Day23 - I2C Sensor with Interrupt (IRQ)

Overview

In this lab, we extend the I2C sensor driver to support hardware interrupt (IRQ).

Previously: - Day19: Basic I2C driver - Day20: hwmon integration - Day22: Polling mode

Now: - Day23: Interrupt-driven update

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Key Concept

1. GPIO Interrupt vs gpiod

Item	gpiod (GPIO driver)	interrupts (IRQ)
Ownership	Driver controls GPIO	Hardware wiring
Direction	Explicit (GPIOD_IN)	Implicit input
Pull-up/down	Often handled	❗ Must use pinctrl
IRQ mapping	gpiod_to_irq()	From Device Tree

👉 interrupts only describes wiring, not configuration.

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2. Role of pinctrl

pinctrl is responsible for:

• Pin mux (GPIO vs ALT)
• Pull-up / pull-down
• Electrical configuration

Example:

my_sensor_pins: my_sensor_pins {
    brcm,pins = <23>;
    brcm,function = <0>;   // GPIO mode
    brcm,pull = <2>;       // pull-up
};

Attach to device:

pinctrl-names = "default";
pinctrl-0 = <&my_sensor_pins>;

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3. Interrupt Configuration

interrupt-parent = <&rp1_gpio>;
interrupts = <23 IRQ_TYPE_EDGE_FALLING>;

• 23 → GPIO number
• IRQ_TYPE_EDGE_FALLING → trigger type

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4. IRQ Flow

STM32 GPIO (PA8)
    ↓
Raspberry Pi GPIO23
    ↓
pinctrl (pull-up, GPIO mode)
    ↓
IRQ subsystem
    ↓
request_irq()
    ↓
ISR (top half)
    ↓
workqueue (bottom half)
    ↓
I2C read sensor

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5. Why Interrupt Didn't Work Initially

Even though IRQ was registered:

/proc/interrupts → counter = 0

Root cause:

• GPIO had no pull-up
• No stable idle high level
• No valid falling edge

Fix:

• Add pinctrl with pull-up

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6. Hardware Design Pattern

Instead of using raw button signal:

Button → (noisy)

Use MCU to generate clean signal:

Button → STM32 → Clean pulse → Pi IRQ

Advantages:

• Debounce handled in MCU
• Stable timing
• Reliable interrupt triggering

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Takeaways

• interrupts ≠ GPIO configuration
• pinctrl is mandatory for stable IRQ
• Always ensure:
• idle level
• correct edge
• proper wiring

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Next Steps

• Combine polling + interrupt mode
• Add irq_enable control
• Optimize workqueue handling