© 1995 Microchip Technology Inc.

DS00610A-page 1

INTRODUCTION

The Microchip Technology Inc. 24LC21 is a 1K-bit

(128 x 8) dual mode serial EEPROM that was

developed primarily for use in computer monitors. This

part was developed with inputs from several computer

(Video Electronics Standards Association) monitor

committee. This committee has developed a serial

communication protocol called Data Display Channel

(DDC

™) which was created to eliminate the need to

change dip switches when configuring a new system

or adding a new monitor or video card. The 24LC21

device is used in the monitor to store and transmit the

EDID (extended display ID) table which contains all

set-up parameters needed by the video card to

operate with a particular monitor. With this system, the

user can now plug any compatible monitor into any

compatible graphics board and the graphics board will

automatically know what type of monitor is being used

and

configure

itself

accordingly. This

automatic

and

Play’

capability

being

built

into

the

new

‘Windows 95

™’ release.

DEVICE OPERATION

The 24LC21 can operate in two modes of operation.

These two modes of operation are the transmit only

mode and bi-directional mode. Upon power-up, the

device will always be in the transmit-only mode.

Transmit only mode is also referred to as DDC1 mode.

The transmit only mode only allows the video card to

read the contents of the 24LC21 in a sequential

manner, one bit at a time. Writing to the device is not

possible in transmit only mode.

The

device

will

automatically

transition

to

the

bi-directional mode whenever a falling edge is seen on

the SCL pin. Bi-directional mode is also referred to as

DDC2 mode, and is implemented as the standard

™ protocol. This allows a controller to read and

write specific addresses in the device like a standard

transitioned to the bi-directional mode, there is no way

to return to transmit only mode other than to reset

(power-down) the device.

Author:

Bruce Negley

Memory Products Division

TRANSMIT ONLY MODE (DDC1)

The 24LC21 will always power-up in the transmit only

mode. In this mode, the 24LC21 will output one bit of

data at the SDA pin for every rising edge on the VCLK

pin. The data will be transmitted in 8-bit words, with

each word followed by a 9th null bit. This null bit will

always be high. A timing diagram for transmit only

mode is shown in Figure 1. As long as VCLK is

present and no falling edges on SCL are received, the

24LC21 will repeatedly cycle through the entire

memory array.

PACKAGE TYPE

Upon power-up, the device will not output valid data

until it has been initialized. This initialization procedure

(Figure 1) data will not be available until after the first

9 clocks are sent to the device. The exact memory

location that the 24LC21 begins to transmit data is

unknown at power-up, and the initialization procedure

only initializes the device, not the starting address or

bit location. In order to for a controller to determine

what address is being read, a ‘framing’ or ‘syncing’

procedure must be executed by the video card.

1

2

3

4

5

6

7

8

VCLK

SCL

SDA

NC

NC

NC

AN610

Using the 24LC21 Dual Mode Serial EEPROM

Thi d

t

t d

ith F

M k

4 0 4