Mobile Device Power Monitor Manual - GitHub Pages

26
Functional Description of the Power Monitor
To measure the current accurately, a dual range, self-calibrating, integrating system is
used. Each channel has two current ranges with a 16-bit analog-to-digital converter (ADC),
one with a high-resolution range, and the other with a low-resolution range. Software
continuously calibrates each of these and selects the proper range during measurement.
The dual range scheme works because mobile devices are usually in standby mode and
drawing only a few milliamps of current, or they are running above 100 milliamps. The
Power Monitor must be very accurate when the current is low, but may be less accurate as
the current increases. Each sample is integrated over its 200-microsecond sample period so
that even a brief high-current pulse is captures. Depending on system capacitance and
other factors, the fastest transient pulses are around 20 microseconds. The integrator sums
up these fast pulses so that an accurate assessment of the average current is maintained.
The unit is self-calibrating. One cycle out of every 1000 cycles is used to run either a zerocurrent
calibration or a reference-current calibration. Software uses these measurements to
null out the offset and gain errors in the system. Because this is done automatically, it
compensates for slow temperature changes during the measurement run.
The only part not included in the self-calibration is the sense resistor. These resistors are
calibrated at the factory and the calibration values may be adjusted and saved by engineers
using the Parameters dialog box. The Power Monitor has an overflow buffer that can hold six
packets of 128 bytes each. During transfer, if the development workstation cannot read the
data fast enough then this buffer will begin to fill up, and samples may be lost. The received
data is concatenated so the line appears continuous. To account for the lost samples, the
protocol contains a 16-bit counter that tracks the number of lost samples. The running total
of lost samples is displayed in the UI, as shown in Figure 26 below.
Figure 26: Counter for dropped samples and dropped connections
Because the dropped samples are less than one percent of a run, the error is minimal.
Usually this loss of data is less than 0.1 percent. However, sometimes on long runs or on
loaded systems the dropped sample count can exceed 65,000. When this occurs, the Power
Tool lost communication for more than 10 seconds. A count of dropped connections is
maintained in the UI, as shown above in Figure 26. Both of these counts are reset to zero
when a run is restarted. These counts are useful to determine how stable the connection is.
If the dropped packets are greater than one percent of all samples, or if there are more
than one or two dropped connections per hour, then it is likely that the development
workstation has other applications running that are causing delays. Closing these
applications or installing the Power Tool software on a clean machine should solve these
issues and reduce the dropped samples and packets.