24LC512-ISLASHST datasheet(14/42 Pages) MICROCHIP

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24AAXX/24LCXX/24FCXX

DS21930B-page 14

5.7

Device Addressing For Devices

With Functional Address Pins

A control byte is the first byte received following the

Start condition from the master device (Figure 5-4).

The control byte begins with a 4-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 Chip Select bits (A2, A1, A0). The Chip Select bits

allow the use of up to eight 24XX devices on the same

bus and are used to select which device is accessed.

The Chip Select bits in the control byte must corre-

spond to the logic levels on the corresponding A2, A1

and A0 pins for the device to respond. These bits are,

in effect, the three Most Significant bits of the word

address.

For 24XX128 and 24XX256 in the MSOP package, the

A0 and A1 pins are not connected. During device

addressing, the A0 and A1 Chip Select bits (Figure 5-4)

should be set to ‘0’. Only two 24XX128 or 24XX256

MSOP packages can be connected to the same bus.

The last bit of the control byte defines the operation to

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

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

selected.

For

higher

density

devices

(24XX32

through

24XX512), the next two bytes received define the

address of the first data byte. Depending on the prod-

uct density, not all bits in the address high byte are

used. A15, A14, A13 and A12 are “don’t care” for

24XX32. A15, A14 and A13 are “don’t care” for

24XX64. A15 and A14 are “don’t care” for 24XX128.

A15 is “don’t care” for 24XX256. All address bits are

used for the 24XX512. The upper address bits are

transferred first, followed by the Less Significant bits.

Following the Start condition, the 24XX monitors the

SDA bus. Upon receiving a ‘1010

’ code, appropriate

device select bits and the R/W bit, the slave device out-

puts an Acknowledge signal on the SDA line. The

address byte(s) follow the acknowledge.

FIGURE 5-4:

CONTROL AND ADDRESS BYTE ASSIGNMENTS FOR

DEVICES WITH ADDRESS PINS

A2 A1 A0 R/W ACK

24XX64

24XX128

24XX256

24XX512

= “don’t care” bit

Acknowledge

Control Code

Start bit

Control Byte

Chip Select bits*

Address High Byte

24XX32

Read/Write bit

xxx

A11 A10 A9 A8

A12 A11 A10 A9 A8

A13 A12 A11 A10 A9 A8

A14 A13 A12 A11 A10 A9 A8

A15 A14 A13 A12 A11 A10 A9 A8

bit

R/W

ACK

R/W

ACK

R/W

ACK

24XX024/025

24C02C

24C01C

Address Byte

. A0

Address Low Byte

* Chip Select bits A1 and A0 must be set to ‘0’ for 24XX128/256 devices in the MSOP package.

Control Byte

(Read = 1, Write = 0)

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