Copyright © Shimmer 2016
Realtime Technologies Ltd External Expansion Board User Guide
All rights reserved Rev 1.6
8
3. Using the External Expansion Board (Typical Applications)
3.1. Connecting an Analog Output Sensor
Using the sensor specification, chose either +3, +5, or +/-5V3as the power supply connections to the
sensor (in order of preference). The output of the sensor should be tied to either the A0 or A7 ADC
inputs4. If the output is outside of the range of the Shimmer ADC (3V max) an attenuator must be
added.
Note: For Shimmer2/2r there is an issue with crosstalk on the accelerometer signals when the ADC is
provided with an un-buffered moderate to high impedance signal. To avoid such issues signals
should be buffered with an appropriate voltage buffer.
If the sensor uses the bipolar power supply, the output must be AC-coupled via a series capacitor to
an appropriate DC bias resistor network connected to the ADC input pin.
Example: A sensor requires +/-5V supply, and has a output of +/- 1.25V. A 10µF capacitor is used to
couple a signal such that 1.5V = zero-signal by connecting a 100k resistor from A0 (ADC0) to +3
(3.0V) and a 100k resistor from A0 (ADC0) to G (GND).
Any remaining GPIO pins can be used to configure control pins on the sensor. Verify the
configuration of any of these pins in the Shimmer application code.
3.2. Connecting a Digital Output Sensor using the SPI interface
Paying attention to the required master-slave relationship in your application (Shimmer is usually
master), connect up the SI,SO, and CLK pins. Use another GPIO pin as a Chip Select to complete the
digital interface. The UART0 module will need to be configured for SPI mode-- there are examples for
Shimmer2/2r in the TinyOS reference code library. For power supply connections chose either +3,
+5, or +/-5V as the power supply connections to the sensor (in order of preference).
Remember that the Shimmer's microSD storage is also connected to the SPI bus. An internal
expansion board may also use this signal. The application must ensure that the bus is correctly
multiplexed using the SPI0 chip-select signals.
Any remaining GPIO pins can be used to configure control pins on the sensor. Verify the
configuration of any of these pins in the Shimmer application code.
Note for Shimmer3 users
When using the External Expansion Board with Shimmer3, the SPI bus is shared with the microSD,
making the interface much more complicated than that for the PROTO3 Deluxe Expansion Board,
which uses the dedicated internal bus. For that reason, Shimmer recommends use of PROTO3 Deluxe
Expansion Board wherever possible. Furthermore, Shimmer recommends that the External
3Do not choose +/- 5V for Shimmer3.
4Note that for Shimmer3, the pin marked A0 on the Expansion Board is tied to the A6 ADC channel.
