User manual

Low-power Application Kit (Oryx) - User’s Guide

Page 29

5.3.6 Accelerometer Sensor

The accelerometer can be placed in a sleep mode where the current consumption is 0.1uA maximum.

Read the datasheet for the accelerometer to get information about active mode and associated current

consumption. There are different sample rates that can be programmed and resulting in different

current consumption.

5.3.7 Capacitive Sensor

The capacitive sensor can be placed in a sleep mode where the current consumption is 0.1uA

maximum. The current consumption in active mode is around 10uA. Read the datasheet for details.

5.3.8 LPC11U37 Brown Out Detector

There is no voltage supervisor/reset generator on the board. The LPC11U37 has functionality for this.

Note however that when enabling the Brown Out Detector (BOD) correct consumption of the

LPC11U37 will increase with about 50uA. This is far more than the 2uA consumption on power down

mode.

5.4 How to Measure Current Consumption

There are 5 ohm resistors on the board for measuring the current consumption at three places:

1) R34 (see schematic page 6) which is placed just after the 3.0V voltage regulator. It is possible

to measure the total current consumption of the board with this resistor.

2) R13 (see schematic page 2) which is connected to the VDDCORE pin of the LPC11U37. It is

possible to measure the current consumption of the LPC11U37 core with this resistor.

3) R11 (see schematic page 2) which is connected to the VDDIO pin of the LPC11U37. It is

possible to measure the current consumption of the IO pins of the LPC11U37 with this

resistor.

a. Most LPC11U37 pins have a (programmable) pull-up resistor. It is possible to

measure the current consumption from each of these resistors. Each enabled

resistor draws about 40uA if the pin is held low.

With a 5 ohm resistor, every 10 mV equals 2mA of current consumption. The value 5 ohm is a trade-off

between measurement resolution and minimizing voltage drop. The maximum current consumption of

the board is about 17 mA, with result in a voltage drop of 85mV. That is an acceptable voltage drop

that will not affect the operation of the board.

The 5 ohm resistor is useful for measuring current consumption on the mA-region when using a normal

low-cost portable multimeter. It will however not work for measuring currents in the uA-region.

If the application switches between many different operating modes with different current consumption,

it can be difficult to measure the real consumption with a cheap multimeter. A more advanced voltage

meter might be required. For fast events and changes, an oscilloscope can with advantage be used to

measure the current consumption.

It is possible to replace the 5 ohm resistors but this shall be done with caution. The LPC11U37 will

have high current consumption during startup and ISP/bootload mode. Make sure the jumpers in JP1

and JP3 short the measuring resistors during for example flashing of the LPC11U37. When the

operating mode of interest has been reached the jumpers can be removed and the voltage drop can be

measured. If the application changes between current consumption modes, it can be impossible to get

it working properly (if the highest current consumption will cause too high voltage drop over the

measuring resistor).

5.4.1 How to Achieve and Measure 2.5uA Current Consumption

2.5uA will generate a 7.5uV voltage drop over a 5 ohm resistor. This is too small to measure reliable

without a very high precision (and high cost) voltage meter. A solution is to replace R34 with a 5K ohm