Insanely Small T402 Atto Is a Complete Microcontroller Development Board

Just under eight millimeters-cubed, you get: six I/O pins, serial, ADC, and 4K of flash memory.

James Lewis
3 years ago
T402 ATto on top of a Z80 (for size comparison)

Creating minimalist solutions within tight constraints is one of Alun Morris' passions. Their latest design is the insanely small T402 Atto microcontroller development board.

"You can buy some absolutely tiny Arduino development boards, e.g. the Beetle (20x22x3.8mm) and the 32-bit Seeeduino XIAO (20x17.5x3.5mm) but I wanted to see how far I could push it without a custom PCB."

T402 Atto forgoes the typical ATtiny85 used in most teeny MCU implementations. Instead, it saves 1.27mm of width by using an ATtiny402 0-series chip in a narrow SOIC package. The microcontroller features 4 kilobytes of program flash, 256 bytes of RAM, and 128 bytes of EEPROM. Like similar AVR chips, its clock can run up to 20MHz.

Even though the microcontroller only has eight pins, it packs significant functionality and flexibility into them. Five pins are available for input and output. Typical buses and peripherals like analog-to-digital converter, touch controller, SPI, I2C, and timers are also available. The sixth pin is unique since it is a UPDI programming pin.

UPDI stands for Unified Program and Debug Interface. Proprietary to AVR XMEGA chips, UPDI replaces the previous generation two-wire physical interface. A benefit to UPDI is that programmers are very inexpensive. For example, here are instructions to use a Nano to create a UPDI programmer for less than $10. If you have a high-voltage UPDI programmer available, you can change the programming pin's function to an input pin instead.

At the base of the T402 Atto is a 1.27mm "matrix" protoboard. This array of prototype holes makes it possible to attach the processor and solder 0805-sized components. Morris added 1.27mm headers to the design to make the I/O pins more accessible.

The entire microcontroller is only 6x6x8 millimeters! Though, the board gets slightly less teeny if you add a programming header.

When you are ready to build one, check out the extensive instructions in this T402 Atto Instructables post. It includes a full circuit description, bill of materials, and step-by-step construction instructions. Morris even covered the software needed to make the ATtiny402 programmable in the Arduino IDE.

James Lewis
Fan of making things that blink, fly, or beep. Host on element14 Presents, baldengineer.com, AddOhms, and KN6FGY.
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