Day34 - Linux IIO I2C Driver (Multi-Channel Direct Mode)¶

Overview¶

This day implements a Linux IIO driver over I2C for a custom STM32-based fake sensor.

The sensor provides:

3 ADC channels (ADC0, ADC1, ADC2)
Register-based interface over I2C
Periodic sample update (time-driven, not request-driven)

The IIO driver exposes these channels via sysfs using direct mode.

System Architecture¶

STM32 (fake sensor)
 ├── sensor_reg (register model)
 ├── sensor_task (periodic update)
 └── i2c_slave (transport)

        ↓ I2C

Linux
 ├── i2c driver
 ├── IIO framework
 └── sysfs interface

IIO Device Interface¶

After driver probe:

/sys/bus/iio/devices/iio:deviceX/
 ├── name
 ├── in_voltage0_raw
 ├── in_voltage1_raw
 ├── in_voltage2_raw
 ├── in_voltage_scale

Channel Mapping¶

Channel	Register	sysfs
ADC0	0x10 / 0x11	in_voltage0_raw
ADC1	0x12 / 0x13	in_voltage1_raw
ADC2	0x14 / 0x15	in_voltage2_raw

Key Implementation¶

1. Channel Specification¶

.type = IIO_VOLTAGE
.indexed = 1
.channel = X

Result:

in_voltageX_raw

2. Shared Scale¶

.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE)

Result:

in_voltage_scale

3. read_raw() Mapping¶

sysfs read
    ↓
IIO core
    ↓
read_raw()
    ↓
I2C register read
    ↓
STM32 sensor

4. Raw Value Conversion¶

STM32 returns:

16-bit register (little endian)
12-bit valid data

Driver:

*val = raw & 0x0FFF;

Sensor Behavior (STM32 Side)¶

Sample Update Rule¶

sample_id++
adc0 = 100 + sample_id
adc1 = 200 + sample_id
adc2 = 300 + sample_id

Key Properties¶

Deterministic pattern (easy debug)
Time-driven (via main loop)
Independent from I2C access

Data Flow¶

Full Path¶

STM32 main loop
    ↓
sensor_task_process()
    ↓
update sample registers
    ↓
Linux read
    ↓
I2C transaction
    ↓
read_raw()
    ↓
sysfs output

Comparison with Previous Work¶

vs hwmon¶

hwmon	IIO
Manual sysfs	Auto-generated sysfs
Single value	Multi-channel
show()	read_raw()

vs Character Driver¶

Char Driver	IIO
read/write	read_raw
protocol-based	measurement-based
user-defined format	framework-defined

vs TTY¶

TTY	IIO
byte stream	structured samples
IRQ RX	trigger-based (future)
flip buffer	IIO buffer (future)

Key Takeaways¶

IIO channel metadata defines sysfs ABI
Multi-channel support is native in IIO
read_raw() bridges sysfs and hardware
Direct mode behaves like multi-channel single-shot sensor
Sensor should be time-driven, not request-driven

Limitations (Current Stage)¶

No buffer support
No trigger
No timestamp
No interrupt (DRDY not used yet)

Next Step (Day35)¶

Add DRDY GPIO (data-ready interrupt)
Introduce IRQ handling in driver
Prepare for IIO trigger/buffer model