What does USBPHYC Tuning register HDRXGNEQEN do?

[Parawizard]

It only gives the bit and the description:

 

Bit 22 HDRXGNEQEN: Enable HS Rx gain equalizer 0: Disable the gain equalizer 1: Enable the gain equalizer

 

 

But what does it actually do? When would you want to use it?

 

Thanks

 

[FBL]

Hi @Parawizard 

Multiple tuning options like Gain Equalizer are present for better adjustment of analog blocks according to package and board environment. They might be useful in case of loss of signal strength or noise etc. So, in my understanding, it amplifies the received signal to restore its original amplitude, making it easier for the receiver to correctly interpret the data. AFAIK, all the tuning bits are not timing critical.

[Parawizard]

Thanks for the reply. Usually these have some kind of configurable EQ profile or at least an EQ profile. I haven't been able to find any additional information at all about it. Seems weird that it has no other configuration bits and no explanation about what curve it uses or how it works otherwise.

I don't really know what it's doing with the RX signal inside the PHY.

[FBL]

Hi @Parawizard 

I am not aware of this EQ profile. Could you provide an example?  

[Parawizard]

I mean in general. You would want to know what it's actually doing to the frequency before you enable it? It's off by default. Circumstances that you would use it don't seem to be documented nor it's effect on the signal. It's possible that the equalization could overshoot or other negative effects in certain scenarios? Redriver chips from TI have compensation curves and equalization but it's quite configurable. I am not sure how I would verify that turning on RX Gain Equalization has a positive or negative or any effect on my device.

[FBL]

Hi @Parawizard 

Check this wiki to adjust settings using Eye diagram

https://wiki.st.com/stm32mpu/wiki/How_to_adjust_USBPHYC_settings_using_USB_Eye_Diagram#Link_between_register_values_and_the_device_tree_description

 

[Parawizard]

Yes. Thanks for the link. I have already read it all before. Unfortunately it doesn't explain the equalization at all.

[FBL]

Hi @Parawizard 

We are still waiting for a confirmation whether new section will be included in AN4879 to include some recommendations for fine tuning USBPHYC (internal ticket 204229)