Step 4: Logic Gates

A computer consists of thousands of logic gates arranged to carry out certain functions. A logic gate is a component in digital electronics whose output depends on the state of its inputs. Most logic gates have two inputs and one output. You can think of logic gates as the decision-makers in digital electronics. The six main logic gates used in digital electronics are:

AND Gate: Output is high when all if its inputs are high.

OR Gate: Output is high when any of its inputs are high.

NOT Gate: Only has one input. Output is high when its input is low.

NAND Gate: Output is high unless all of its inputs are high.

NOR Gate: Output is high when none of its inputs are high.

XOR Gate: Output is high when an odd number of inputs are high.

Tri-State Buffer: A buffer that is controlled by a third logic signal.

It is important to mention now the difference between a high "1" signal and a low "0" signal. A high signal can either be a connection to positive voltage or it can be a floating input. A floating input is one that is not connected to any output. An example of a floating input would be one that is not connected at all or one that is connected to the output of a 3-state buffer that is not activated. A low signal is present when an input is at ground.

Logic gates can be fed into each other to produce almost any function imaginable. For instance, two NOR gates can be fed into each other to store one bit of data in a RS_NOR latch while power is supplied to the circuit.
<p>Great manual!</p><p>I've made it using Your design, but on an unusual medium: pixels :D Here's the simulator if you wanna see this in motion operating: <a href="https://realhet.wordpress.com/2015/09/02/bitmap-logic-simulator/" rel="nofollow">https://realhet.wordpress.com/2015/09/02/bitmap-lo...</a></p><p>I programmed it to calculate the Fibonacci series, it has an extra 16 byte ROM for the program, the fetch is only 2 cycles and different instructions can break the micro-word sequence earlier as when they finish, so instruction times are ranging from 3 to 5 cycles.</p><p>It was fun to input a program only using switches, like in ancient sci-fi movies. Well, it was fun for the 1st time... After I rather made a ROM.</p>
<p>Good job bro...Nice software</p>
<p>Realhet, that is spectacular! I really like your idea of using simple CA rules to make a logic sim. It is almost like Redstone in Minecraft, but so much more powerful and less latent (and also devoid of a jungle of wires, that must be nice). Congrats on the great project. Do you know of any other users of your program yet? I'll make sure to keep an eye on your site for updates, and I'll share your work with my friends. I'm sure they will really enjoy it.</p>
<p>what if you made it using leds instead of wires. could u see where the signals were going </p>
<p>How would you program an if-statement?</p>
<p>You don't need a new opcode. What you need is a value that represents true and false as 1 and 0. You load that value into the accumulator and subtract 1. Then you do a conditional jump. If the value was 1 (true), the subtraction will cause an overflow and the conditional jump performs. If the value was 0 (false), no overflow and thus no jump occurs.</p>
<p>Since there is a register (accumulator) that stores the previous sum from the ALU you can check if the result gives a negative, positive, or zero number then with that you can add a new opcode that checks that state and jumps if the condition is true.<br><br>for example</p><p>4 - 10 = -6 -&gt; -6 is negative so any branch command with the N bit set to 1 would jump</p><p>assembly: BRN RA This would branch on negative to location stored in A</p><p>binary assuming opcode is 1000: 10001000</p><p>Where this is the bit breakdown of the opcode</p><p>BR: 1000</p><p>N: 1 | 0</p><p>Z: 1 | 0</p><p>P: 1 | 0</p><p>0: 0 | 0</p><p>Also if you want to make it more versatile you can try adding the ability for B to set the bus value then the last bit '0' could be for A or B as 0 or 1 respectively.</p>
Crikey, you've got some patience!
<p>I've got a question. All of the parts in the list are there with the exception of 2.<br>the tri state buffers and the jk flip flops<br>There was a different website with the same model as the tri state buffers<br>but<br> there wasn't a JK flip flop replacement, do I have to use the exact <br>same model as you listed or can I use a different model? and if so which<br> model should I use?<br></p>
<p>Hey! I know that this is an older post but could you give me some updated links for the Tri State Buffers and the JK Flip Flops? I tried researching the products but I don't know which models to use. Thanks in advance!</p>
<p>I'm interested in making a simple CPU that I can use to type on an LED or even LCD screen. Is this the project for me or can you point me in better direction please?</p><p>I am a complete novice at this. I'm a carpenter trying to make some kind of wooden computer!</p>
<p>Instead of building a CPU you should just build a computer with a wooden case. Computers may cost quite a bit, but you can actually get them under \$300. Try using this site: pcpartpicker.com</p>
<p>Thanks for this. I hope there are parts thin enough. I want it to be like a calculator for words. I'll check out the link :D</p>
<p>If all you want it to do is act as a calculator, you might want to look into Raspberry Pi. It'll be much smaller and cheaper, Pi's are like the size of a phone.</p>
