CN - Change Notification (Pin Interrupt)

dsPIC30F dsPIC33F dsPIC33E dsPIC33C PIC32

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Change Notification - Pin State Change Interrupts and Timing

Overview

The MCHP_CN block provides an interface to the Change Notification (CN) peripheral, which detects pin state changes and measures transition timing. While primarily used for GPIO interrupts (buttons, switches), CN can also serve as a simple encoder interface or pulse counter. Key Features:

• Multi-pin state change detection (up to 32 CN pins)
• Edge detection: rising, falling, or both
• Transition timing measurement (time between edges)
• Period measurement (rising-to-rising or falling-to-falling)
• Programmable pull-up/pull-down resistors
• Interrupt generation on pin change
• Wake from sleep on pin change
• Can be used for simple rotary encoders or pulse counting

Device Support

Block Parameters

Channel Selection

Measurement Configuration (per channel)

Change Detection (per channel)

Advanced Configuration

CN Peripheral Registers

dsPIC30F/33F (Type 0)

// Enable CN module CNCONbits.ON = 1; // Enable specific CN pins CNEN1bits.CN0IE = 1; // Enable CN0 CNEN1bits.CN1IE = 1; // Enable CN1// Enable pull-ups CNPU1bits.CN0PUE = 1; // Pull-up on CN0// Interrupt configuration IFS1bits.CNIF = 0; // Clear interrupt flag IEC1bits.CNIE = 1; // Enable CN interrupt IPC4bits.CNIP = 5; // Priority level// Read current state state = PORTBbits.RB0; // Read pin state (if CN0 is on RB0)

dsPIC33E/33C (Type 1)

// All GPIO pins have CN capability// Enable change notification on specific pins// For port A, pin 0 (PA0/RA0): CNENAbits.CNIEA0 = 1; // Enable CN on RA0 CNPUAbits.CNPUA0 = 1; // Pull-up on RA0// Interrupt per port IFS3bits.CNAIF = 0; // Clear Port A CN interrupt IEC3bits.CNAIE = 1; // Enable Port A CN interrupt

PIC32MK (Type 2)

// One CN interrupt per port CNENAbits.CNIEA0 = 1; // Enable CN on RA0// Edge select (positive/negative/both) CNNEAbits.CNNEA0 = 0; // 0 = positive edge, 1 = negative edge// Interrupt control IFS3bits.CNAIF = 0; IEC3bits.CNAIE = 1;

Examples

Example 1: Button Interface with Debouncing

Application: 4-Button User Interface

Hardware: 4 push buttons on CN0-CN3 with pull-ups

Configuration:

• Channels: [0 1 2 3]
• Measurement: [0 0 0 0] (change detect only)
• Change Detection: [1 1 1 1] (rising edge = button press)
• Output Port Value: [1 1 1 1] (read button state)

Example 2: Rotary Encoder (Simple, No Quadrature Decoding)

Application: Rotary Switch Position Detection

Hardware: Mechanical rotary encoder on CN0 (A) and CN1 (B)

Configuration:

• Channels: [0 1]
• Measurement: [0 0]
• Change Detection: [3 3] (both edges)

Example 3: Pulse Width and Period Measurement

Application: External PWM Signal Analysis

Configuration:

• Channels: [0]
• Measurement: [5] (UP + Period on rising)
• Max Channel: [0.1] (100 ms max) // Measure external PWM duty cycle and frequency Pulse_Width_High = CN0_Up; // Time high (timer counts) Period = CN0_Per; // Time between rising edges// Convert to physical units (CNxmax variable set by block) Time_Resolution = Max_Channel / CN0max; Pulse_Width_seconds = Pulse_Width_High * Time_Resolution; Period_seconds = Period * Time_Resolution; // Calculate duty cycle and frequency Duty_Cycle_percent = (Pulse_Width_seconds / Period_seconds) * 100; Frequency_Hz = 1 / Period_seconds; // Example: CN0max=65535, Max_Channel=0.1s// Resolution = 0.1/65535 = 1.53 µs// If CN0_Up=32768, Pulse_High=50ms// If CN0_Per=65535, Period=100ms → 10 Hz, 50% duty

Example 4: Wake from Sleep on Button Press

Application: Low-Power Wakeup

Configuration:

• Channels: [0] (wakeup button)
• Change Detection: [1] (rising edge)
• Enable pull-down (button connects to VDD)

Output Scaling

Workspace Variables

The block creates workspace variables for scaling calculations:

Scaling Formula

Output values are in raw timer ticks. To convert to physical time:

Time_Resolution = Max_Channel / CNxmax
Time_seconds = Timer_Ticks × Time_Resolution

Example: With CN0max = 65535 and Max_Channel = 0.1s:

Time_Resolution = 0.1 / 65535;        % ≈ 1.53 µs per tick
Pulse_Width_s = CN0_Up * Time_Resolution;
Period_s = CN0_Per * Time_Resolution;
Frequency_Hz = 1 / Period_s;

Troubleshooting

Issue: Multiple False Triggers (Button Bounce)

Cause: Mechanical switch contacts bounce

Solutions:

• Implement software debouncing (ignore edges for 10-50ms after first edge)
• Add external RC filter (e.g., 10kΩ + 100nF)
• Use hardware Schmitt trigger inputs

Issue: Missed Events at High Frequency

Cause: CN interrupt rate too high or interrupt latency

Solutions:

• Use dedicated counter peripheral (QEI, Input Capture) for high-speed signals
• Increase interrupt priority
• Reduce other interrupt sources

Issue: Timer Overflow (CNxmax exceeded)

Cause: Signal period exceeds Max Channel setting

Solutions:

• Increase “Max Channel” parameter
• Increase “Safe Margin” for more headroom
• Use larger timer prescaler (block automatically selects best timer)

Performance Considerations

Timing Resolution

// Timer resolution depends on Max_Channel and timer selection// Block automatically chooses best timer to meet requirements// Example: Max_Channel = 1ms, CNxmax = 65535 Time_Resolution = 1e-3 / 65535 = 15.3 ns; // For pulse width measurement accuracy:// ±1 count error = ±15.3 ns// For longer periods, resolution decreases:// Max_Channel = 1s → Resolution = 15.3 µs

Interrupt Overhead

Typical CN Interrupt Overhead:

• Interrupt latency: 10-20 instruction cycles
• Handler execution: 50-100 cycles (depends on measurements enabled)
• At 70 MIPS: ~1-2 µs total per CN event
• For high-frequency signals (>100 kHz), consider Input Capture or QEI

Related Blocks

• MCHP_QEI - Hardware quadrature decoder for precise encoder counting
• MCHP_IC - Input Capture for high-speed pulse/period measurement
• MCHP_DIO - Digital I/O for reading/writing GPIO pins
• MCHP_INT - External interrupt blocks

References

• dsPIC30F/33F Family Reference Manual - Section: Change Notification Module
• dsPIC33E/33C Family Reference Manual - Change Notice (CN) peripheral
• PIC32 Family Reference Manual - Change Notice/Interrupt-on-Change
• dsPIC33A Family Reference Manual - CN peripheral

Last Updated: 2024 | MCHP Blockset for MATLAB/Simulink