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
monitor manufacturers, in accordance with the VESA
(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
configuration is the cornerstone for Microsoft
’s ‘Plug
and Play’ capability being built into the new
‘Windows 95
’ release. AN610
Using the 24LC21 Dual Mode Serial EEPROM
TRANSMIT ONLY MODE (DDC1)
Author: Bruce Negley
Memory Products Division 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
INTRODUCTION pin. The data will be transmitted in 8-bit words, with
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