WB_TSPENDOWN - Wishbone Touch Screen Pen Controller

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Figure 1. WB_TSPENDOWN - Wishbone Touch Screen Pen Controller.

The TFT LCD panel on the Desktop NanoBoard features an analog resistive touch screen. Control for the touch screen is provided through an AD7843ARU Touch Screen Digitizer (from Analog Devices). This device is essentially a programmable 8-/12-bit ADC. Input to the Digitizer is the four-wire analog output from the touch screen, which relates to the position touched on the screen.

The Digitizer provides an interrupt signal, wired to an I/O pin of the daughter board FPGA. This signal – DAU_TFT_IRQ – is sourced from the device's \PENIRQ output. This signal is kept normally High through use of an external pull-up resistance to the 3.3V power supply. When the touch screen is touched, the signal will be taken Low.

The Touch Screen Pen Controller (WB_TSPENDOWN) is a simple front-end control block used to alert a processor within the FPGA design whenever the touch screen is touched. It takes as input the interrupt signal from the Digitizer, and reflects the state of this signal in bit 0 of an 8-bit word made available on the data bus to a connected processor. The embedded code running on the processor can poll this bit and when the screen is found to be touched, enter a routine to send a control word to the Digitizer, and hence initiate conversion.


From an OpenBus System document, the Touch Screen Pen Control component can be found in the Peripherals region of the OpenBus Palette panel.

From a schematic document, the WB_TSPENDOWN component can be found in the FPGA Peripherals (Wishbone) integrated library (FPGA Peripherals (Wishbone).IntLib), located in the \Library\Fpga folder of the installation.

Designing with the WB_TSPENDOWN

Use the following links to explore use of the WB_TSPENDOWN in more detail:

See Also

For more information on the AD7843 device, refer to the datasheet (AD7843.pdf) available at www.analog.com.

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