Input Capture / Output Compare

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Overview

Input Capture and Output Compare blocks provide hardware-based event timing and waveform generation capabilities for precise measurement and control applications. These peripherals operate independently of CPU execution, ensuring deterministic timing for frequency measurement, pulse width detection, servo control, and custom waveform generation.

Available Blocks

IC - Input Capture for Event Timing

All Families

Hardware event capture that records timer values when external events occur on input pins. - **Edge Detection**: Rising, falling, or both edges

OC HW - Hardware Output Compare

dsPIC30F dsPIC33F dsPIC33E dsPIC33C PIC32

Hardware-based output generation using Output Compare (OC) peripheral with zero CPU overhead. - **Autonomous Operation**: Hardware toggles/sets/clears output pins

OC SW - Software Output Compare

dsPIC30F dsPIC33F dsPIC33E dsPIC33C

Interrupt-driven software output compare for flexible output control with CPU overhead. - **Interrupt-Based**: Timer interrupt triggers output changes

Block Selection Guide

By Application

By Performance Requirements

🟢 Zero CPU Overhead (Hardware-based, autonomous)

• IC - Event capture happens automatically
• OC_HW - Output changes controlled by hardware

🟡 Interrupt-Driven (Flexible but with CPU overhead)

• OC_SW - Software logic for complex waveforms

IC vs OC Comparison

Hardware Resource Considerations

Input Capture (IC) Modules

• Dedicated IC peripherals on most dsPIC/PIC32 devices
• Typical availability: IC1-IC8 (device-dependent)
• Cannot be used with PWM or OC on same timer
• Requires timer resource for counting

Output Compare (OC) Modules

• Dedicated OC peripherals on dsPIC/PIC32 devices
• Typical availability: OC1-OC16 (device-dependent)
• Can share timer with other OC modules
• Requires timer resource for time base

Timer Sharing Rules

❌ Cannot Share:

• IC and PWM on same timer
• IC and OC on same timer
• Multiple IC modules on same timer

✅ Can Share:

• Multiple OC modules on same timer
• OC and Timer Config blocks

Timing Accuracy

Input Capture Accuracy

• Capture Latency: Typically 1-2 instruction cycles
• Jitter: ±1 timer tick (depends on timer resolution)
• For high precision: Use fastest prescaler possible

Example: dsPIC33CK @ 100 MIPS, Prescaler=1

• Resolution = 1 / 100 MHz = 10 ns
• Max measurable period = 10 ns × 65536 = 655 µs

Output Compare Accuracy

• Hardware OC: ±1 timer tick (deterministic)
• Software OC: ±1 timer tick + interrupt latency (5-20 cycles typical)
• For critical timing: Use Hardware OC (MCHP_OC_HW)

Recommendation: For motor control PWM, use dedicated PWM blocks instead of OC for better performance and features.

Typical Use Cases

Input Capture Applications

Frequency Measurement:

• Tachometer for motor RPM sensing
• External clock frequency detection
• Pulse counting for flow meters

Pulse Width Measurement:

• RC servo position feedback
• Ultrasonic ranging (echo pulse width)
• Duty cycle monitoring

Event Timing:

• Encoder interface (non-quadrature)
• Time-of-flight sensors
• Timing between external events

Output Compare Applications

Simple Waveform Generation:

• Servo control pulses (1-2 ms pulses at 50 Hz)
• Stepper motor step pulses
• General-purpose PWM (when PWM module unavailable)

Timed Events:

• One-shot pulses for triggering external circuits
• Delayed output changes
• Periodic interrupts for software timing

Complex Waveforms:

• Multi-phase waveforms with software coordination
• Custom pulse patterns
• Dynamic timing sequences

Device Family Support Summary

Legend: ✅ Full Support | ⚠️ Different Peripheral (TC/TCC capture/compare) | ❌ Not Available

Note: SAM ARM devices use different timer peripherals (TC/TCC blocks with capture/compare functionality, not dedicated IC/OC modules).

See Also

Related Block Categories:

• Timers - For timer configuration and resource management
• PWM Blocks - For advanced PWM generation (preferred for motor control)
• Encoders - For quadrature encoder interfaces (QEI block)

Recommended Reading:

• Input Capture: Use for non-quadrature encoders, tachometers, frequency counters
• Output Compare: Use for servo control, simple pulse generation
• For motor PWM: Use dedicated PWM blocks instead of OC

Example Projects:

• Board Templates - Pre-configured IC/OC setups

💡 Quick Selection Guide:

• Need to measure something? → Input Capture (IC)
• Need to generate simple pulses? → Output Compare Hardware (OC_HW)
• Need complex waveforms? → Output Compare Software (OC_SW)
• For motor control PWM → Use dedicated PWM blocks