
REF: BB_SRM_xM
BeagleBoard-xM System
Reference Manual
Revision C2
Page 53 of 163
7.4
Meter Current Measurement
Jumper
J2
is a header that allows for the voltage drop across the resistor to be measured
using a meter, providing a way to measure the current consumption of the BeagleBoard
from the main voltage rails, either USB or DC. The resistor,
R13
, is a .1 ohm resistor
across which the voltage is measured. The reading you get is .1mV per mA of current.
You will need to make sure you have a sensitive meter to make your measurements.
Please keep in mind, that this current reading does not include any current consumed by
the USB HUB, USB ports, or the Expansion headers.
7.5
Processor Current Measurement
The resistor across
J2
can also be used to measure the current of the board by reading the
voltage drop across
R13
from software. There are two pairs of resistors provided on the
TPS65950
that measure the voltage on either side of
R13
. This is done via the I2C
control bus to the
TPS65950
from the processor. These values along with resistance of
R
13 are used to calculate the current consumption of the board.
Figure 24
is the
schematic of the measurement circuitry. The maximum value that can be input to the
ADC inputs is based on the setting of the
VINTANA2.OUT
voltage rail which defaults
to 2.5V. In order to prevent the voltage levels from exceeding this value a pair of resistors
of
12K
and
10K
is used to scale the voltage down.
C83
0.1uF,10V
R52
10K,1%
C84
0.1uF,10V
R53
10K,1%
ADCIN5
ADCIN3
R48
12K,1%
R49
12K,1%
VBAT_FB
VBAT
R14
56.2K,1%
R15
22.6K,1%
C5
10uF,CER,0805,6.3V
DC_5V
4.2V
VBAT_MAIN
C7
0.1uF,10V
R13
.1,0805
+
J2
HDR2_.1x.1
1
2
U3
TL1963A
IN
2
SHDN
1
OUT
4
ADJ
5
GND
3
GND
6
U7A
TPS65950
CTSI/BERDATA/ADCIN3
P11
RTSO/CLK64K/BERCLK/ADCIN5
N11
Figure 23.
Processor Current Measurement
This results in a value that is 46% of the actual value. So, for a maximum value of 5.25V,
the voltage read would be 2.415V which keeps it below the 2.5V point. The voltage drop
across
R13
will be small as the value of the resistor is 0.1 ohms. For every 100 mA of
Summary of Contents for XM C2
Page 152: ...REF BB_SRM_xM BeagleBoard xM System Reference Manual Revision C2 Page 152 of 163 ...
Page 153: ...REF BB_SRM_xM BeagleBoard xM System Reference Manual Revision C2 Page 153 of 163 ...
Page 154: ...REF BB_SRM_xM BeagleBoard xM System Reference Manual Revision C2 Page 154 of 163 ...
Page 155: ...REF BB_SRM_xM BeagleBoard xM System Reference Manual Revision C2 Page 155 of 163 ...
Page 156: ...REF BB_SRM_xM BeagleBoard xM System Reference Manual Revision C2 Page 156 of 163 ...
Page 157: ...REF BB_SRM_xM BeagleBoard xM System Reference Manual Revision C2 Page 157 of 163 ...
Page 158: ...REF BB_SRM_xM BeagleBoard xM System Reference Manual Revision C2 Page 158 of 163 ...
Page 159: ...REF BB_SRM_xM BeagleBoard xM System Reference Manual Revision C2 Page 159 of 163 ...
Page 160: ...REF BB_SRM_xM BeagleBoard xM System Reference Manual Revision C2 Page 160 of 163 ...
Page 161: ...REF BB_SRM_xM BeagleBoard xM System Reference Manual Revision C2 Page 161 of 163 ...