Encoders

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Overview

Encoder blocks provide hardware-based position and velocity measurement using quadrature encoders, essential for motor control and motion feedback applications. The MPLAB Blockset offers dedicated encoder interfaces (QEI, PDEC, QDEC) that decode incremental encoder signals with zero CPU overhead, enabling precise position tracking and velocity calculation for motor control feedback loops.

Available Encoder Blocks

QEI - Quadrature Encoder Interface for dsPIC

dsPIC30F dsPIC33F dsPIC33E dsPIC33C

Dedicated QEI peripheral for dsPIC with hardware quadrature decoding, velocity measurement, and index pulse support. - **Hardware Quadrature Decoding**: No CPU overhead for edge counting

PDEC - Position Decoder for SAM ARM

SAM

Advanced position decoder for SAM with angular position tracking and configurable filtering. - **Angular Position Output**: Direct angle calculation (0-360°)

QDEC SAMx - Quadrature Decoder using TC for SAM

SAM

Timer/Counter-based quadrature decoder for SAM devices without dedicated PDEC. - **TC Module Quadrature Mode**: Uses Timer/Counter peripheral

ChangeNotification - Software Encoder Decoding

dsPIC30F dsPIC33F dsPIC33E dsPIC33C PIC32

GPIO interrupt-based encoder decoding for devices without dedicated QEI/PDEC peripherals. - **Software Decoding**: Interrupt-driven quadrature state machine

Block Selection Guide

By Device Family

By Encoder Speed

PPR Calculation: Maximum edge frequency = (RPM × PPR × 4) / 60

• Example: 10000 RPM × 1024 PPR × 4 = 682 kHz edge rate (challenging for most MCUs)

By Application Requirements

Encoder Fundamentals

Quadrature Encoding Basics

Quadrature encoder signals:

• Channel A: Primary position signal
• Channel B: Phase-shifted signal (90° ahead for forward rotation)
• Index (Z): One pulse per revolution (optional, for absolute reference)

Decoding Modes:

• 1x Mode: Count edges of Channel A only (lowest resolution)
• 2x Mode: Count rising and falling edges of Channel A
• 4x Mode: Count all edges of A and B (highest resolution, 4× PPR)

Direction Detection:

• Forward: Channel A leads Channel B by 90°
• Reverse: Channel B leads Channel A by 90°

Position and Velocity Calculation

Position:

Position (counts) = Edge_Counter / 4   (for 4x mode)
Position (mechanical degrees) = (Position_counts / PPR) × 360°
Position (electrical degrees) = Position_mechanical × Pole_Pairs

Velocity:

RPM = (Delta_Position / Delta_Time) × 60 / PPR

Example:
- PPR = 1024
- Delta_Position = 5000 counts (in 4x mode)
- Delta_Time = 10 ms (0.01 s)
- RPM = (5000 / (1024 × 4)) / 0.01 × 60 = 7324 RPM

Hardware velocity measurement (QEI, PDEC):

• Uses timer to measure interval between encoder edges
• More accurate at low speeds than position differentiation
• Automatically handles direction

Encoder Wiring and Electrical Considerations

Encoder Types

Noise Immunity

Digital Filter (QEI/PDEC):

• Purpose: Reject switch bounce and electrical noise
• Setting: 3-7 clock cycles typical
• Trade-off: More filtering = slower response to high-speed edges

Hardware Recommendations:

• Twisted pair wiring: Reduces EMI pickup
• Shielded cable: Ground shield at one end only
• Bypass capacitors: 100 nF at encoder connector (close to MCU pins)
• Series resistors: 100Ω on A/B/Z lines (current limiting, ESD protection)

Troubleshooting Common Issues

Encoder Not Counting

Check:

• ✅ Encoder power supply (5V or 12V/24V for industrial encoders)
• ✅ Ground connection between encoder and MCU
• ✅ QEI/PDEC peripheral enabled in block
• ✅ Pin assignments correct (use Port Info block to verify)
• ✅ Encoder actually rotating (use Digital_Input block to check raw signals)

Wrong Direction

Check:

• ✅ Swap Channel A and B in block configuration
• ✅ Or use “Invert Direction” option if available
• ✅ Verify encoder wiring matches datasheet

Position Jumps or Noise

Check:

• ✅ Increase digital filter setting (5-7 clock cycles)
• ✅ Add external 100 nF capacitors on A/B/Z lines
• ✅ Use shielded twisted pair cable
• ✅ Check for EMI sources (PWM switching, motor cables)
• ✅ Reduce maximum encoder speed (lower PPR encoder)

Velocity Measurement Unstable

Check:

• ✅ Use hardware velocity measurement (QEI/PDEC) instead of position differentiation
• ✅ Increase velocity measurement time window
• ✅ Add low-pass filter on velocity output (cutoff ~10-50 Hz)
• ✅ Check encoder PPR not too high for motor speed range

Device Family Support Summary

Legend: ✅ Full Support | ⚠️ Available but PDEC preferred | ❌ Not Available

See Also

Related Block Categories:

• PWM Blocks - For motor control PWM generation
• Timer Blocks - For velocity measurement timing
• Digital I/O Blocks - For software encoder decoding

Application Notes:

• AN1078 - Sensorless Field-Oriented Control with Encoder Option
• AN1292 - Sensorless FOC with Encoder Backup
• DS70601B - QEI Module Family Reference Manual

Example Projects:

• Motor Control Examples - FOC with encoder feedback
• Board Templates - Pre-configured encoder setups

💡 Quick Selection Guide:

• Using dsPIC for motor control? → QEI (hardware, zero overhead)
• Using SAM E5x/E7x for motors? → PDEC (angular output, hardware velocity)
• Using SAM C2x cost-optimized? → QDEC_SAMx (TC-based)
• Need encoder on any device? → ChangeNotification (software, universally available)
• High-speed encoder (> 5000 RPM)? → QEI or PDEC only (hardware required)