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 are 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).

The AT24CS01 utilizes a separate memory block containing a factory-programmed 128-bit serial number. Access to this memory location is similar to the EERPOM region with the exception that the device type identifier is set to ‘1011’ (Bh) in bits 7 through 4 of the device address byte (see Table 6‑1).

Following the 4-bit device type identifier are the hardware client address bits, A2, A1 and A0. These bits can be used to expand the address space by allowing up to eight Serial EEPROM devices on the same bus. These hardware client address bits must correlate with the voltage level on the corresponding hardwired device address input pins A0, A1 and A2. The A0, A1 and A2 pins use an internal proprietary circuit that automatically biases the pin 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 A0, A1 and A2 pins to a known state whenever possible.

When using the SOT23 package, the A2, A1 and A0 pins are not accessible and are left floating. The previously mentioned automatic pull‑down circuit will set these pins to a logic ‘0’ state. As a result, to properly communicate with the device in the SOT23 package, the A2, A1 and A0 software bits must always be set to logic ‘0’ for any operation. 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 AT24CS01 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 contains a 7-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.