I’m still just starting to get into understanding the hardware side of making stuff, and having a blast with it. I’m trying to solve a problem and have been unable to come up with a solution yet.
I have design that for various reasons would be best to use an ATTiny85 for, and therefore I get to have the fun of working with limitations. On the code side I have no problems, and getting protocols to work are no issue either, but getting multiple devices connected to the limited pins is my current challenge.
For this problem:
I have an SPI device connected, taking up 4 lines. Then I have two devices that still need to connect on one line each. One of these needs a dedicated line, no sharing, but the other is only ever sent data when the SPI device is not active(ie SS is high). Due to the completely different data formats between devices, I cannot let SPI data get to the second device, it has to get nothing when I am doing SPI conversations.
So, my thoughts:
Since I have to take the SS line low to select the SPI chip, is there a basic circuit that I can build out of my limited parts collection(think a couple of different transistors, resistors of every type, caps and diodes - but I cannot use LEDs because this needs to be dark) that will take the MOSI line and output only to the non-spi device when SS is high, and either both or just the SPI device when SS is low?
I tried to figure out how to make this work with an NPN transistor, but got nowhere, as my understanding of them was and still is limited.
A 1:2 demux would work, but I don’t have a chip that can do that, and while I can find and understand the logic diagrams for them everwhere I look, no one has a basic circuit design for a 1:2 demux that I could breadboard and try out. I don’t know that I need to be fully in a demux, as the SPI device should not care about data coming in on the MOSI line when SS is high.
Any suggestions, I’ve been googling around and reading a tonne of docs for a while now, but still nothing that makes sense.
Thanks