PDEC - Position Decoder for SAM Devices

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Position Decoder - Hall Sensors and Stepper Motor Interface

Overview

The MCHP_PDEC block provides an interface to the Position Decoder (PDEC) peripheral on SAM E5x and E7x microcontrollers. Unlike traditional quadrature encoders, PDEC is optimized for Hall effect sensors (3-phase BLDC motors) and stepper motor position tracking, with specialized features for these applications.

When to use:

• Hall sensors — Decode 3-phase Hall sensor signals for BLDC motor position (6-step)
• SAM E5x/E7x only — Dedicated PDEC peripheral on these ARM Cortex-M devices
• 9-bit angular resolution — Fine position within each Hall sector for smooth FOC
• Stepper motors — Track step/direction inputs for open-loop stepper position
• Auto-correction — Built-in phase error correction for Hall sensor glitches

When NOT to use:

• Quadrature encoders — Use QDEC SAMx or QEI blocks (optimized for A/B/Index signals)
• dsPIC/PIC32 targets — PDEC only on SAM E5x/E7x (use QEI for dsPIC)
• Other SAM families — PDEC not available on SAM C2x/D2x (use QDEC SAMx)
• No Hall sensors — If using incremental encoder, use QDEC SAMx block instead
• Software decoding sufficient — If Hall speed low, GPIO + software may be simpler Key Features:
• 3-input position decoder (Hall sensors: A, B, C)
• Quadrature encoder mode (X2/X4)
• Counter with direction input mode
• Configurable position counter and revolution counter
• Auto-correction for phase errors
• Programmable digital filtering
• Index pulse support
• Angular position calculation (9-bit + configurable resolution)

Device Support

Block Dialog

Inputs Tab

Outputs Tab

Options Tab

Parameters

Operating Mode

Position Configuration

Index and Reset

Signal Configuration

Hall Sensor Operation

3-Phase Hall Sensor Interface

PDEC decodes 3-wire Hall sensor signals for BLDC motor position: Hall Sensor States (6-step commutation): State | Hall A | Hall B | Hall C | Sector ——-|——–|——–|——–|——– 1 | 0 | 0 | 1 | 0-60° 2 | 0 | 1 | 1 | 60-120° 3 | 0 | 1 | 0 | 120-180° 4 | 1 | 1 | 0 | 180-240° 5 | 1 | 0 | 0 | 240-300° 6 | 1 | 0 | 1 | 300-360° Valid transitions: 1→2→3→4→5→6→1 (CW) 1→6→5→4→3→2→1 (CCW)

Angular Position Calculation

// PDEC provides 9-bit angular resolution within each sector Angular_bits = 9; // Fixed 9-bit per sector (0-511) Sector = 0 to 5; // From Hall state// Total position = (Sector * 512) + Angular_position Counts_per_rev = 6 * 512 = 3072; // Convert to electrical angle Electrical_angle_deg = (Position_count / 3072) * 360; // For motor with 4 pole pairs: Mechanical_angle_deg = Electrical_angle_deg / 4;

PDEC Registers (SAM E5x)

// Control Register PDEC->CTRLA.bit.MODE = 0; // QDEC mode PDEC->CTRLA.bit.CONF = 1; // X4 quadrature (secure) PDEC->CTRLA.bit.PINEN = 0b111; // Enable all 3 inputs PDEC->CTRLA.bit.PINVEN = 0b000; // Invert mask PDEC->CTRLA.bit.SWAP = 0; // No pin swap PDEC->CTRLA.bit.PEREN = 1; // Enable period/modulo// Position Counter (16-bit) count = PDEC->COUNT.reg; // Current count (9-bit angular + N-bit revolution)// Angular position (9-bit within sector) angular = count & 0x1FF; // Lower 9 bits// Revolution counter (upper bits) revolution = count » 9; // Upper N bits// Period register (modulo limit) PDEC->CC[0].reg = max_position; // Position wraps at this value// Digital Filter PDEC->FILTER.bit.FILTER = filter_value; // 0=disabled, 1-255=filter clocks

