© 1995 Microchip Technology Inc.

DS00610A-page 3

AN610

BI-DIRECTIONAL MODE (DDC2)

protocol and allows the controller to read and write to

the device. The 24LC21 supports byte and page writes

and byte and sequential reads in the bi-directional

mode. This mode will be used primarily before the

monitor leaves the factory to load the EDID table into

the device, but it also provides a means of updating

the table if necessary. It is also used for faster (up to

100 kHz) data transmission, or transmission of only

protocol allows the host to request data from a specific

portion of the EDID table rather than waiting for the

entire table.) When writing to the device, the VCLK pin

must be held high while the write command is being

loaded or the write will be aborted and no data will be

written. Note that this is the opposite of the 24LC01B,

where the WP pin must be held low for the device to

be written.

EDID TABLE

The EDID table is the Extended Display ID table,

specified by VESA, that will be stored in the 24LC21

and contains information about what type of display it

is and the capabilities of the display. The basic EDID

table consists of 128 bytes of data. A breakdown of the

table

is

shown

below

in

Table 1. A

complete

description of the table can be found in the VESA DDC

Specification.

TABLE 1:

EDID TABLE DESCRIPTION

Bytes

Description

8

Header

10

Vendor/Product Description

2

EDID Version/Revision

15

Basic Display Parameters/Features

19

Established/Standard Timings

72

Detailed Timing Descriptions

(18 bytes each)

1

Extension Flag

1

Checksum