24LC21A

DS21160F-page 6

 2003 Microchip Technology Inc.

FIGURE 3-3:

DISPLAY OPERATION PER DDC STANDARD PROPOSED BY VESA

Communication

is idle

Is Vsync

present?

No

Send EDID continuously

using Vsync as clock

High-to-low

transition on

SCL?

No

Yes

Yes

Stop sending EDID.

Switch to DDC2 mode.

Display has

transition state

?

optional

Set Vsync counter = 0

Change on

VCLK lines?

SCL, SDA or

No

Yes

High - low

transition on SCL

?

Reset Vsync counter = 0

No

Yes

Valid

received?

DDC2 address

No

No

VCLK

cycle?

Yes

Increment VCLK counter

Yes

Switch back to DDC1

mode.

DDC2 communication

idle. Display waiting for

address byte.

DDC2B

address

received?

Yes

Receive DDC2B

command

Respond to DDC2B

command

Is display

Yes

Valid Access.bus

address?

No

Yes

See Access.bus

specification to determine

correct procedure.

Yes

No

Yes

No

No

No

The 24LC21A was designed to

Display Power-on

or

DDC Circuit Powered

from +5 volts

or start timer

Reset counter or timer

(if appropriate)

Counter=128 or

timer expired?

High-to-low

transition on

SCL?

No

Yes

comply to the portion of flowchart inside dash box

Note 1:

The base flowchart is copyright

 1993, 1994, 1995 Video Electronic Standard Association (VESA) from

VESA’s Display Data Channel (DDC) Standard Proposal ver. 2p rev. 0, used by permission of VESA.

2: The dash box and text “The 24LC21A and... inside dash box.” are added by Microchip Technology Inc.

3: Vsync signal is normally used to derive a signal for VCLK pin on the 24LC21A.

capable?