Examples

Example 1: BLDC Motor with Hall Sensors

Application: 3-Phase BLDC Motor Position Feedback

Hardware: BLDC motor with 3 Hall sensors, 4 pole pairs

Configuration:

• Mode: Quadrature Encoder
• Counter Output: on (unsigned)
• Counter Length: 16-bit (9+7, allows 128 revolutions)
• Index: Not used // Position calculation Position_count = PDEC_Output; Angular_9bit = Position_count & 0x1FF; // 0-511 within sector Revolution = Position_count » 9; // Revolution count// Hall sector (0-5) from upper 3 bits of angular position Sector = Angular_9bit » 6; // Approximate sector// Electrical angle (0-360°) Elec_angle_deg = (Position_count % 3072) / 3072 * 360; // Mechanical angle (4 pole pairs) Mech_angle_deg = Elec_angle_deg / 4; // Commutation sector for 6-step drive Commutation_sector = floor(Elec_angle_deg / 60); // 0-5

Example 2: Stepper Motor Position Tracking

Application: Open-Loop Stepper with Position Feedback

Configuration:

• Mode: Counter with direction
• QDI0: Step pulse input
• QDI1: Direction input (HIGH=CW, LOW=CCW)
• Counter Output: 32-bit signed // For 200 steps/rev stepper (1.8° per step) Steps_per_rev = 200; Position_steps = PDEC_Counter; // Convert to angle Angle_deg = (Position_steps % Steps_per_rev) * 1.8; // Full revolutions Revolutions = floor(Position_steps / Steps_per_rev); // Microstep mode: 1/16 microstepping = 3200 steps/rev Microsteps_per_rev = 200 * 16; Microstep_angle_deg = (Position_steps % Microsteps_per_rev) / Microsteps_per_rev * 360;

Example 3: Index-Based Homing

Application: Absolute Position Reference Using Index Pulse

Configuration:

• Mode: Quadrature Encoder
• Index: Reset counter once only
• Auto Correction: on (for Hall sensor noise immunity)

Example 4: Dynamic Modulo Limit

Application: Variable Position Range

Configuration:

• Max Position Input: on
• Counter wraps at value from MAXPOS input port

Troubleshooting

Issue: Incorrect Hall Sequence Detection

Causes:

• Hall sensors wired incorrectly (wrong A/B/C assignment)
• Hall sensor polarity inverted
• One Hall sensor not working (stuck at 0 or 1)

Solutions:

• Use “Invert QDI” options to correct polarity
• Enable Auto Correction for minor glitches
• Verify Hall wiring with multimeter or oscilloscope
• Check Hall sensor power supply (typically 5V or 12V)

Issue: Position Jumps or Glitches

Solution: Enable digital filter to suppress noise. Start with 1 µs filter period and increase if needed.

Issue: Wrong Direction

Solution: Enable “Swap Pin Phase A & B” to reverse counting direction.

References

• SAM E54 Family Datasheet (DS60001507) - PDEC Position Decoder section
• SAM E70 Family Datasheet (DS60001527) - PDEC peripheral
• Atmel Application Note: BLDC Motor Control with Hall Sensors (search Microchip Documentation Portal )
• PDEC Angular Position Calculation: Family reference manual chapter on position decoding (see datasheets above)

Last Updated: 2024 | MCHP Blockset for MATLAB/Simulink

• SAM E54 Family Datasheet (DS60001507)

• SAM E70 Family Datasheet (DS60001527)

• Microchip Documentation Portal

Related Blocks

• MCHP_QEI - Quadrature Encoder Interface for dsPIC/PIC32
• MCHP_QDEC_SAMx - Quadrature Decoder using TC modules
• MCHP_PWM_SAMx - PWM for BLDC motor drive (works with PDEC Hall inputs)