Using Shift Register

[toyracer]

My class project is to determine the specs and design a circuit using a mystery chip given to us. We got a MC54HC595 which turns out to be a 16 pin Shift Register.

I've searched the archives for a circuit that will help my understanding of this chip but didn't come up with much. My deadline is by Friday. Can anyone help me please?

Thanks,
Toyracer

 

[laroche73]

shift registers

54 is the military/industrial prefix for 74xx logic. It's guaranteed to work over a greater temperature range. When doing a web search, delete the prefix and search for the part # and a short description. Ex: "595 shift register". Phillips has datasheets for most standard logic parts.

https://www.electro-tech-online.com/threads/how-to-drive-several-leds.4396/

 

[toyracer]

Thanks for the info, laroche. Do you have any simple schematics where this chip is used in practice? I guess I would have to display its use by outputing to LEDs? Thanks again.

 

[laroche73]

'595

The chip is often used for output expansion in a microcontroller environment. Probably the most interesting thing you can do with a shift-register is to turn it into a "maximal-length" linear feedback shift register by adding an exclusive-OR/NOR gate. Given "n" bits, it can sequence through "2n-1" unique states (All "0"s is the excluded state with an XNOR gate, all "1"s with an XOR). An 8-bit LFSR would require a 4-input X(N)OR gate. Interesting note - a '595 can emulate a 9-bit LFSR (with one inaccessible bit) using a single 2-input gate. That's an interesting project, I'll leave it to you.

For a simple demonstration, tie the clock pins together, and use bits 5 and 6 as the feedback terms (0-7 being the bit numbers, this gives a count of 127 and will drive the LEDs with a pseudo-random pattern. Since the output register is always one clock behind the shift register, outputs Q4 and Q5 correspond to bits 5 & 6). The reset state of the SR is all "0"s, so use a XNOR gate as the feedback element. The first two clocks after a chip reset produce all "0"s on the output pins, then the outputs will go through the normal LFSR sequence.

There is a lot of info concerning LFSRs on the web. You can start here:

https://www.xilinx.com/bvdocs/appnotes/xapp210.pdf

 

[toyracer]

I will try and experiment with what you suggested. I found out that I have plenty of time still to do the project...it got moved to Wednesday.

Thank you very much for your insight!

 

[laroche73]

correction

You're welcome. There was an error in my last post, which I've corrected.