3.1 Resistive touch: a press becomes a voltage

Slide 16 of 157 · Resistive touch: a press becomes a voltage

Welcome to chapter 3: Resistive touch: a press becomes a voltage. I'm Diggy Breiling with FPD.DEV, The Future of Display and Systems Development. Our goal is to explain four-, five- and eight-wire measurement, calibration, and why reporting resolution is not accuracy.

Slide 17 of 157 · The voltage-divider mechanism

Two conductive surfaces face each other across a small separation. Pressing the flexible top surface makes contact. In a four-wire panel, the controller drives a voltage gradient along one layer and uses the other as a voltage probe. It then swaps roles to measure the other axis. For an ideal uniform sheet, x/L = Vtouch/Vdrive. With a 100 mm span, a 3.0 V gradient and a 1.2 V sample, the inferred position is 40 mm from the zero-voltage edge. This calculation assumes a high-impedance measurement and ignores edge-field distortion, contact resistance and settling. The animation separates pressing, X acquisition, Y acquisition and release. Real controllers must allow switching transients to settle and reject contact bounce.

Slide 18 of 157 · Why the wire count matters (1 of 2)

A five-wire design drives both coordinate gradients on the lower resistive layer. The upper layer senses voltage rather than defining an axis gradient. This changes how upper-sheet wear affects measurement. Eight-wire panels add sensing connections to a four-wire arrangement to help compensate drive-path voltage drops; they are not eight-touch panels. Analog panel wiring is controller-specific. Identify electrode names and connector orientation from the actual drawing; never infer a pinout from wire count. A controller may deliver raw samples over SPI or I2C, while another adds a USB interface and HID firmware.

Slide 19 of 157 · Why the wire count matters (2 of 2)

Conventional analog resistive is often used for single touch. Specialized controllers and panel structures can support additional contacts; verify the exact supported gestures rather than claiming all resistive systems are single-touch.

Slide 20 of 157 · What to validate

The practical trade is reliable physical actuation versus the force, wear and optical consequences of the compliant stack. Test the intended glove, rounded stylus, diagonal drag and edge targets. Compare the first and last taps after temperature exposure and repeated use. Calibration estimates a mapping between sensor coordinates and displayed targets. A three-point affine fit can correct translation, rotation, scale and shear, but cannot repair arbitrary nonlinear distortion or intermittent contact. Use independent validation points after fitting. For the voltage-divider example, a 12-bit converter has 4,096 codes: the nominal 100 mm/4,095 code spacing is about 0.0244 mm. This is quantization, not a claim of 0.0244 mm touch accuracy.

Slide 21 of 157 · Apply the idea

A 150 mm ideal axis is driven with 3.3 V and sampled at 2.2 V. Find x. Explain why an accurate answer does not prove an accurate panel. Pause here and consider your answer. x = 150 x 2.2/3.3 = 100 mm. Sheet uniformity, calibration, noise, mechanics and the actual contact location still affect accuracy.

Slide 22 of 157 · Chapter 3 complete

Complete the chapter exercise and review any open questions. Next is Chapter 4: Surface capacitive: current paths across a sheet. I'm Diggy Breiling with FPD.DEV, The Future of Display and Systems Development. Thank you for joining me.

x = 150 x 2.2/3.3 = 100 mm. Sheet uniformity, calibration, noise, mechanics and the actual contact location still affect accuracy.

Accuracy

Closeness of a reported coordinate to a defined reference position.

Precision

Repeatability or spread of results under specified conditions.

Resolution

Smallest coordinate increment or representable change; not equivalent to accuracy.

Latency

Time between explicitly defined input and output events.

PCAP

Projected capacitive touch; patterned electrodes sense electrical coupling changes.

Self-capacitance

Measurement of an electrode relative to an electrical reference.

Mutual capacitance

Measurement of coupling between transmit and receive electrodes.

Baseline

Reference measurement used to identify changes associated with touch.

SNR

Signal-to-noise ratio; specify the signal, noise measure and conditions.

SAW

Surface acoustic wave sensing through controlled propagation on a surface.

APR

Acoustic pulse recognition; a touch-generated mechanical response is used to locate contact.

DST

Dispersive signal technology; a bending-wave sensing implementation.

EMR

Electromagnetic resonance, used by compatible pen digitizers.

Haptics

Tactile feedback produced by the system.

HID

Human Interface Device class/protocol family; the descriptor defines report interpretation.

Report descriptor

Data describing the structure and meaning of HID reports.

Contact ID

Identifier used to distinguish and track a contact.

I2C

Two-wire synchronous bus; raw I2C communication does not itself implement HID.

SPI

Synchronous serial peripheral interface, with device-specific protocol behavior.

UART

Asynchronous serial hardware; electrical levels and framing must be specified.

Optical bonding

Joining optical layers with a transparent material to manage the interface.

Parallax

Apparent relative position shift caused by viewing separated planes from an angle.

NVIS

Night Vision Imaging System; pronounce 'en-vis' in this course.

Qualification

Evidence against a defined set of requirements for an identified configuration.

Validation

Evidence that the system satisfies its intended use.

Verification

Evidence of conformance to specified requirements.

Ghost touch

Reported contact that does not correspond to an intended physical touch.

Palm rejection

Classification or suppression of unintended hand contact.

On-cell / in-cell

Touch structures at a display-cell surface / integrated within the display structure; verify supplier construction.

Touch plane

Geometric region in which the sensing system detects an object.