration is default.