24LC21A datasheet(4/26 Pages) MICROCHIP

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24LC21A

DS21160G-page 4

2.0

FUNCTIONAL DESCRIPTION

The 24LC21A is designed to comply to the DDC

Standard proposed by VESA (Figure 3-3) with the

exception that it is not Access.bus capable. It operates

in two modes, the Transmit-Only mode and the

Bidirectional mode. There is a separate 2-wire protocol

to support each mode, each having a separate clock

input but sharing a common data line (SDA). The

device enters the Transmit-Only mode upon power-up.

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

pin in response to a clock signal on the VCLK pin. The

device will remain in this mode until a valid high-to-low

transition is placed on the SCL input. When a valid

transition on SCL is recognized, the device will switch

into the Bidirectional mode and look for its control byte

to be sent by the master. If it detects its control byte, it

will stay in the Bidirectional mode. Otherwise, it will

revert to the Transmit-Only mode after it sees 128

VCLK pulses.

2.1

Transmit-Only Mode

The device will power-up in the Transmit-Only mode at

address 00h. This mode supports a unidirectional

2-wire protocol for continuous transmission of the

contents of the memory array. This device requires that

it be initialized prior to valid data being sent in the

Transmit-Only mode (Section 2.2 “Initialization Pro-

cedure”). In this mode, data is transmitted on the SDA

pin in 8-bit bytes, with each byte followed by a ninth,

null bit (Figure 2-1). The clock source for the Transmit-

Only mode is provided on the VCLK pin, and a data bit

is output on the rising edge on this pin. The eight bits in

each byte are transmitted Most Significant bit first.

Each byte within the memory array will be output in

sequence. After address 7Fh in the memory array is

transmitted, the internal Address Pointers will wrap

around to the first memory location (00h) and continue.

The Bidirectional mode Clock (SCL) pin must be held

high for the device to remain in the Transmit-Only

mode.

2.2

Initialization Procedure

After VCC has stabilized, the device will be in the Trans-

mit-Only mode. Nine clock cycles on the VCLK pin

must be given to the device for it to perform internal

sychronization. During this period, the SDA pin will be

in a high-impedance state. On the rising edge of the

tenth clock cycle, the device will output the first valid

data bit which will be the Most Significant bit in address

00h. (Figure 2-2).

FIGURE 2-1:

TRANSMIT-ONLY MODE

FIGURE 2-2:

DEVICE INITIALIZATION

SCL

SDA

VCLK

Tvaa

Bit 1 (LSB)

Null Bit

Bit 1 (MSB)

Bit 7

Tvlow

Tvhigh

Tvaa

Bit 8

Bit 7

High-Impedance for 9 Clock Cycles

Tvpu

SCL

SDA

VCLK

Vcc

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