AT24C21

5

Functional Description

The AT24C21 has two modes of operation: the Transmit-

Only Mode and the Bidirectional Mode. There is a separate

2-wire protocol to support each mode, each having a sepa-

rate clock input (SCL and VCLK) and both modes sharing a

common Bidirectional data line (SDA). The AT24C21

enters the Transmit-Only Mode upon powering up the

device. In this mode, the device transmits data on the SDA

pin upon a clock signal on the VCLK pin. The device will

remain in the Transmit-Only Mode until a valid high-to-low

transition takes place on the SCL pin. The device will

switch into the Bidirectional Mode when a valid transition

on the SCL pin is recognized. Once the device has transi-

tioned to the Bidirectional Mode, there is no way to return to

the Transmit-Only Mode, except to power down (reset) the

device.

Transmit-Only Mode (DDC1)

The AT24C21 will power up in the Transmit-Only Mode. In

this mode, the device will output one bit of data on the SDA

pin on each rising edge on the VCLK pin. Data is transmit-

ted in 8 bit words with the most significant bit first. Each

word is followed by a 9th “don't care” bit which will be in

high impedance state (refer to Figure 1). The AT24C21 will

continuously cycle through the entire memory array in

incremental sequence as long a VCLK is present and no

falling edges on SCL are received. When the maximum

address (7FH) is reached, the output will wrap around to

the zero location (00H) and continue. The Bidirectional

mode clock (SCL) pin must be held high for the device to

remain in the Transmit-Only mode.

Upon power-up, the AT24C21 will not output valid data until

it has been initialized. During initialization, data will not be

available until after the first nine clocks are sent to the

device (refer to Figure 2). The starting address for the

Transmit-Only mode can be determined during initializa-

tion. If the SDA pin is held high during the first eight clocks

(refer to Figure 2), the starting address will be 7FH. If the

SDA pin is low during the first eight clocks, the starting

address will be 00H. During the ninth clock, SDA should be

in high impedance.

Figure 1. Transmit-Only Mode

Figure 2. Device Initialization for Transmit-Only Mode