Capacitive Touchpad Using MCU GPIO Charge Time Analysis

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Solution Overview

Problem

Existing touch-sensitive interfaces in electronic devices are costly due to the requirement of specialized circuits for translating user motion and position to screen coordinates.

Innovation Solution

A method utilizing microcontroller GPIOs connected to sensors that vary in capacitance to detect touchpad contact locations by measuring voltage charge times, with a resistor-capacitor network and firmware code to analyze charge times and map them to a two-dimensional contact location.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If specialized circuits are used for touch-sensitive interfaces, then measurement precision and reliability are improved, but device complexity and cost increase

Engineering Contradiction:

Improvetouch detection accuracyVSAvoidcircuit complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent replaces specialized touch detection circuits with a microcontroller that uses general-purpose I/O pins and software-based capacitance sensing algorithms. The microcontroller measures capacitance changes through charge/discharge timing of RC networks formed by pull-up resistors and parasitic capacitances, eliminating the need for dedicated touch controller hardware while achieving comparable measurement precision.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The invention makes general-purpose microcontroller I/O pins perform the specialized function of touch sensing. By configuring standard GPIO pins as inputs with internal pull-up resistors enabled, the microcontroller uses the same hardware resources for multiple purposes (general I/O and touch detection), thereby reducing overall device complexity and component count.

Inventive Principle:

Principle #6Universality (Multi-functionality)

2Reliability

If specialized circuits are used for touch-sensitive interfaces, then reliability is improved, but cost increases

Engineering Contradiction:

Improvetouch detection reliabilityVSAvoidmanufacturing cost

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The patent employs inexpensive standard microcontroller components (GPIO pins with pull-up resistors) instead of expensive specialized touch controller ICs. The solution uses software-based detection algorithms that run on commodity microcontrollers, significantly reducing bill of materials cost while maintaining sufficient reliability for consumer applications through careful signal processing and threshold detection.

Inventive Principle:

Principle #27Cheap short-living objects (Disposable)

Solution Approach 2:

The invention substitutes hardware-based specialized circuits with a software-based detection system running on a standard microcontroller. The reliability is maintained through careful measurement of RC time constants and comparison against threshold values, with the software able to adapt to different touch conditions and compensate for environmental variations.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Device complexity

If standard microcontroller components are used instead of specialized circuits, then device complexity and cost are reduced, but measurement precision may deteriorate

Engineering Contradiction:

Improvecircuit complexityVSAvoidtouch detection accuracy

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The patent performs preliminary calibration by measuring and storing threshold values during system initialization or manufacturing. These pre-determined thresholds account for variations in parasitic capacitances and are used during operation to accurately distinguish between touched and untouched states, compensating for the lower inherent precision of standard microcontroller components.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The system continuously monitors capacitance changes and compares measured values against stored threshold values. The microcontroller adjusts its detection logic based on the difference between measured capacitance and threshold, enabling accurate touch detection despite using standard components with lower inherent precision compared to specialized circuits.

Inventive Principle:

Principle #23Feedback

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This approach reduces costs by using standard microcontroller components to accurately determine user contact locations on a touchpad, providing a cost-effective and efficient touch-sensitive interface.

Implementation Method 1

sensors that vary in capacitance based upon whether a user touches the sensors

Methodology Applied

Scientific EffectCapacitance: Capacitance

Implementation Method 2

measuring voltage charge times, with a resistor-capacitor network

Methodology Applied

Scientific EffectRC time constant:

Data Source

PatentUS8212159B2Capacitive touchpad method using MCU GPIO and signal processing

Publication Date: 2012.07.03 NXP USA INC

AI summary

A microcontroller identifies a maximum x-axis charge time and a maximum y-axis charge time, along with x-axis sensor locations and y-axis that correspond to the maximum charge times (maximum charge time sensor locations). Next, the microcontroller identifies sensor locations that are adjacent to the maximum charge time sensor locations and computes adjacent charge time ratios based upon their corresponding charge times. In turn, the microcontroller uses the largest adjacent charge time ratios and the maximum charge time sensor locations to compute and map an x-axis position and a y-axis position of a user's contact location on a two-dimensional grid.