24LC256-ISLASHST datasheet(13/42 Pages) MICROCHIP

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DS21930B-page 13

24AAXX/24LCXX/24FCXX

5.6

Device Addressing For Devices

Without Functional Address Pins

A control byte is the first byte received following the

Start condition from the master device (Figure 5-3).

The control byte begins with a four-bit control code. For

the 24XX, this is set as ‘1010

’ binary for read and write

operations. The next three bits of the control byte are

the block-select bits (B2, B1, B0). They are used by the

master device to select which of the 256-word blocks of

memory are to be accessed. These bits are in effect the

three Most Significant bits of the word address. Note

that B2, B1 and B0 are “don’t care” for the 24XX00, the

24XX01 and 24XX02. B2 and B1 are “don’t care” for

the 24XX04. B2 is “don’t care” for the 24XX08.

The last bit of the control byte defines the operation to

be performed. When set to ‘1’, a read operation is

selected. When set to ‘0’ a write operation is selected.

Following the Start condition, the 24XX monitors the

SDA bus. Upon receiving a ‘1010

’ code, the block

select bits and the R/W bit, the slave device outputs an

Acknowledge signal on the SDA line. The address byte

follows the acknowledge.

FIGURE 5-3:

CONTROL AND ADDRESS BYTE ASSIGNMENTS FOR

DEVICES WITHOUT ADDRESS PINS

R/W

ACK

R/W

ACK

R/W

ACK

R/W

ACK

B1 B0

R/W

ACK

B2 B1 B0

R/W

ACK

24XX01

24XX02

24XX04

24XX08

24XX016

= “don’t care” bit

Acknowledge

Control Code

Start bit

Control Byte

Block Select bits

Address Byte

24XX00

Read/Write bit (read = 1, write = 0)

bit

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