Designing and building a 1-bit register - 8 bit register - Part 3

Before we build the 8-bit registers for our computer, let's design and build a 1-bit register.

The 8-bit registers we'll actually use in our 8-bit computer will be simplified. We're going to take a big shortcut—using the 74LS173—described at the end of this video. But before we get to that, this video shows how we could build a 1-bit register using more basic components.

Complete parts list (everything in this video):
- 1x 74LS04 (Hex inverter)
- 1x 74LS08 (Quad AND gate)
- 1x 74LS32 (Quad OR gate)
- 1x 74LS74 (Dual D flip-flop)
- 1x LED
- 22 gauge wire
- 5 volt power source (e.g., a USB phone charger)

[TheActualG]

The values on the bus go round and round.

[stephenc]

I love that you completely demystify any "magic" going on inside the IC by building the circuit. You could have just whiteboarded what a register does and why we need a couple of them, then showed the schematic from the LS173 datasheet and how to use the data enable and tri-state out pins, and finally told us to Google "D-type flip flops" if we want to know more. Thanks for showing what's going on under the hood.

[MissyEmz]

I can't get over how good this playlist is. Already ordered all the parts, can't wait to start building this.

[hareknowsnone]

Thank you. I have always wanted to understand a computer from scratch...without software.

[kingcrimson]

Super video! I applauded for £10.00 👏👏👏

[kdmq]

Can't you just AND the LOAD and CLK values together so if the LOAD is disabled none of the D flip-flops get a pulse from the clock?

[TerabyteDragon]

8:00 - Watching this series over again. Wonderful content. Why not AND the load signal and the clock signal. So the flip-flop only sees the clock signal when load is high? Wouldn't that be easier?

[bcal]

This is a seriously incredible video that has totally blown my mind. So helpful to see these things put together like this

[angelorondini]

I'm going "super extreme", I'm building it out of transistors

[keiyakins]

I always felt like building one of something the hard way and understanding that is good. After that though, building up further complexity using components that implement that for you is fine!

[CodeCrafterPro]

I will build this completely without ready-made circuits!

[jrldds]

4:24 you can replace all of the gates with NAND Gates, hook it up the same way (but connect the load signal to both the inputs of the NAND Gate taking the place of the inverter), and everything works the same. And as an added bonus, you get to save yourself a couple of chips, as now you only need one instead of three.

[brucewoods]

Excellent description of the D type register

[stephenjacks]

FYI the National 8900 16-bit microprocessor actually had a 4-bit ALU. Also the 8070 (8073 tiny basic) processor used a one bit ALU hence a lot of clock cycle per CPU cycle. Rem with older silicon the carry propagation over 32 bits that breaking up an arithmetic operation into many cycles didn't cause longer instruction times.

[JulieanGalak]

Note for folks trying to build this 1-bit register circuit: I built it just like in the video, and not only didn't it work, the AND chip started massively overheating - hot enough to burn skin.

Spent a while checking every connection, got out the vom to check that the right voltages were present where they were supposed to, everything looked good.

I eventually tracked down the problem to the Clear and Preset lines of the D flip-flop chip. I left them unconnected as in the video, which should have worked. These are both active-low, and unconnected pins usually float high. But for some reason, mine didn't. Maybe slightly different version of the chip? Not sure. But the datasheet says that pulling both low at once puts the chip into an unstable configuration, and I think that's what happened, which also led it to somehow source way too much current into the AND chip.

Pulling both lines high with a pair of jumpers solved the issue. Circuit worked fine, and no more overheating. Amazingly, the overheating didn't damage the chip.

Posting this in case someone else runs into this problem.

[diegoalbertomamaniquispe]

Este tipo es un genio y de los humildes.

[dev-toe]

Dude you are a god! thank you so much!!

[danielbernhard]

and now build this just with some transistors and resistors :D i think we could need a much larger breadboard

[TheAnachronist]

Even using the same approach, you could cut down on the number of NOT gates by just having a "LOAD" line (as shown) *as well as* a "NOT LOAD" line. As long as the fan-out for the NOT gate is good enough (it likely is for CMOS chips).

[daveo]

When I was at Universaty I was taught by Profeser Hall the guy that invented the tristate chip. Have to say you explane it better.