HDMI signal filtering on page 10 (BBB schematics)

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Gerald:
what was the issue that you solved with 47 pF caps on page 10 of the BBB schematics? I am guessing you wanted to slow down the edges. Why and by how much? Was it a serious problem or a sort of optimization?

I am tempted to replace these caps with serial resistors to save some power. Any comment on this idea?

Thank you,
Wojtek

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Too many loads on the signal. The HDMI encoder is a 1.8V input that is 5V tolerent. So we had to add the filters to make the output of the HDMI work.

Serial resistors did not work. We tried that first. Remember you have the HDMI device, the boot resistors, and the LCD load to contend with.

Gerald

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Gerald:
thank you for the answer. I am a bit surprised that caps on the LCD_DATAx are helpful. My understanding is that they must slow down the edges because the AM335x drivers are forced to charge them, what just takes time at a given AM335x buffer strength, 6 mA in this case (data sheet Table 2-7, p.26).

I was thinking that much the same effect can be had by adding series resistors in front of the LCD inputs of the TDA19988. The slowing down would be due to charging the TDA19988 pin capacitance through the resistor. I am curious why it did not work in your design. Perhaps I will give it a shot…

Thank you,
Wojtek

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Well, it works. Remember, these signals are 3.3V and the threshold on the HDMI chip is for a 1.8V rail.

Feel free to try the series resistors if you like.

Gerald

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Gerald:
thank you. Just a small point. According to TDA19988-product-datasheet-rev3-jul2011.pdf, only some inputs on TDA19988 are 5V-tolerant. The ones connected to the AM335x are not. These are listed on page 38, table 30 as “Not 5 V tolerant CMOS 1.8 V and CMOS 3.3 V tolerant”. I wanted to clarify because you said in the earlier post that “The HDMI encoder is a 1.8V input that is 5V tolerant.”

BTW, the TDA19988 is a closed-source chip. Its data sheet is missing vital information. Is there any replacement out there whose data sheet would be available?

Thank you – Wojtek

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I spent four weeks looking for one. So not that I know of or I would have used it. These guys were the only ones that would take it as far as they did. Everyone else wants and NDA to get what I got form NXP without one…

Gerald