The four pins

Four pins, all four wired. GND is the square pad, VCC goes to your board's logic supply, and SDA and SCL are the two wires of an I2C bus. There is no light on this board: the only sign it is alive is the chip answering at 0x50.

Updated 2026-09-244 min readEEPROM memory

What is on the board

One memory chip, four small parts beside it, and a four-pin header along the bottom edge. The header is right-angled, so the pins point out past the edge of the board and it lies flat in a breadboard or on a cable.

The TK31 seen from the parts side: a black board with a wide gold border, a black eight-legged chip near the top with a small dot by its top-left leg, two brown capacitors under it, a white memory-chip icon with MEMORY in the middle, two big round mounting holes, two small black resistors at the lower right, lonely binary up the left edge, and a right-angle header labelled GND, VCC, SDA and SCL from left to right. — The parts side. The chip is at the top; GND is the left-hand pin.

The chip keeps 16,384 bytes with the power off. A sketch writes a setting, the board is unplugged, and a week later the setting is still there. That is the whole job, and it takes two signal wires.

The four names

The four pins, and the back

Show

Pins to wire

4 of 4

Address

0x50

Pitch

2.54 mm

SDA, serial data, carries every bit in both directions: your board's addresses and bytes one way, the chip's acknowledges and the bytes it reads back the other. Nothing drives it high; a device pulls it low and the pull-up lets it back up.

GND is on the square pad. Count from it.

VCC powers the chip, and it is also where the two pull-up resistors end. So VCC decides the voltage on SDA and SCL. Wire it to the voltage your board's pins run at: 5V on an Uno, 3V3 on an ESP32, an ESP32-S3 or a Pico. VCC sets the bus is the reason.

SDA is serial data and SCL is serial clock: the two wires of an I2C bus, the same two wires a temperature sensor or a display on I2C uses. Your board drives the clock; data goes both ways on SDA.

Every pin is wired. Unlike most TinkerBlock blocks, this one has no NC.

The back of the board

The back of the TK31: a black board with a gold border, lonely binary across the top, TK31 and EEPROM MEMORY in a boxed label, I2C PULL-UP printed over two small gold pads joined by a thin strip, a white memory-chip icon, four header pads in a row with the square one on the right, and NON-VOLATILE MEMORY along the bottom. — The back. The square pad is on the right now, and it is still GND.

Turned over, the order reverses: the square pad is on the right. The back prints TK31, the number on the box, but no pin names. The two gold pads under I2C PULL-UP are joined by a thin strip of copper, and that strip is the subject of the pull-up jumper. Leave it alone for now.

When it does not work

Which pin is GND?

Hold the board with the parts facing you and the header pins pointing down. Read GND, VCC, SDA, SCL from left to right. GND is also the square pad, which is the one mark that survives when the labels are hidden in a breadboard.

Nothing lights up when I plug it in.

Nothing is meant to. The TK31 has no power LED. Run the I2C scan from the troubleshooting article: if it lists 0x50, the board is powered and wired, whatever it looks like.

Can SDA and SCL go to any two pins?

On an Uno, no: I2C is A4 and A5 and nowhere else. An ESP32, an ESP32-S3 or a Pico can move I2C to other pins, but this book uses each board's default pins so that Wire.begin() needs no arguments.

What if I swap SDA and SCL?

Nothing is damaged. The chip simply never sees a valid start, and the scan finds nothing at 0x50. Swap the two wires back.

Alongside this page

I2C, the two-wire bus What a pull-up resistor does

Where this goes next

The chip, its address pins, and the jumper on the back.

What is on it →

Edit this page — content/books/eeprom-memory/the-four-pins.mdx

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