 2003 Microchip Technology Inc.

DS21160F-page 5

24LC21A

3.0

BIDIRECTIONAL MODE

Before the 24LC21A 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 it enters the Transition mode, it looks

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

tion 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

in 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. This mode supports a two-wire

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 24LC21A 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 24LC21A 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 =

1

2

3

4

127 128

Transition mode with possibility to return to Transmit-only mode

Bidirectional

permanently

SCL

SDA

VCLK count =

1

2

n

0

VCLK

Transmit

Only mode

MODE

S1

0

1

0

0

00

0

ACK

n < 128