<p>Now that I think about it, the Pi can do pretty much anything a full desktop computer can, it's just much smaller. There's probably some sort of software you can use for it.</p>
<p>Thank you for your reply :D</p><p>Shortly after my original comment I actually found this guy: <a href="https://www.etsy.com/shop/writeotron?ref=unav_listing-r" rel="nofollow"> https://www.etsy.com/shop/writeotron?ref=unav_lis...</a></p><p>We're going to work together on a new one. It's a Pi!</p>
<p>Its wonderful and encouraging me to make one of my own..</p><p>sure i will press &quot; I made it &quot;. I kinda in process to build it. hee hee its awesome digital electronics</p>
<p>can someone please tell me the part list with the total no. of them used. </p><p>i did saw the part list but not the total number. I havnt read the whole tutorial yet so before i start would love to have evrything beforehand. and also please can anyone tell me based on this how much would i be able to increase its functionality . </p>
<p>Hi, don't start counting how many ic's you need... Just study the first component (I recommend clock, mar or program counter), design it and buy the integrated circuits you need. Do the same for all the components.</p><p>Not giving you the exact number and id of integrated circuits means that you can develop YOUR solution</p>
<p>How did you decide between active-low and active-high bits?</p>
<p>I mean for the control words that you used.</p>
<p>Subscribed just to push &quot;I Made it!&quot; button and to say a <strong>BIG</strong> thank you to K.H.! :D</p>
<p>Wow! I love the modular setup of the boards. That is a fantastic idea both aesthetically and functionally. I ran into EM interference issues with my breadboards' close proximity. I would imagine a setup like this has much less of a problem. Congrats again!</p>
<p>and what do you do with the carry out of the lasy 1-bit adder?</p>
It gets connected to the carry in for subtraction operations
<p>Help, I am trying to build a 8 bit computer from this but I don't understand, im building this so I can show off to my little brother... I don't understand how many chips he used and I dont understand anything past the ALU's basic add and subtract, and the register, any help would be great, please email me at hurster100@gmail.com... Once again thanks</p>
<p>What do you recommend for a power supply? I really want to do this project, but I have no idea what to do for a power supply.</p>
<p>Hi, I'm thinking about starting this project, I find it really intresting and my question is Should I make it, because I'm 17, I know binary and some eletronics, please help!</p>
<p>Since its 8-bit, you can do anything that requires 16 bytes of memory ( depending on your RAM ). Unless you add some external drive... which would allow you to do so much more. But just with 16 bytes of RAM you can probably do operations like:</p><p>A+B=C, then</p><p>C+B=D,then</p><p>D+A=Z,</p><p>I think.</p><p>But with an external drive of some sort your 8 bit computer will be able to do stuff like this https://www.youtube.com/watch?v=qYvr0b8jqbg ...i think?-e-dah-puzi-1998</p>
<p>16 bytes? How did you get that? Surely an 8-bit address buffer should be able to address 2^8=256 bytes of memory?</p>
<p>i am new in this topic </p><p>i think we can point to 16 Bytes memory with 5 bit address.</p><p>16 Bytes = 4 * 4 (bytes) = 32 * 32 (bit) =&gt; 2^5 = 32</p><p>am i wrong ?</p>
<p>or is it right?</p><p>to say memory we calculate this way !!!</p><p>4*16 or 5*32 or 6*64 </p><p>so if this is true </p><p>4*16 = 64 bit </p><p>64/8 = 8 byte</p><p>so for 8 byte memory we can address with 4 bit .??!!!!</p>
<p>Yeah....some people like to have a 4 bit memory address and 4 bit op code...I would personally do an 8 bit op code and memory address.</p>
<p>I mean I guess if you're not addressing much memory then it's Ok, but it would severely limit what you could do with the thing. With 256 bytes, you could even do some kind of really simple pong game.</p>
<p>Yeah....some people like to have a 4 bit memory address and 4 bit op code...I would personally do an 8 bit op code and memory address.</p>
<p>how many wires do I need? </p>
A thousand?
<p>That is a lot of wires! COOL!!!! Do you play Super Mario Bros. on it?</p>
<p>How do I use the CD4029 as a ring counter? I bought it and all I've been able to get it to do is count up/down in binary, resetting at either 15 or 9. Any help?</p>
<p>I am looking forward to making this project! I couldn't find anything like it no matter how hard I looked! Thank you for making this project available. :3</p>
<p>How do you ensure that the program counter starts out at zero?</p>
<p>What type of PROM are you using? I didn't see any parts listed for the PROM?</p>
<p>good jop, i motivated by your project, i am gonna make one sooner . i want to know how much this projects coast you ? is there any additional advice to build one like any mistakes must avoid or things that may save time ?</p>
Sure! Try using shorter breadboard wires, and plan out the layout of your computer beforehand. This will save you a bunch of grief in the future of dealing with EMF related issues and dirty clock signals.
<p>Very well structured into topics. Very well explained and crafted. Awesome job!</p>
<p>I think someone forgot to cable manage :P</p>