The AD7156 delivers a complete signal processing solution for capacitive sensors, featuring an ultralow power converter with fast response time. The AD7156 uses an Analog Devices, Inc., capacitance-to-digital converter (CDC) technology, which combines features important for interfacing to real sensors, such as high input sensitivity and high tolerance of both input parasitic ground capacitance and leakage current. The integrated adaptive threshold algorithm compensates for any variations in the sensor capacitance due to environmental factors like humidity and temperature or due to changes in the dielectric material over time.
System: Microblaze, AXI, UART
The bit file provided in the project *.zip file combines the FPGA bit file and the SDK elf files. It may be used for a quick check on the system. All you need is the hardware and a PC running a UART terminal and the programmer (IMPACT).
If you are not familiar with LX9 and/or Xilix tools, please visit
http://www.xilinx.com/products/boards-and-kits/AES-S6MB-LX9.htm for details.
If you are not familiar with Nexys™3 and/or Xilix tools, please visit
http://www.digilentinc.com/Products/Detail.cfm?NavPath=2,400,897&Prod=NEXYS3 for details.
Extract the project from the archive file (AD7156_<board_name>.zip) to the location you desire.
To begin, connect the PmodCDC1 to J5 connector of LX9 board (see image below). You can use an extension cable for ease of use. Connect the USB cable from the PC to the USB-UART female connector of the board for the UART terminal. The board will be programmed through its USB male connector.
To begin, connect the PmodCDC1 to JA connector of NEXYS3 board (see image below). You can use an extension cable for ease of use. Connect the USB cables from the PC to the board, one for programming (Digilent USB device) and one for the UART terminal (FT232R USB UART).
Start IMPACT, and double click “Boundary Scan”. Right click and select Initialize Chain. The program should recognize the Spartan 6 device (see screenshot below). Start a UART terminal (set to appropiate baud rate) and then program the device using the bit file provided in the project *.zip archive, located in the “sw” folder (../ad7156/sw/AD7156.bit).
If programming was successful, the Main Menu will apear in your UART terminal, as seen in the picture below. There are 7 options. Pressing [d], [c], [a], [t], [r], [m], [s] or [q] key will allow you to select the desired option.
Select Channels for display allows selecting which channel(s) to display and measure.
Display Capacitance prints the selected Channel(s) measured capacitance.
Display Average Capacitance prints the selected Channel(s) measured average capacitance.
Set Threshold allows setting the threshold for each channel (feature available only in fixed threshold mode).
Set Sensitivity allows setting timeout for each channel.
Set Timeout prints Threshold mode and values for each channel.
Set Input Range allows setting the input range for each channel.
Threshold menu allows selecting between Fixed Threshold and Adaptive Threshold.
Display current settings prints Threshold mode, type and also the values for each channel.
Stop and return to menu will stop any ongoing action and will display the menu with all the available options.
The reference design is a SPI interface used to communicate with the device. The software programs the AD7156s internal registers, and afterwards reads desired data from the device and prints it via UART. Interrupts are used to turn on LEDs and start a timer (LEDs stay ON for 1 second).