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Baffled - Please help - Quad buffer / line driver tri state circuit issue

Discussion in 'General Electronics Discussion' started by Experimentalist, May 19, 2016.

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  1. Experimentalist


    May 18, 2016
    Hi all, this is my first post so please be gentle. I have a circuit comprising of an East Rising 7" display (ER-TFTM070-5), a PJRC Teensy 3.1 micro controller and I am also using the on board SD card slot of the display. The display is based around the RAIO RA8875 TFT driver, an SPI device. This chip has a well known fault whereby it does not tristate the MISO pin so it does not play nicely with other SPI devices on the bus. For this reason if you need to use other SPI devices, as in my case, the SD card, you need to provide the tristate isolation yourself. For this purpose I am using the 74HC125 IC as advised in the previously linked posts. I am using a circuit that fully isolates the RA8875 by using 3 of the 4 channels on the 74HC125. I have not worked on this for some time and decided to build a second version as the first never made it off the bench and I want to build an enclosure and make it portable. The second build, which as best as I can is a duplicate of the first, had an issue which causes a QR code like display corruption that appears on start up and never goes away. After much testing I finally tried to swap the 74HC125 chip from the good board to the bad and low and behold it fixed the issue. Great, I thought, just drop a new chip in and we are there. No. I had already ordered another batch of these chips some time ago from Little Diode. However, I have tried many of the batch and the problem remains. If I move my "good" chip between the boards they work fine but I have tried 5 or 6 of the new batch with no joy so far. I am guessing that something is right on the limit and a slight variation is all it takes to stop it working, a guess at best as I am well out of my depth here. I tried dropping the pull up in the isolation circuit from 10K to approx. 3K by putting a 4K7 in parallel as some reading suggested this could make sense to me but with no joy.

    As I have said, if I move the original chip from one board to the other they both work fine. As soon as I replace it with the one of the other chips I have I get the QR code problem back. I have double checked the chips and to be honest the working one looks like a clone if any of them are. I had 2 of the originals and then bought 10 from Little Diode as spares and quite feasibly a useful one to have to hand.

    Can anyone offer any kind of advice? I intend to look for a mechanism of testing the chips tomorrow. I was thinking of running bread boarding it and putting the 1KHz test tone from my oscilloscope through each channel of the chip in turn as a test. By the way the oscilloscope came out the box yesterday and I am a hobbyist really with some dim and distant training in the past some 25-30 years ago.

    If anybody has any advice or pointers I would be very grateful

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