Capacitative Input Buttons
The Digilent PmodCDC1 demonstrates capacitative-to-digital conversion through the use of Analog Devices AD7156. With its high input sensitivity and adaptive threshold capability, this Pmod is able to easily handle variance in environmental changes while still reporting accurate information.
Functional Description
The PmodCDC1 utilizes a 12-bit capacitance-to-digital converter (CDC) to determine if the measured capacitance on a particular channel (conveniently labeled as BTN1 and BTN2 on the Pmod surface) is above, below, inside, or outside the user definable threshold level(s). Users are able to read the digital capacitance level on the most recently converted channel through I2C or can simply use the output pins on header J2 if they are only interested in knowing if a threshold has been crossed or not.
Interfacing with the Pmod
The PmodCDC1 communicates with the host board via the I2C protocol. The byte consisting of the 7-bit address for the on-board chip is 0x90 (10010000) for a write command and is 0x91 (10010001) for a read command.
On power-up the Pmod enters into continuous conversion mode with both of the channels alternately reporting their data. It also uses the adaptive threshold mode where the output is active (high) only when the sensed capacitance is lower than the difference of the average measured capacitance and the sensitivity (a sensitivity of about 12 fF). Users can choose to either detect when the output of a channel crosses its threshold by “watching” its respective output pin on header J2 for a high voltage or can get more specific information by querying the device through the I2C interface.
Physical Dimensions
The pins on the main pin header are spaced 100 mil apart. The PCB is 2.3 inches long on the side parallel to the pins on the pin header and 0.8 inches long on the side perpendicular to the pin header.