TI中文支持网Part Number:AM5708Other Parts Discussed in Thread: AM3354
Q:We use am708 to drive lcd display, and at the same time use adv712 to convert the signal to VGA and then connect to an external pc monitor, the image displayed by the external monitor is jittery, and the text and image displayed in some areas are blurred. After our analysis, it is caused by the jitter of the am5708 LCD pixel clock, as shown in the following figure:
The test conditions in the above picture: pixel clock input channel 1, the center of the oscilloscope screen is 5μs from the left trigger edge, and the *** jitter of the pixel clock is about 15ns;
The device tree settings are as follows：
dss_pll_calc_a prints as follows:
We suspect that the division by 40 is causing clock jitter, and the jitter range is amplified when the frequency is doubled. So we modified the pixel clock to 40Mhz, the crystal frequency of arm to 20MHz, and modified the pll frequency multiplication scheme to 20MHz / 2 * 4 = 40MHz. At this time, the pixel jitter is significantly improved, the 5μs *** jitter is less than 2.5ns, and the VGA display effect is also significantly improved. As shown below:
But we try to keep the original pll frequency multiplication scheme, that is, first divide the crystal frequency to a lower frequency (less than 1MHz) and then multiply to the set target frequency; there is no obvious change or improvement in the pixel clock jitter range.
At the same time, compared with AM3354, similar hardware peripheral circuits, no matter how the frequency of the pixel clock is set, the jitter range is less than 200ps.
Therefore, we suspect that it has something to do with the hardware design of AM5708. Now that the product has reached the stage of mass production, this problem causes the VGA extended display quality to be substandard. What is the solution please?
Nancy Wang：
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