Preliminary
...the world's most energy friendly microcontrollers
2010-04-09 - t0005_1.10
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9 Advanced Energy Monitor
9.1 AEM Display
To enter AEM from the main page, press the pushbutton under the display labeled AEM. If the EFM
is using the display, press the AEM button at the right side of the display, and the board controller will
take control of the display and show the AEM. To return to EFM control, simply press the AEM button
once more.
When the AEM is entered, you will get a real-time graphical display of the current consumption of the
EFM and other circuits powered by the VMCU power rail. The AEM display mainly features a plot of
current the consumption over time. It also displays the latest sampled current consumption and voltage.
9.2 AEM configuration
There are several parameters that can be configured on the AEM. To configure AEM, first enter the
AEM page. Then push the button labeled CFG. In the CFG page, you can adjust the scale of the time
axis of the current plot.
9.3 AEM theory of operation
In order to be able to measure currents ranging from 0.1uA to 50mA (114dB dynamic range), two current
sense amplifiers are utilized. The amplifiers measure voltage drop over a small series resistor and
translates this into a current. Each amplifier is adjusted for current measurement in a specific range.
The ranges for the amplifiers overlap and a change between the two occurs when the current is 200uA.
To reduce noise, averaging of the samples is performed before the current measurement is presented
in the AEM GUI.
During startup of the kit and when VMCU is changed, a calibration of the AEM is performed. This
calibration compensates for the offset error in the sense amplifiers. In order for the calibration to be
correct, no load should be connected between the pins of ST6 during calibration.
9.4 AEM accuracy and performance
The Advanced Energy Monitor is capable of measuring currents in the range of 0.1uA to 50mA. For
currents above 200uA, the AEM is accurate within 0.1mA. When measuring currents below 200uA, the
accuracy increases to 1uA. Even though the absolute accuracy is 1uA in the sub 200uA range, the AEM
is able to detect changes in the current consumption as small as 100nA. The measurement bandwidth
of the AEM is 60Hz when measuring currents below 200uA and 120Hz when measuring currents above
200uA. The table below summarizes accuracy of the two current sense amplifiers in different ranges.
Table 9.1. AEM accuracy
Current range
Low gain amplifier accuracy
High gain amplifier accuracy
50mA
0.1mA
-
1mA
0.1mA
-
200uA
0.01mA
1uA
10uA
-
0.1uA
1uA
-
0.1uA
Note
In order for the AEM to work correctly, VMCU should be 3.0V or higher.
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