6.1 Device Addressing

Accessing the device requires an 8‑bit device address byte following a Start condition to enable the device for a read or write operation. Since multiple client devices can reside on the serial bus, each client device must have its own unique address so the host can access each device independently.

The Most Significant four bits of the device address byte is referred to as the device type identifier. The device type identifier ‘1010’ (Ah) is required in bits 7 through 4 of the device address byte (see Table 6‑1).

Following the 4-bit device type identifier is the hardware client address bits, A2. This bit can be used to expand the address space by allowing up to two Serial EEPROM devices on the same bus. The A2 value must correlate with the voltage level on the corresponding hardwired device address input pin A2. The A2 pin uses an internal proprietary circuit that automatically biases it to a logic ‘0’ state if the pin is allowed to float. In order to operate in a wide variety of application environments, the pull‑down mechanism is intentionally designed to be somewhat strong. Once the pin is biased above the CMOS input buffer's trip point (~0.5 x V_CC), the pull‑down mechanism disengages. Microchip recommends connecting the A2 pin to a known state whenever possible.

When using the SOT23 package, the A2 pin is not accessible and is left floating. The previously mentioned automatic pull‑down circuit will set this pin to a logic ‘0’ state. As a result, to properly communicate with the device in the SOT23 package, the A2 software bit must always be set to logic ‘0’ for any operation.

Following the A2 hardware client address bit are bits A9 and A8 (bit 2 and bit 1 of the device address byte), which are the two Most Significant bits of the memory array word address. Refer to Table 6‑1 to review these bit positions.

The eighth bit (bit 0) of the device address byte is the Read/Write Select bit. A read operation is initiated if this bit is high and a write operation is initiated if this bit is low.

Upon the successful comparison of the device address byte, the AT24C08D will return an ACK. If a valid comparison is not made, the device will NACK.

For all operations except the current address read, a word address byte must be transmitted to the device immediately following the device address byte. The word address byte consists of the remaining eight bits of the 10-bit memory array word address, and is used to specify which byte location in the EEPROM to start reading or writing. Refer to Table 6-2 to review these bit positions.