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LM1270 データシート(PDF) 8 Page - National Semiconductor (TI) |
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LM1270 データシート(HTML) 8 Page - National Semiconductor (TI) |
8 / 27 page Functional Description VIDEO SECTION The LM1270 gives the monitor designer the ability to add a highlighted window to the CRT monitor. Special software provided manufacturer enables the user to select the area he wants to have highlighted. National Semiconductor refers to this feature as Hi-Brite. Referring to Figure 1, the Block Diagram of the LM1270, note that there are two channels for each video input. Of the three video channels only the blocks of the blue channel are shown since all three channels have the same blocks. A buffer stage is located at the video inputs and this is followed by a contrast control. Referring to chan- nel one (B1 input), the contrast control is followed by an emphasis block. The combination of the emphasis control and the contrast control gives the desired highlighted video inside the Hi-Brite window. The output of the emphasis stage and the output of the contrast control in channel two go to a high-speed video switch, which selects the video channel going to the output stage. Having an independent contrast control in each video chan- nel allows the user to adjust the video gain, normally having the higher gain in channel one, giving a “brighter” picture within the Hi-Brite window. The Emphasis stage is used to give more “sparkle” to the highlighted video. Video going into the emphasis stage has peaking added to the video. Both the amplitude and the duration of the peaking are adjustable through the I 2C compatible bus, optimizing the emphasis for different video resolutions. Maximum peaking is 20%. Al- though most uses of the Hi-Brite would be for the video inside the selected window, the reverse may be selected where all the video outside the window is processed through the emphasis stage. Please remember that for all video inputs the corresponding channel one input and the channel two input must be shorted together. Standard AC coupling such as used with the LM123X or LM124X pre-amps is to be used with the LM1270. I 2C COMPATIBLE INTERFACE Pins 13 and 14 receive signals from an I 2C compatible bus. The interface section is used to decode the I 2C compatible signal and update the registers. Note that all functions of the LM1270 are controlled through an I 2CoranI2C compatible bus. Details on the internal registers are covered in the I 2C compatible Interface Registers Section. PLL A phase locked loop (PLL) is required for the LM1270 to sync all the timing circuits to the incoming horizontal sync. Proper operation of both the Hi-Brite window and the video detec- tion requires a proper lock of the PLL reference pulse to the horizontal sync. Careful layout of the loop filter at pin 10 is necessary for keeping noise out of the PLL section. The register to the PLL is used to set the number of pixels per line for the internal counters that set the Hi-Brite window, gener- ate the internal clamp pulse, and set the counters for the video detection. The maximum number of pixels per line is 2,047, or 7Fh. For good resolution of the window, 640 is the recommended minimum pixels per line. The VCO has a running frequency range of 110 MHz to 160 MHz when using prescaler 7. For the other pre-scaler settings the frequency range is 120 MHz to 220 MHz. The output of the VCO goes to a pre-scaler with a 3 bit register, dividing the VCO output by up to 8. Minimum division is 1, just a straight pass through the pre-scaler. Calculating the VCO frequency: F VCO = VCO frequency F H = Horizontal line frequency N = Number of pixels per line M = PLL pre-scaler Using the above terms gives the following formula for the PLL: F VCO =N * M * FH It is easiest to run the PLL in AUTO mode, setting bit 7 of address 0E to a “1”. Here the user programs in the desired pixels per line and the part will select the preferred pre-scaler number. If one is using manual mode below is a table showing the relationship between the pre-scaler and the VCO frequency. The chart shows the pixel frequency, F P. F P =FVCO/M The necessary pixels per line is calculated by the following formula: N=F P/FH PRES C2 PRES C2 PRES C2 PRES DIV RATIO F P(max) MHz F p(min) MHz 1 1 1 1 160.0 110.0 1 1 0 2 110.0 60.0 1 0 1 3 73.3 40.0 1 0 0 4 55.0 30.0 0 1 1 5 44.0 24.0 0 1 0 6 36.7 20.0 0 0 1 7 31.4 17.1 0 0 0 8 27.5 10.0 TIMING AND WINDOW GENERATION SECTION All timing is referenced to the vertical and horizontal sync inputs that come in on pins 11 and 12 respectively. The two registers near the inputs sets the start of the window for both the horizontal and vertical direction. The window generator and the following logic are used to generate a pulse to control the video switch used to switch between the video with emphasis and the standard video. The switching be- tween the two video channels is the generation of the win- dow seen by the user. There is the option to switch off the Hi-Brite window. An additional switch located just before the video switch is used to turn off the control signal. Video detection is another important function inside the LM1270. Note that both video channels, the PLL, and both vertical sync and horizontal flyback signals all go to this block. From these inputs the video detection is used to detect the length of the horizontal and vertical front porch as well as the length of the active video for both the horizontal and vertical directions. This information allows the monitor designer to offer auto alignment in his monitor. Further pro- cessing of the video information in the video and window timing data section generates an internal clamp pulse. This is the DC restoration pulse for the LM1270. Pin 24 is the output for the clamp pulse, allowing the monitor designer to use it for the clamp input of the pre-amp. There is also the option to program the LM1270 to accept an external clamp pulse instead of using its own internally generated pulse. The video detection section also has a data receiver for receiving data encoded on the video. When this feature is used, the MCU in the monitor is used to control the LM1270, keeping all communication with the LM1270 within the moni- tor. This results in a faster response time by eliminating the need for the computer to control the LM1270. Control by the computer requires communication through the monitor MCU. www.national.com 8 |
同様の部品番号 - LM1270 |
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同様の説明 - LM1270 |
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