Witty Pi 4 Mini – Realtime Clock and Power Management for Raspberry Pi

The Witty Pi 4 Mini is an add-on board that adds a real-time clock (RTC) and power management to your Raspberry Pi. It can define your Raspberry Pi’s ON/OFF sequence and significantly reduce energy usage. The Witty Pi 4 Mini is the fourth generation of Witty Pi and has these hardware resources onboard:

• Factory calibrated and temperature compensated real-time clock with ±2ppm accuracy
• Dedicated temperature sensor with 0.125 °C resolution
• AVR 8-bit microcontroller (MCU) with 8 KB programmable flash

The Witty Pi 4 Mini has a very similar design to the Witty Pi 4, however, it doesn’t come with a DC/DC converter onboard, and it uses supercapacitor for RTC time keeping when no power supply is connected. The Witty Pi 4 Mini has solder pads on the back which allow you to solder a DC/DC converter (LDO or switching DC/DC converter).

Accurate Real-time Clock and On/Off Scheduling

The real-time clock (RTC) on Witty Pi 4 Mini has been calibrated in the factory and Witty Pi 4 Mini’s firmware also makes temperature compensation for the crystal. This makes the RTC very accurate and the actual annual error is limited to within ±2ppm.

When your Raspberry Pi boots up, the time stored in the RTC will overwrite the system time. As a result, your Raspberry Pi knows the correct time even without accessing the Internet. You can schedule the startup and/or shutdown of your Raspberry Pi, and make it a time-controlled device. You can even define a scheduled script to schedule complicated ON/OFF sequence for your Raspberry Pi.

Scheduling the ON/OFF sequence for Raspberry Pi is the most popular feature of Witty Pi, and it is extremely useful for systems that are powered by a battery. By only turning on Raspberry Pi when necessary, the battery can be used way much longer with Witty Pi installed.

Temperature Controlled Device

The temperature sensor on the Witty Pi 4 Mini has a 0.125°C resolution. The temperature data is used for compensating the crystal and making the RTC more accurate.

You can also specify the action (startup or shutdown) when the temperature goes above or below the preset threshold. This means you can also make your Raspberry Pi a temperature-controlled device.

E-latching Power Switch

The Witty Pi 4 Mini implements an e-Latching power switch, which is very similar to the power switch on your PC/laptop computer. You can gracefully turn on/off your Raspberry Pi with a single tap on the button. The software running in the background will execute the shutdown command before the power is cut, avoiding potential data corruption caused by “hard” shutdowns.

Single I2C Device

The Witty Pi 4 uses an MCU to emulate a single I2C device with default address 0x08, and also maps all I2C registers in the real-time clock and temperature sensor as virtual I2C registers in the same device. You can access all I2C registers in the real-time clock and temperature sensor via the single I2C device emulated by the Witty Pi 4.

The advantage of this new design is that Witty Pi 4 hides other I2C devices (real-time clock, temperature sensor) and becomes the proxy for them to talk to Raspberry Pi. Because the I2C address used by Witty Pi 4 can be changed to any value, you can always avoid the I2C address conflicting.

UWI Support

The Witty Pi 4 Mini is fully supported by UWI (UUGear Web Interface), and you can access your Witty Pi 4 Mini from any device that has network access.

Software Installation

The Witty Pi 4 Mini uses the same software as the Witty Pi 4. You just need to run these two commands in your Raspberry Pi to install Witty Pi 4 Mini’s software:

wget https://www.uugear.com/repo/WittyPi4/install.sh
sudo sh install.sh

The software and firmware of Witty Pi 4 Mini are open-sourced. You can find the source code on GitHub.

Package Contents

Each Witty Pi 4 Mini package contains:

• 1x Witty Pi 4 Mini board
• 4x M2.5 x 10mm plastic screws
• 4x 4mm plastic spacer
• 4x M2.5 plastic nut

USB cable/power supply not included

Specifications

Dimension	65mm x 30mm x 4mm
Weight	9g (net weight without accessories)
Microcontroller	ATtiny841 (datasheet)
Realtime Clock	PCF85063A (datasheet), calibrated in the factory
Temperature Sensor	LM75B (datasheet)
MOSFET Switch	AO4616 (datasheet)
Super Capacitor	0.07F
Power In	DC 5V (via USB type C connector)
	or 5V in unpopulated header P2
Output Current	Up to 2.5A for Raspberry Pi and its peripherals
Standby Current	~0.5mA
Operating Environment	Temperature -30°C~80°C (-22°F~176°F) Humidity 0~80%RH, no condensing, no corrosive gas

Resources

• User manual
• GitHub
• Schedule Script Generator – You can now use our web app to create and debug the schedule script for Witty Pi