Mystery FPGA Circuit Feels the Pressure

You have an FPGA circuit and you want the user to interact with your circuit by pushing a button. Clearly, you need a button, right? Not so fast! [Clifford Wolf] recently found a mysterious effect that lets him detect when someone pushes on his iCEstick board.

The video below shows the mystery circuit (which is just the stock iCEstick board), which appears to react any time you flex the PC board. The Verilog implements a simple ring oscillator (basically an inverter with its output tied to its input).

Here’s an excerpt from his Verilog code:

// ring oscillator

wire [99:0] buffers_in, buffers_out;
 assign buffers_in = {buffers_out[98:0], chain_in};
 assign chain_out = buffers_out[99];
 assign chain_in = resetn ? !chain_out : 0;

SB_LUT4 #(
 .LUT_INIT(16'd2)
 ) buffers [99:0] (
 .O(buffers_out),
 .I0(buffers_in),
 .I1(1'b0),
 .I2(1'b0),
 .I3(1'b0)
 );

He compares the output frequency to a known frequency from the onboard crystal oscillator and registers large shifts as a push. At first we thought it might be mechanical flex on the crystal, but if you watch the video, the output of the ring oscillator is clearly shifting on a touch. Presumably (and this is a guess), small changes in the capacitance and other circuit parameters have an effect on the ring oscillator’s frequency. [Clifford] says he has many theories, but doesn’t know which one (if any) is correct.

We can’t decide if we’d depend on this effect or not in practice. But it does seem repeatable in the video. Maybe [Clifford] has invented a new product category: the Field Programmable Gate Array and Switch (FPGAS). We’ve mentioned [Clifford’s] work before on project iCEstorm and even used his open source tool chain in our FPGA tutorial (look for another one soon, by the way).

Thanks for the tip [James Bowman].

Filed under: FPGA