The infamous cheap AT24C256 I2C EEPROM board provides 32K bytes of storage for your projects:

  • Cost: ~$2

  • Size: 32768 bytes

  • VCC: 2.7 - 5.5V

  • It includes I2C pull-ups

  • Speeds: 1 MHz (5V), 400 kHz (2.7V, 2.5V)

  • 64-byte Page Write Mode (Partial Page Writes Allowed).

  • I2C address ranges from 0x50 to 0x57 and can be configured using the A jumpers (A0-A2)

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Connections

Bus PirateAT24C256 boardDescription
SDASDAI2C Data
SCLSCLI2C Clock
Vout/VrefVCC5volt power supply
GNDGNDGround

Setup

Considering:

  • AT24C256’s datasheet max speed
  • AT24C256 board pull-ups are 10k
  • You’ve purchased a low-quality clone from AliExpress or any other source
  • The length of the Bus pirate cable

We’re going to be very conservative and operate at:

  • 5V, 100kHz.
  • Max current: 50ma.
Bus Pirate [/dev/ttyS0]
HiZ> m

Mode selection
 1. HiZ
 2. 1-WIRE
 3. UART
 4. I2C
 5. SPI
 6. LED
 x. Exit
Mode > 4
I2C speed
 1KHz to 1000KHz
 x. Exit
KHz (400KHz*) > 100
Data bits
 1. 8*
 2. 10
 x. Exit
Bits (1) > 
Mode: I2C
I2C> W
Power supply
Volts (0.80V-5.00V)
x to exit (3.30) > 5
5.00V requested, closest value: 5.00V
Set current limit?
y

Maximum current (0mA-500mA)
x to exit (100.00) > 50
50.0mA requested, closest value: 50.0mA

Power supply:Enabled

Vreg output: 4.9V, Vref/Vout pin: 4.9V, Current sense: 9.2mA

I2C>
  • Use the m mode command and select I2C
  • Configure I2C for 100kHz and 8bits of data
  • Enable the onboard power supply with the W command, and configure it for 5volts output.
  • Select a current limit of at least 50mA.

Partial write

We’ll write three bytes - 0x41, 0x42, 0x43 - to the EEPROM at memory location 0x69.

Bus Pirate [/dev/ttyS0]
I2C> [0xA0 0x00 0x69 0x41 0x42 0x43]

I2C START
TX: 0xA0 ACK 0x00 ACK 0x69 ACK 0x41 ACK 0x42 ACK 0x43 ACK 
I2C STOP
I2C>
  • [ Begin with an I2C START
  • 0xA0 is the I2C device address.
  • 0x00 0x69 is the memory address where we intend to write. The AT24C256 has a 16bit/2byte address range.
  • 0x41 0x42 0x43 are the three data bytes we want to write.
  • ] End with an I2C STOP

Reading three bytes

We’ll read three bytes from the EEPROM at memory location 0x69. To achieve this, two commands are necessary.

Bus Pirate [/dev/ttyS0]
I2C> [0xA0 0x00 0x69]

I2C START
TX: 0xA0 ACK 0x00 ACK 0x69 ACK 
I2C STOP
I2C>

First, use a write command with no data to to indicate the memory address we want to read.

  • [ Begin with an I2C START
  • 0xA0 is the I2C device write address.
  • 0x00 0x69 is the 16bit/2byte memory address to read.
  • ] End with an I2C STOP
Bus Pirate [/dev/ttyS0]
I2C> [0xA1 r:3]

I2C START
TX: 0xA1 ACK 
RX: 0x41 ACK 0x42 ACK 0x43 NACK 
I2C STOP
I2C>

Second, use the I2C read address to read the bytes from that memory location.

  • [ Begin with an I2C START
  • 0xA1 is the I2C device read address to access data at the location we set with the previous command.
  • r:3 reads back three bytes of data. Bytes read: 0x41 0x42 0x43.
  • ] End with an I2C STOP

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