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MAX3816A

I2C 2-Wire Extender for DDC in DVI, HDMI, and VGA Interfaces

Maxim Integrated 

I2C 2-Wire Extender for DDC in DVI,

HDMI, and VGA Interfaces

meter (> 3000pF) cable with a controlled slew-rate.

Similarly, the display side is pulled down to VSS with

a controlled slew rate, open-drain n-channel MOS

device. These buffers stay on for 1.75µs.

3) Low state. Level sensing off (Holdoff).

Level sensing remains off until the completion of the

holdoff period, which is 2.5µs on both the clock and

data channels.

In series mode, drivers maintain low level (at or

below VHOLD). Either the client pulldowns sustain

the level below VOL, or the MAX3816A sustains the

level at VHOLD if no other driver on the same node

is pulling down. This action is in support of the

“wired-AND” function across source and display

sides.

4) Low state, drivers off. Level sensing on.

After the MAX3816A holdoff time completes, level

sensing resumes.

In series mode, the MAX3816A supports a “wired-

AND” connection between source and display

sides; returning to the high state is supported only

when all client sources turn off. If either the source

or display side releases the bus, but not both, a

MAX3816A level-sensing buffer senses the transi-

tion at VTRIGIL, supporting the existing low state by

clamping the voltage to VHOLD and waiting for the

remaining side to release the bus.

5) Low-to-high transition. Drivers on. Level sensing

off (Holdoff).

A change of state is initiated when no device is

holding the bus low on both source and display

sides. When both sides exceed their respective

VTRIGIL levels, the source side turns on a slew-rate

controlled open-drain p-channel device, pulling up

to VCC for 1.75µs. Simultaneously, the display side

is released and the pullup resistors pull the display-

side bus up to VDD, as per normal I2C operation.

6) High state. Drivers off. Level sensing off

(Holdoff).

During holdoff, no transitions are sensed. The high

state is maintained by external pullup resistors. Upon

the end of holdoff, when the cable and display levels

are above 85%, the state machine transitions to state

(1); otherwise, it waits until levels raise above 85% to

transition to state (1). Data, but not clock, has anoth-

er exit from state (6) to (1) upon data source or data

display levels dropping below 60%.

I2C continuous clock applications are not recommend-

ed for the MAX3816A. The MAX3816A is optimized for

DDC applications with a noncontinuous clock.

Detailed Description

The MAX3816A DDC/I2C 2-wire extender consists of

two controllers with level-shifters, cable drivers, display

drivers, and level-sensing circuitry (Figure 2).

Controllers and Level Shifters

The MAX3816A functionality is governed by two con-

trollers, one for CLOCK and one for DATA. Bidirectional

signaling is fully supported on both CLOCK and DATA.

The primary function of the controllers is to receive the

state-change information from the source- and display-

side level-sense circuitry and support the “wired-AND”

function between the two. When the state changes, a

holdoff period is timed during which the source and

display drivers assert the next state, high or low, and all

input sensing is ignored while I/O transients settle

(Figure 3). The holdoff period is approximately 2.5µs.

The cable transmission-line termination feature is active

only during the first 1.75µs of holdoff, sufficiently long

enough to absorb roundtrip reflections from a 60m

cable.

In series mode, the CLOCK and DATA controllers iso-

late the source electronics from the display electronics.

The cable side of the MAX3816A is referenced to VCC

and GND_REF, and the display side is referenced to

VDD and VSS. This power scheme provides tolerance to

offset and noise between the source and display

devices.

Cable Drivers

The low-impedance cable drivers (Figure 10) can

charge and discharge at least 3000pF of capacitive

cable load within the I2C rise and fall time limits. The

drivers each incorporate a slew-rate limiter to control

the amount of high-frequency energy transmitted. The

cable drivers also provide a back termination imped-

ance of approximately 60Ω to absorb transmission-line

reflections returning to the driver. The cable drivers

each include a high-state current-limiting feature to

clamp the output current to less than 16mA.

After 1.75µs of driver assertion, following a decision to

transition, the low-impedance drivers are turned off.

Subsequently, when another device asserts a new

state, it does not have to work against the low imped-

ance of the MAX3816A.

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