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Interface design

A project log for Repurposing an old nixie thermometer

Returning to gear neglected for 35 years

Ken YapKen Yap 01/08/2022 at 09:520 Comments

I already have an idea of what the shift register GPIO should look like. In a previous project #An uninnovative linear LED display I put the 74HC595 8-bit serial in parallel out shift register to work. This is a remarkably useful and cheap chip. It contains a latch so that the data shifted in can be held until the next update. It can be cascaded to any practical length. It can source or sink current. It's is very simple to drive, the software routine is easy to understand.

4 digits means 28 lines are needed. The next multiple is 32 = 4 shift registers, so I can allocate a whole byte to each digit. The length of register chain is not a problem as it doesn't take long to shift in 32 bits, much shorter than the update period for the envisaged use as a display.

While I only need one interface for this project, the horrors of perfboard assembly have put me off anything more complicated than a chip or two. 100x100 mm PCBs are so cheap, only a couple of bucks each delivered, so I will make a design that I can reuse for other projects. It will be a general purpose 32 line GPIO, or rather GPO, board.

This project will go into hibernation until I have finished the SRGPO project.

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