QDEC SAMx - Quadrature Decoder for SAM Devices

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Quadrature Decoder Using TC Modules - SAM Device Encoder Interface

Overview

The MCHP_QDEC_SAMx block implements quadrature encoder decoding using the Timer/Counter (TC) modules on SAM microcontrollers (E5x, E7x, C2x). Unlike dedicated QEI peripherals, this implementation uses TC modules in quadrature mode to decode encoder signals for position and speed measurement. Key Features:

• Quadrature position decoding using TC modules
• 32-bit position counter with revolution counter
• Index pulse support for position reset
• Direction detection
• Speed measurement using time base
• Phase error detection (quadrature integrity check)
• Missing pulse error detection
• Configurable digital filtering
• Rotation change flag

Device Support

Important: Each QDEC instance requires 3 TC channels (one complete TC module). For example, TC0 uses channels 0, 1, and 2 for phase A/B counting, index counting, and speed time base respectively.

Block Parameters

Module Selection

Operating Mode

Position Mode Configuration

Speed Mode Configuration

Index Counter (Revolution Counter)

Signal Configuration

Advanced Features

Error Detection Outputs

TC Module Configuration

Quadrature Decoding Mode

The TC module is configured in quadrature decoder mode (QDEC): // Channel 0: Phase A/B counter (position) TC0->COUNT16.CTRLA.bit.MODE = 0x01; // 16-bit counter TC0->COUNT16.CTRLA.bit.ENABLE = 1; TC0->COUNT16.CTRLBSET.bit.DIR = 0; // Count up (default)// QDEC Configuration TC0->COUNT16.DRVCTRL.bit.INVEN0 = invert_pha; TC0->COUNT16.DRVCTRL.bit.INVEN1 = invert_phb; // Channel 1: Index counter (revolutions) TC1->COUNT16.CTRLA.bit.MODE = 0x01; TC1->COUNT16.EVCTRL.bit.EVACT = 0x01; // Count on event// Channel 2: Speed time base TC2->COUNT16.CTRLA.bit.MODE = 0x00; // 16-bit mode TC2->COUNT16.CTRLA.bit.PRESCALER = speed_prescaler;

SAM E70/S70/V71 TC Registers

// TC Channel 0 (Position Counter) TC0->TC_CHANNEL[0].TC_CMR = 0x00000006; // QDEC mode, QDPHASEL=0, QDPHBSEL=1 TC0->TC_CHANNEL[0].TC_BMR = …; // Block mode config// Position counter value position = TC0->TC_CHANNEL[0].TC_CV; // Direction dir = (TC0->TC_CHANNEL[0].TC_SR & TC_SR_DIR) ? 1 : 0; // Channel 1 (Index Counter) TC0->TC_CHANNEL[1].TC_CMR = …; // Capture mode for index index_count = TC0->TC_CHANNEL[1].TC_CV; // Channel 2 (Speed Time Base) TC0->TC_CHANNEL[2].TC_CMR = …; // Timer for speed measurement

Examples

Example 1: Optical Encoder Position Tracking

Application: Robotic Joint Position Feedback

Hardware: 1024 PPR optical encoder

Configuration:

• QDEC Module: TC0
• Mode: Position
• Position Output: 32-bit signed
• Index Output: 16-bit signed (revolution counter)
• Reset Mode: on each IDX signal
• IDX Pin: Enabled, rising edge // Quadrature mode provides 4× resolution Counts_per_rev = 1024 * 4 = 4096; // Absolute position calculation Position_within_rev = Position_Output; // 0 to 4095 Total_revolutions = Index_Counter; Absolute_position = (Total_revolutions * 4096) + Position_within_rev; // Convert to angle Angle_deg = (Position_within_rev / 4096) * 360; Angle_rad = (Position_within_rev / 4096) * 2 * pi; Tip: Use 32-bit signed position counter for bidirectional motion. The revolution counter (index output) tracks full rotations in both directions.

Example 2: Velocity Estimation Using Position Difference

Application: Motor Speed Control

Configuration:

• Mode: Position
• Position Output: 32-bit signed
• Sample Rate: 1 kHz // In Simulink: Position → [z^-1] → Subtract → Speed (counts/sample)// └────────────────────┘// Convert to RPM (4096 counts/rev, 1 kHz sample) Speed_counts_per_sample = Position_current - Position_previous; Speed_RPM = Speed_counts_per_sample * (1000 Hz) * (60 s/min) / (4096 counts/rev); Speed_RPM = Speed_counts_per_sample * 14.65; // Low-pass filter for noise reduction Speed_filtered = 0.9 * Speed_filtered_prev + 0.1 * Speed_RPM;

Example 3: Position-Triggered ADC Sampling

Application: Synchronous Motor Current Sampling

Goal: Trigger ADC at specific rotor positions

Configuration:

