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AN610
Using the 24LC21 Dual Mode Serial EEPROM
Author:
Bruce Negley
Memory Products Division
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
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.
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
I²C™ protocol. This allows a controller to read and
write specific addresses in the device like a standard
I²C Serial EEPROM device. Once the device has
transitioned to the bi-directional mode, there is no way
to return to transmit only mode other than to reset
(power-down) the device.
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
NC 1
NC 2
NC 3
VSS 4
5 VCC
6 VCLK
7 SCL
8 SDA
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.
© 1995 Microchip Technology Inc.
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DS00610A-page 1