Day35 - IIO + DRDY GPIO + IRQ Integration

Overview

This day extends the IIO I2C driver by introducing data-ready (DRDY) interrupt signaling.

Key idea:

• Move from polling (pull model) to interrupt-driven (push model)
• Establish the pipeline:

Sensor → DRDY GPIO → Linux IRQ → driver handler

This prepares the system for IIO trigger/buffer mode (Day36).

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System Architecture

STM32 (fake sensor)
 ├── sensor_reg (register model)
 ├── sensor_task (sample update)
 ├── sensor_gpio (DRDY pulse)
 └── i2c_slave (transport)

        ↓ I2C + GPIO

Linux
 ├── I2C driver
 ├── IRQ handler
 └── IIO framework (direct mode)

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DRDY Signal Design

Design Choice

• Active-low pulse
• Triggered on new sample
• Non-blocking, time-controlled

HIGH → LOW → HIGH

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Implementation Flow

sensor_task_process()
    ↓
generate sample
    ↓
set DATA_READY
    ↓
trigger DRDY pulse

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GPIO Pulse Model

STM32 Side

• DRDY asserted via GPIO
• Pulse width controlled in sensor_gpio_process()

LOW (assert)
→ delay (pulse width)
→ HIGH (deassert)

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Why Pulse Instead of Level?

Mode	Problem
Level	IRQ storm or no re-trigger
Pulse	Clean edge trigger

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Linux IRQ Integration

Device Tree

interrupt-parent = <&rp1_gpio>;
interrupts = <23 2>;   // GPIO23, falling edge

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Driver Integration

probe()
    ↓
get irq from client->irq
    ↓
request_threaded_irq()

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IRQ Handler

IRQ
→ threaded handler
→ log / debug

(Current stage: no data consumption yet)

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Data Flow

STM32 sample update
    ↓
DRDY pulse
    ↓
Linux IRQ
    ↓
driver handler
    ↓
(user still reads via sysfs)

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Comparison with Day34

Day34	Day35
polling read	interrupt notification
CPU pulls data	sensor pushes event
no IRQ	GPIO interrupt

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Key Concepts

1. Sensor must be explicitly configured

Driver must enable:

• measurement engine
• DRDY interrupt

CONFIG = ENABLE | DRDY_EN

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2. IRQ is only a trigger, not data path

IRQ → notify
read_raw() → fetch data

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3. Separation of concerns

Layer	Responsibility
sensor_task	generate data
sensor_gpio	signal event
driver	receive IRQ
IIO	expose interface

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Limitations (Current Stage)

• No data consumption in IRQ
• No buffer / trigger
• DATA_READY not cleared by host
• No timestamp

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Next Step (Day36)

• Introduce IIO trigger
• Push data into buffer
• Enable /dev/iio:deviceX
• Add timestamped samples

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