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ADV7125 데이터시트(PDF) 11 Page - Analog Devices |
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ADV7125 데이터시트(HTML) 11 Page - Analog Devices |
11 / 16 page 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. 2. VREF = 1.235V, RSET = 530Ω. 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 |
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