Capacitive Touch Sensing Design Guide
Capacitive touch sensing requires careful hardware and software design. This guide covers best practices for Hangshun Touch Control MCUs.
Touch Sensing Fundamentals
capacitive Touch Principle: Sensor pad forms capacitor with ground; Finger touch adds capacitance; MCU measures capacitance change; Threshold detection determines touch.
Hangshun Touch Features: 6-24 configurable channels; Water rejection technology; Self and mutual capacitance modes; Integrated touch library.
Sensor Design
button Design: Circular pads: 8-12mm diameter; Square pads: 8x8mm to 12x12mm; Keep-out from ground: 0.5-1mm; Trace width: 0.2-0.3mm.
Slider Design: Interlocking triangles; 3-5 channels for linear slider; 4-8 channels for circular wheel; Calculate position by interpolation.
Overlay Materials: Glass: Up to 6mm with proper tuning; Plastic/Acrylic: Up to 5mm; Uniform thickness critical; No air gaps between sensor and overlay.
PCB Layout Guidelines
Layer Stackup: Top layer: Sensor pads and traces; Inner layer: Ground plane with hatched pattern; Bottom layer: Components and routing.
Trace Routing: Keep traces short (<100mm); Route on opposite side from touch pads; Maintain 0.5mm spacing between traces; Use guard rings for noise reduction.
Ground Design: Solid ground on layer below sensors; Hatched ground directly under sensors (20% fill); Connect guard ring to driven shield.
Water Rejection
water Rejection Technology: Hardware: Guard ring and driven shield; Software: Multi-channel water detection; Dynamic threshold adjustment.
Implementation: Enable water rejection in touch library; Configure sensitivity for wet conditions; Test with actual water exposure scenarios.
Tuning and Calibration
Sensitivity Tuning: 256 levels per channel (0-255); Higher value = higher sensitivity; Target SNR > 10:1; Auto-calibration at startup.
Threshold Settings: Touch threshold: ~80% of signal; Release threshold: ~60% of signal; Hysteresis prevents oscillation.
Environmental Compensation: Auto-calibration handles drift; Periodic baseline updates; Temperature compensation.
Testing and Validation
Test Procedures: Sensitivity with actual overlay; Water rejection testing; ESD immunity (±8kV contact); Noise immunity in target environment.
Common Issues: False triggers �?Reduce sensitivity, add filtering; Missed touches �?Increase sensitivity, check overlay; Water issues �?Enable water rejection, check sealing.
Development Tools
touch Tuning Tool: Real-time signal monitoring; Sensitivity adjustment; Export configuration; Graphical visualization.
Development Kits: Pre-tuned reference designs; Example touch patterns; Documentation and guides.
💡 FAE Insights
📋 Customer Cases
Appliance Manufacturer
Home Appliances
Challenge
Customer experienced false triggers and missed touches in high-humidity washing machine environment
Solution
FAE guided implementation of water rejection algorithms and optimized PCB layout with guard rings. Tuned sensitivity for wet conditions.
Customer Feedback
"The water rejection technology worked perfectly - no false triggers even with water spray. Customer satisfaction improved significantly with the new touch interface."
Frequently Asked Questions
1. How many touch channels do I need for my application?
Channel count depends on interface design: Simple buttons - One channel per button; typical appliances need 4-12 buttons. Sliders - 3-5 channels for linear; 4-8 for circular wheels. Keypads - Can use matrix to reduce channels; 3x4 keypad uses 7 channels. Proximity - 1-2 channels for wake-on-approach. Add 20% margin for future expansion. Hangshun offers 6, 12, 16, and 24 channel variants.
2. What overlay materials work with Hangshun touch MCUs?
Supported overlay materials: Plastic/Acrylic - Best sensitivity; up to 5mm; lowest cost. Glass - Premium appearance; up to 4mm; requires higher sensitivity. Ceramic - High-end; up to 3mm. Wood - Natural look; up to 2mm; moisture sensitive. Material properties: Higher dielectric constant = better coupling; Thicker = lower signal; Uniformity critical. Water rejection enables operation even with water droplets.
3. How do I implement water rejection for kitchen appliances?
Water rejection implementation: Hardware - Guard ring around pads connected to driven shield; Proper overlay sealing; Drainage channels. Software - Enable water rejection algorithm; Set sensitivity for wet conditions (150-200); Configure multi-channel detection. Testing - Test with water droplets, spray, continuous flow; Verify with soapy water; Test temperature range. The HS32T024RGT6 includes enhanced water rejection with submersible detection.
4. What is the power consumption of touch sensing?
Touch power consumption: Active scanning - 50-100μA per channel at 10Hz; depends on sensitivity and overlay. Sleep with wake-on-touch - 5-10μA total; MCU in deep sleep. Wake response - <100ms from sleep to active. Optimization - Reduce scan frequency in standby; Use proximity wake-up; Disable unused channels. Example: 6-button remote with wake-on-touch: <10μA standby.
5. How do I design touch sensor PCBs?
PCB design guidelines: Sensor shapes - Circular pads 8-12mm for buttons; Interlocking triangles for sliders. Trace routing - Keep <100mm; Route on opposite side from pads; 0.2-0.3mm width; 0.5mm spacing. Ground plane - Solid on layer below; 0.5-1mm gap to sensors; Hatched directly under sensors. Shielding - Guard ring around sensors; Driven shield for water rejection. Components - Place MCU close to sensor array; Minimize vias in traces.