Control a PWM fan over the I²C bus.
Tests: 
ATtiny I²C Fan Control is made as a PlatformIO project.
- Set fan speed by using a PWM signal
- Set fan speed via I²C bus
- Read the current fan RPM via I²C bus
- Auto adjust PWM range by testing the connected fan at startup (can be re-triggered by I²C bus)
- Override min PWM level by I²C bus
Adjustments for other AVR microcontrollers should be easy.
| Register | Description |
|---|---|
| 0x00 | Status - By writing a 1 to bit 0 of this register you can trigger a re-calibration of the fan. |
| 0x01 | Fan speed - By writing to this register you can set the fan speed. A value of 0 means "fan off" and 255 is full speed. |
| 0x02 | Minimal PWM level - Read or write the minimal PWM level at which the fall will rotate when fan speed is set to 1. This will be detected automatically at startup. |
| 0x03 | Current fan speed in revolutions per second (RPS) - Read the current fan speed in RPS. This is calculated from the fan tacho signal. |
| 0x04 and 0x05 | Current fan speed in revolutions per minute (RPM) - Read the current fan speed in RPM. This is calculated from the fan tacho signal. 0x04 is the low-byte and 0x05 the high-byte. |
Since the ATtiny microcontrollers use a USI (Universal Serial Interface) to provide the I²C bus, there may be some problems reading or writing more than one byte at once.
To overcome this issue you should read/write byte by byte, even if want to read the fan speed in RPM.
- Active cooling of the "HomePi" (German)
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