24LCS22AT datasheet(5/26 Pages) MICROCHIP

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DS21682E-page 5

24LCS22A

3.0

BIDIRECTIONAL MODE

Before the 24LCS22A can be switched into the

Bidirectional mode (Figure 3-1), it must enter the

Transition mode, which is done by applying a valid

high-to-low transition on the Bidirectional mode clock

(SCL). As soon as it enters the Transition mode, it looks

for a control byte ‘1010 000X’ on the I2C™ bus, and

starts to count pulses on VCLK. Any high-to-low

transition on the SCL line will reset the count. If it sees

a pulse count of 128 on VCLK while the SCL line is idle,

it will revert back to the Transmit-Only mode, and

transmit its contents starting with the Most Significant

bit in address 00h. However, if it detects the control

byte on the I2C bus (Figure 3-2), it will switch to the

Bidirectional mode. Once the device has made the

transition to the Bidirectional mode, the only way to

switch the device back to the Transmit-Only mode is to

remove power from the device. The mode transition

process is shown in detail in Figure 3-3.

Once the device has switched into the Bidirectional

mode, the VCLK input is disregarded, with the

exception that a logic high level is required to enable

write capability. In Bidirectional mode the user has

access to the entire 2K array, whereas in the Transmit-

Only mode, the user can only access the first 1K. This

mode

supports

two-wire

bidirectional

data

transmission protocol (I2C). In this protocol, a device

that sends data on the bus is defined to be the

transmitter, and a device that receives data from the

bus is defined to be the receiver. The bus must be

controlled by a master device that generates the

Bidirectional mode clock (SCL), controls access to the

bus and generates the Start and Stop conditions, while

the 24LCS22A acts as the slave. Both master and

slave can operate as transmitter or receiver, but the

master device determines which mode is activated. In

the Bidirectional mode, the 24LCS22A only responds

to commands for device ‘1010 000X’.

FIGURE 3-1:

MODE TRANSITION WITH RECOVERY TO TRANSMIT-ONLY MODE

FIGURE 3-2:

SUCCESSFUL MODE TRANSITION TO BIDIRECTIONAL MODE

TVHZ

SCL

SDA

VCLK

Transmit-Only

MODE

Bidirectional

Recovery to Transmit-Only mode

Bit8

(MSB of data in 00h)

VCLK count =

127 128

Transition mode with possibility to return to Transmit-Only mode

Bidirectional

permanently

SCL

SDA

VCLK count =

VCLK

Transmit-Only

MODE

ACK

n < 128

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