24LCS21AT-SLASHP datasheet(5/20 Pages) MICROCHIP

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DS21161G-page 5

24LCS21A

3.0

BIDIRECTIONAL MODE

Before the 24LCS21A can be switched into the

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

sition mode, which is done by applying a valid high-to-

low transition on the Bidirectional mode Clock (SCL).

As soon it enters the Transition mode, it looks for a con-

trol 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 con-

tents 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 Bidirec-

tional 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. This mode supports a 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 24LCS21A 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 24LCS21A 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

MODE

ACK

n < 128

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