24VL024HSLASHMS datasheet(8/28 Pages) MICROCHIP

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24VL024H

DS22109A-page 8

5.0

DEVICE ADDRESSING

A control byte is the first byte received following the

Start condition from the master device (Figure 5-1).

The control byte consists of a four-bit control code; for

the 24VL024H 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 24VL024H devices on

the same bus and are used to select which device is

accessed. The Chip Select bits in the control byte must

correspond 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.

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. Following the Start condition, the 24VL024H

monitors the SDA bus, checking the control byte being

transmitted. Upon receiving a ‘1010’ code and appro-

priate Chip Select bits, the slave device outputs an

Acknowledge signal on the SDA line. Depending on the

state of the R/W bit, the 24VL024H will select a read or

write operation.

FIGURE 5-1:

CONTROL BYTE FORMAT

5.1

Contiguous Addressing Across

Multiple Devices

The Chip Select bits (A2, A1, A0) can be used to

expand the contiguous address space for up to 16K bits

by adding up to eight 24VL024H devices on the same

bus. In this case, software can use A0 of the control

byte as address bit A9, A1 as address bit A10, and A2

as address bit A11. It is not possible to sequentially

read across device boundaries.

SACK

R/W

Control Code

Chip Select

Bits

Slave Address

Acknowledge Bit

Start Bit

Read/Write Bit

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