Firefox OS/Performance/Power/Sample Delay Calibration of COTS Ammeter

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Calibrating for Initial Sample Dealy of COTS Ammeter

The Yoctopuce Ammeter we're using for the initial baselining of power consumption of Firefox OS has a small delay before we receive the first sample over the USB connection. This page documents the method used to measure the delay so that we can use it to time-corelate the power usage data with other profiling data for analysis.

Setup and Methodology

I used a Goodfet digital I/O peripheral to toggle on a digital I/O pin and then measure the delay until the ammeter measures current. I wrote some custom firmware for the Goodfet and a python client that interfaced with the yoctopuce ammeter to measure the delay between the goodfet pin going high and the ammeter seeing a measurable increase in current.

The code work is in the fxos branch of my fork of the goodfet repo.

Data and Results

To get a good idea of what the expected delay to first sample will be for subsequent power consumption test runs, I ran 4 trials, each taking 100 measurements. I merged the data into File:Ammeter delay measurements.xls. The important numbers to come out of this measurement are:

Mean: 31.76 ms
Std Dev: 6.20 ms
Variance: 38.47 ms

I think we can assume a 32 ms delay when time correlating the power usage samples with profiling data.