Tip: Make the USB-OTG connection more stable

[grixm]

I have long struggled with a problem with my Rock S0: If using the USB OTG port in device mode, the connection would suddenly drop randomly, minutes or sometimes hours after initialization. I spent months on and off looking for solutions, and I finally found one, so here it is if you ever find yourself having the same problem.

Basically, it is caused by the SoC being very sensitive to voltage drops before the USB controller interprets it as the connection being lost. It's far more sensitive than it needs to be, given that my device is bus-powered and so the OS never actually needs to worry about a connection loss. If the connection is lost it is because I have unplugged the cable and so the device would be shut off regardless.

Anyway, the solution lies deep within the RK3308's control registers, where I found the following: Three bits named "B_validsession reference tuning". There's no other explanation on what these bits does in the manual, but it sounded vaguely promising since I had been able to establish that "b-session valid" is actually the name of the status/interrupt that the vbus voltage affects in OTG device mode. Other registers related to the b-session valid status didn't work, however, including filter times, interrupt masks, force high bits, etc. But luckily this one register finally did the trick.

The default value of these three bits are 000. Without any documentation I basically just had to guess what to do, so I tried setting them to 111 instead. And that seemed to work. It significantly lowered the voltage on the vbus pin required to trigger a disconnect signal. My connection was finally stable.

Here's how you can do it yourself. First install memtool via apt, then run: 

memtool mw 0xFF008018 0x1C001C00

 

[laibsch]

Thank you for sharing.  Would be good to have your findings corroborated by some of official documentation.  Thanks again for sharing.