Tristate Miso Pin (RA8875)

Hello, I am trying to figure out how to tristate the miso pin on an RA8875 breakout board from Adafruit.

I want to connect an SD card and the RA8875 lcd driver at the same time, both using SPI

I tried following this diagram:


However, i noticed in its diagram it uses a 74HC125. I do not have one of those handy, so i used a 74AHC125 but it doesn't seem to be working.... the LCD screen still works, but only if i have the SD card disconnected... just like it did before i attempted the fix.

Is there are difference between the 74AHC125 and the 74HC125 and if there is (i'm assuming the A is there for a reason) is there a way to modify my connections to make it work?

Alternately, i heard that the Teensy has multiple SPI busses, but from what i can tell the libraries for SD card to allow you to specify which SPI bus to use???

AHC and HC families behave the same - the AHC is somewhat faster (we are talking ns).

If the diagram was a circuit diagram it would be easy to see what it does.... A wiring diagram is opaque.

I believe the issue is that the miso pin stays high even when chip select is low, it's a design flaw of the chip

"RA8875 has a bug in MISO pin that doesn't work as tristate as it should be and remain high, the chip works as SPImode3 so the SCLK and MOSI are HIGH, the I/O of the RA8875"

so, basically, the drawing shows a way of doing this, to isolate the ra8875 spi lines from the rest of the spi lines.

Just in case anyone comes across this, the issue is that my SPI speed was too high for the SD card. I fixed that by lowering the Teensy's SPI speed to 24mhz