Digilent 410-247P-KIT - Manual

PmodCDC1™
Capacitance-to-Digital Converter
Reference Manual
Revision: October 4, 2012
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Doc: 502-247 page 1 of 2
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Overview
The PmodCDC1 demonstrates capacitance-to-
digital proximity sensing through two “buttons.”
Digilent engineers designed and built this
Pmod around the Analog Devices AD7156
Capacitance – to – Digital Converter.
Features include:
 I 2 C™ communication interface
 two capacitance input channels
 adaptive proximity threshold
Functional Description
The PmodCDC1 has two capacitive buttons for
user input and two output pins on header J2
that signal when they reach a proximity
threshold. (See Table 1) The PmodCDC1
measures this threshold in capacitance, the
users can program the threshold, and it can be
either positive or negative. A logic high output
on header J2 will indicate which channel has
reached its threshold.
Table1. Interface Connector Signal Description
Capacitance to Digital Conversion
The PmodCDC1 uses the AD7156 to measure
the capacitances of the two channels
separately by exciting the capacitance and
sampling the response. It stores data as a 16-
bit word that you can read by accessing the
correct data registers via I
2
C in the AD7156.
The PmodCDC1 averages the capacitance of
each channel, which allows users to configure
the PmodCDC1 to adapt its threshold level.
The adaptive threshold is a programmable
feature of the AD7156.
For more detailed information on controlling
the AD7156, please see the device’s data
sheet, available from
www.analog.com .
I
2
C Interface
The Analog Devices AD7156 acts as a slave
device using I
2
C communication protocol. The
following instructions provide procedures for
reading and writing to the device.
The host device is the master when reading
from the PmodCDC1. If the host device
acknowledges the receipt of data, then the
AD7156 will auto increment the address
pointer and send the next byte of data in its
memory. The host device must assert a No
Acknowledge
to the PmodCDC1 to cease
receiving data.
When reading a conversion value, assert an
Acknowledge
between the first and second
byte in order to receive the bytes back to back.
Receiving them back to back will ensure that
data is not skipped between conversions.
Writing to the device is similar to reading from
it. After sending the first data byte, the address
pointer in the AD7156 will auto increment. If
the master sends another byte, then the
Connector J2 – AD7156 Output Pins
Pin Signal Description
1
OUT1 Threshold detected on CIN1
2 OUT2 Threshold detected on CIN2