ADV7125

Rev. C | Page 11 of 16

CIRCUIT DESCRIPTION AND OPERATION

The ADV7125 contains three 8-bit DACs, with three input

channels, each containing an 8-bit register. Also integrated

on board the part is a reference amplifier. The CRT control

functions, BLANK and SYNC, are integrated on board the

ADV7125.

DIGITAL INPUTS

There are 24 bits of pixel data (color information), R0 to R7,

G0 to G7, and B0 to B7, latched into the device on the rising

edge of each clock cycle. This data is presented to the three 8-bit

DACs and then converted to three analog (RGB) output wave-

forms (see Figure 4).

CLOCK

DATA

DIGITAL INPUTS

(R7 TO R0, G7 TO G0,

B7 TO B0,

SYNC, BLANK)

ANALOG OUTPUTS

(IOR, IOR, IOG, IOG,

IOB, IOB)

Figure 4. Video Data Input/Output

The ADV7125 has two additional control signals that are latched

to the analog video outputs in a similar fashion. BLANK and

SYNC are each latched on the rising edge of CLOCK to maintain

synchronization with the pixel data stream.

The BLANK and SYNC functions allow for the encoding of

these video synchronization signals onto the RGB video output.

This is done by adding appropriately weighted current sources

to the analog outputs, as determined by the logic levels on the

BLANK and SYNC digital inputs.

Figure 5 shows the analog output, RGB video waveform of the

ADV7125. The influence of SYNC and BLANK on the analog

video waveform is illustrated.

Table 7 details the resultant effect on the analog outputs of

BLANK and SYNC.

All these digital inputs are specified to accept TTL logic levels.

CLOCK INPUT

The CLOCK input of the ADV7125 is typically the pixel clock

rate of the system. It is also known as the dot rate. The dot rate,

and thus the required CLOCK frequency, is determined by the

on-screen resolution, according to the following equation:

Dot Rate = (Horiz Res) × (Vert Res) × (Refresh

Rate)/(Retrace Factor)

where:

Horiz Res is the number of pixels per line.

Vert Res is the number of lines per frame.

Refresh Rate is the horizontal scan rate. This is the rate at which

the screen must be refreshed, typically 60 Hz for a noninterlaced

system, or 30 Hz for an interlaced system.

Retrace Factor is the total blank time factor. This takes into

account that the display is blanked for a certain fraction of the

total duration of each frame (for example, 0.8).

Therefore, for a graphics system with a 1024 × 1024 resolution,

a noninterlaced 60 Hz refresh rate, and a retrace factor of 0.8,

Dot Rate = 1024 × 1024 × 60/0.8 = 78.6 MHz

The required CLOCK frequency is thus 78.6 MHz. All video

data and control inputs are latched into the ADV7125 on the

rising edge of CLOCK, as previously described in the Digital

Inputs section. It is recommended that the CLOCK input to the

ADV7125 be driven by a TTL buffer (for example, the 74F244).

RED AND BLUE

NOTES

1. OUTPUTS CONNECTED TO A DOUBLY TERMINATED 75Ω LOAD.

3. RS-343 LEVELS AND TOLERANCES ASSUMED ON ALL LEVELS.

mA

V

18.67

0.7

00

mA

V

26.0

0.975

WHITE LEVEL

BLANK LEVEL

SYNC LEVEL

7.2

0.271

00

GREEN

Figure 5. Typical RGB Video Output Waveform