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LM1270 データシート(PDF) 10 Page - National Semiconductor (TI) |
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LM1270 データシート(HTML) 10 Page - National Semiconductor (TI) |
10 / 27 page Functional Description (Continued) The LM1270 has the counters required to locate the clamp pulse to any position on the horizontal line. Normally this pin would be used as an output and be connected to the video pre-amps. Now the clamp pulse location can be optimize for the video channel. Note: When using this pin as an output a 1k Ω series resistor must be used to protect the output stage of the clamp circuit. It is also recommended to connect a 10 k Ω resistor between this pin and ground for more reliable operation. Video Detection MODES OF OPERATION Three modes of operation are possible with the video detect circuit: 1. Autosizing mode: in this mode the LM1270 takes the measurements necessary for the monitor to perform the autosizing operation. The horizontal flyback is used as the reference for timing. The resultant horizontal outputs are the flyback time, the position of the start of video relative to the flyback end and the time from the end of the active video to the start of the flyback time. Since the total line time is known the microcontroller can calculate the active video time. Now the microcontroller can cen- ter the video between the start and end of flyback for best image centering, and to calculate the duty cycle of the video with respect to the forward scan time, thus giving a measure of the relative size of the image. For best performance in autosizing mode, it is recom- mended that the maximum pixels per line mode is used when measurements are made. 2. Window calibration mode: in this mode, the horizontal sync is used as the reference for timing. The resultant horizontal outputs are the horizontal sync time and the back porch and active image video time that can subse- quently be used in conjunction with the image position data. The image pixel per line value should be used to set the PLL. 3. Video data transfer: in this mode, data is sent from the PC to the monitor on the first active video line. A 72 pulse clock string is sent on one video channel. Data is sent on a second video channel. A window signal that tracks the actual highlight window is sent on the third video chan- nel. The timing parameters of this window signal are timed by the video detect logic and stored in a special register set. The relationship between these three sig- nals is shown in Figure 3. The selection of which video channel contains which signal is programmed by the D_CNTL register. IMAGE CALIBRATION MODE Two registers are used for each axis to control the window enable signal, WIN_EN. The first register HW_START (VW_START) defines the period in pixels (lines) from the leading edge of the Hsync (Vsync) to the start of the window. The second register HW_END (VW_END) defines the dura- tion of the highlight window. Each of these registers is de- fined as two byte values, as per the table below: The microcontroller must write the values of these registers, after it has received the window position information from the PC. However, the PC may reference this information in any of the following ways: 1. Leading edge of sync 2. Trailing edge of sync 3. Start of the video image In order to accommodate different modes of operation, the video detect function may be used to measure the sync width, and the back porch (i.e., trailing edge of sync to start of video) periods in horizontal pixels, or vertical lines for window position calibration. The video detect function must be set to measure with reference to the horizontal sync. If the leading edge of sync is used as the reference for the window start timing from the PC application, then the start values can be directly written to the registers HW_START and VW_START. If the trailing edge of sync is used as the reference for the window start timing from the PC application, then the micro- controller should add the measured sync durations from the video detect registers to the timing values supplied by the PC application before writing to the HW_START and VW_START registers. If the start of video image is used as the reference for the window start timing from the PC application, then the micro- controller will add the measured sync duration and the back porch duration from the video detect registers to the timing values supplied by the PC application before writing to the HW_START and VW_START registers. In each case, it may also be necessary to modify the window position values by adding or subtracting a value to compen- 20063520 FIGURE 3. Data, Clock and Window Signals www.national.com 10 |
同様の部品番号 - LM1270 |
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同様の説明 - LM1270 |
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