USB Voltage: 4.93V -> normal ?

DoNcK

Hi,

I'm powering my UP board through the GPIO using an OpenCR board as a PSU. It has a 5V@4A regulator fed from a LiPo or an AC adapter.
UP board specs says it should run fine even with 5VDC@3A, which it actually does BUT I encounter connection stability issues with both the WiFi adapters I could try.
Using a powered hub: I get muuuuch less connection drops.

I investigated on voltages using a simple voltmeter with two probes and an USB voltmeter widget. Here are the results:

• 5.02V out of OpenCR (up board running)
• 5.00V on UP board GPIO powered between pin 4(5V) and 6(GND) (-> 0.02V loss along the tiny cable between the two boards -> very acceptable)
• 5.00V also measured between pin 2 and 6 of OpenCR
• 4.93V on each of the UP board USB ports (only the voltmeter widget is connected to the USB ports)
• I did not check the USB 3.0 OTG connector

Are these results within the acceptable range regarding both the USB 2.0 specifications and the actual performance that can be expected from the UP board ?

nukular

The tolerance for 5V USB is +- 0.25V, so it's fine.

DoNcK

Experimenting with a USB hub (UUGear Big7) between the Up board and the WiFi USB dongle:

https://twitter.com/jfdonck/status/1064829926690430976

My USB voltmeter widget displays 4.93V out of the USB hub, which is the same voltage than cames out of the Up board USB ports. So there is no improvement, at least on the static voltage reading.
Regarding the stability of my connections, I could see a little improvement, but still I encounter some occasional drops. Something must be wrong in the software domain.