• Position Output: 16-bit unsigned
• Reset Mode: at user defined value (e.g., 4095 for modulo)
• Use comparison match events (via TC compare registers) // Set TC compare value for trigger position// Example: Trigger at 60° electrical (for 6-step commutation) Electrical_angle_deg = 60; Pole_pairs = 4; Mechanical_counts = (Electrical_angle_deg / 360) * (4096 / Pole_pairs); TC0->COUNT16.CC[0].reg = Mechanical_counts; // Enable event output on compare match TC0->COUNT16.EVCTRL.bit.MCEO0 = 1; // Match/Compare Event Output// Connect to ADC via EVSYS// ADC trigger = TC0 Match Compare 0

Example 4: Speed Measurement Mode

Application: Low-Speed Motor Velocity Feedback

Configuration:

• Mode: Speed
• Speed Output: 32-bit unsigned
• Speed Time Base: Resol: 125 µs - MaxPer: 8.19 s // Speed output is time-based (period measurement)// Time = (Speed_Output) * Time_Base_Resolution Time_between_counts = Speed_Output * 125e-6; // seconds// Convert to frequency Frequency_Hz = 1 / Time_between_counts; // Convert to RPM (for 4096 counts/rev encoder) RPM = (Frequency_Hz * 60) / 4096; // Example: Speed_Output = 20000// Time = 20000 * 125µs = 2.5 seconds per count// Freq = 1/2.5 = 0.4 Hz// RPM = (0.4 * 60) / 4096 = 0.0058 RPM (very slow!)Speed Mode Best For: Very low speeds where position-based velocity calculation has poor resolution. At high speeds, position difference method is more accurate.

Error Detection and Handling

Quadrature Error Detection

The QDEC monitors phase transitions for validity. Invalid transitions indicate: Valid Quadrature Sequence: 00 → 01 → 11 → 10 → 00 (CCW) 00 → 10 → 11 → 01 → 00 (CW) Invalid Transitions (Quadrature Error): 00 → 11 (both changed simultaneously) 01 → 10 (both changed simultaneously)

Quadrature Error Causes:

• Encoder mechanical misalignment
• Electrical noise causing glitches
• Loose encoder connections
• Encoder speed exceeds maximum input frequency

Solution: Enable digital filter (Max Period Filter) and/or enable Auto Correction to automatically fix minor phase errors.

Missing Pulse Error

Detects when expected encoder transitions don’t occur within a time window:

Direction Change Detection

Troubleshooting

Common Issues

Issue: Position Counter Drifts

Symptoms: Position slowly increases/decreases at standstill

Causes & Solutions:

• Electrical noise → Enable digital filter
• Mechanical vibration → Increase filter period
• Encoder alignment → Check 90° phase relationship
• Enable Auto Correction for minor phase errors

Issue: Index Counter Not Incrementing

Check:

• IDX pin configuration (not “not used”)
• Index pulse polarity (Invert IDX if needed)
• Index pulse width (must be wider than input filter period)
• Physical encoder rotation through index position

Issue: Quadrature Error Flag Always Set

Solutions:

• Enable Auto Correction (handles minor glitches)
• Increase digital filter period (filters noise)
• Check PHA/PHB wiring (may need to swap or invert)
• Verify encoder signals have 90° phase shift (use oscilloscope)

Performance Optimization

// Maximum encoder frequency depends on MCU clock and filter// SAM E54 @ 120 MHz: MCU_Clock = 120e6; Filter_Period = 1e-6; // 1 µs filter Max_Encoder_Freq = 1 / (Filter_Period * 4); // 4 edges in quadrature Max_Encoder_Freq = 250 kHz; // For 4096 counts/rev encoder: Max_RPM = (250e3 * 60) / 4096 = 3662 RPM; // Disable filter for higher speeds (if noise is low)// Filter = disabled → Max freq limited only by TC clock

Related Blocks

• MCHP_QEI - Quadrature Encoder Interface for dsPIC/PIC32
• MCHP_PDEC - Position Decoder for SAM E5x/E7x (Hall sensors, stepper)
• MCHP_TC - Timer/Counter block (underlying peripheral used by QDEC)
• MCHP_EVSYS - Event System (for position-triggered events)

References

• [SAM E54 Family Datasheet (DS60001507)] - Timer/Counter (TC) peripheral section
• [SAM E70/S70/V71 Family Datasheet (DS60001527)] - TC in QDEC mode
• [SAM C21 Family Datasheet (DS60001479)] - TC quadrature decoder
• Atmel Application Note: Using TC for Quadrature Decoding (search [Microchip Documentation Portal])
• [SAME54 Xplained Pro User Guide (70005321)] - Hardware connections for encoder

Last Updated: 2024 | MCHP Blockset for MATLAB/Simulink

See Also

Datasheets

• SAM E54 Family Datasheet (DS60001507)
• SAM E70/S70/V71 Family Datasheet (DS60001527)
• SAM C21 Family Datasheet (DS60001479)

Other

• Microchip Documentation Portal
• SAME54 Xplained Pro User Guide (70005321)