building USB power measurement equipment

USB power measurement

USB power measurement set-up

It is generally good to know what is consumed power all the time during USB driver development, especially because power is drained from the very limited capacity USB bus. Reliable testing of power management features  without a measurement equipment is also quite impossible.

Sometimes it is nice to compare Linux driver and Windows driver from the power consumption point of view. It is good indicator that something could be wrong if your driver is eating more power on Linux than on Windows in similar use case.  Undesirable power usage is always kinda problem, especially when device is running on battery. The other problem is heating. USB devices and chips are small and there is no heat-sinks used. Over heating could cause stability problems or even break your device in worst case.

old USB power measurement set-up

Problem with that old set-up was bad cable and cheap multimeter. Cheap unshielded cable causes communication errors with some USB-bridge chips, like Afatech AF9015. Resolution of that multimeter meter was also too low, 3.5 digit / 10A scale. Use always good quality shielded cable and multimeter which could display more than 500mA in reasonable scale.

testing wiring before cover install

Could you see the mistake in that picture? .... Meter was set to DC A whilst red cord is connected to DC uA/mA hole. Unfortunately it show quite realistic values and I installed connector cover using a glue. Later I found it shows negative numbers when meter was set correctly, which means black and red cable are cross connected. It is only visual problem as you can still connect black cable to red hole and vice versa, it was still annoying me... The other small problem I found was red and black cable were not tied together. So I decided open connector again and fix those problems eventually :-)

Pictures