8-bit computer build: Connecting the bus


In a previous video we built this decimal decoder that takes an 8-bit binary value here and will display Whatever that value is equal to as a decimal number here either as a two’s complement in which case It’s positive or negative or we can switch it to Be signed or unsigned rather so that it just displays the normal whatever the binary value here is in In decimal now use this as the output for our computer We [don’t] want the number to be just [hard-coded] like this We want to have a register that we can write to and whatever values in that register is what gets displayed here so let’s get rid of these and [this] is kind [of] fun, because how these pins are floating and we can just kind of mess around with it, but Anyway, I’ll [just] connect power and what we want to do is we want to build a register that connects to our [computer’s] bus that We can then store a value in and whatever value is stored in their register is what gets [displayed] so I’m going to add another breadboard here, but first I’m going to remove the power strip from the top here Because we’ve already got the power strip for this other breadboard and we can just snap this in here now [we’ve] built 8-bit registers in Previous videos and this is an example here of an 8-bit register for the a registry that we built and we’ve got the 8-bit bus connected here and you can see that that bus goes into the 74 LS 173 s and it also comes out through the 74 LS 245 and In each of the 74 LS 173 stores four bits of Data and so to soar data in this Register you put whatever value you want to store on the bus that [would] come in here and we go to these inputs On both of these chips, and then we’d have [our] a in signal which is right here We take this low because it’s an active low signal and then on the next clock pulse because these chips are Clock Edge triggered on the next clock pulse it would take whatever data is on the bus and store it here And then we’d be able to see the output here on these leds and of course for an output register We could just replace these leds with our nice decimal display And in fact we could build our output register using only the 74 LS 173 s we don’t need the 74 LS 245 That’s an extra feature so this register has that we don’t need and in this case the 74 LS 275 is used so that the data can [be] read the register, and if the 74 loss [245] is enabled using this a output signal Then that data that’s in the register will then be sent out [onto] the bus but for the output register We really only need Data to come in from the bus and be displayed. We don’t really need to take that data that’s already been displayed and put it back on [the] bus so we can do away with this mu for all s 245 and We could build it with just the [two] 74 LS 173 s instead, but I’m actually going to do something different instead of using two 74 LS [173] s I’m going to use a 74 LS 273 which has 8 d flip-flops in a single chip and so within [this] single chip you can see there’s 8 inputs and 8 outputs, and there’s a clock pulse input here And [there’s] also a reset pin over here that just sets everything to 0 But otherwise this just has 8 d flip-flops, so you can see there’s 8 8 Flip-flops here each of them has an input, and so these are just the pin numbers here for [each] of those inputs And then each has an output as well, and then there’s a single clock pulse Which is active on the rising edge, so on the rising edge of the clock pulse? whatever is on the input goes to the output whether it’s high goes high or if it’s low it goes low and [then] there’s a master reset which just clears everything to zero [so] just to do something different I’m going to use this chip instead of building it the same way that we built the other registers So here’s the 74 LS [273] and of course I’ll start by connecting power and ground So next I’ll just hook the outputs of the register up [to] the address line input of our eeprom here The pin out for the 74 LS 273 is a little bit wacky here. So these outputs are kind of spread around in kind of weird places But you have to make sure [that] the outputs line up to the appropriate inputs and right order here and so that’s it now so all of the output pins here should be going to the appropriate address lines here, so Q0 through qk 0 through a [7] address lines here to provide the input for our display Now in addition to the inputs and outputs. There’s also the clock pulse and the master reset. [I’ll] just tie I’ll just tie the master reset [hi] for now, so that it’s not active [in] the clock input I don’t want to hook that directly to the [computer] system clock because then The register would latch in whatever is on the bus every clock pulse, and we don’t want to do that We want to have some control over when it latches in whatever is on the bus So what I want to do is end the system clock With some sort of control signal that says whether or not we want to change the output display So I am actually going to need a second ship to go with this 273, so I thought I was saving [a] chip by by not using two of the 74 Ls 173 s, but I am going to need a nand gate [too] to be able to end the clock pulse, and we didn’t need [that] with the 74 LS 173 s because we’ve got the clock input, but then we also have an enable input here or an input enable I should say but oh well that’s fine. It’s easy enough to add a nand gate This is just a 74 Ls 0 8 quad en gate chip, so we’ll add that and we’ll hook up power and ground for that and so the clock pulse for our Register we want that to be the output of one of these an gates so we can just use the one that’s closest here The output then to the clock pulse and then the [input] One of those inputs is going to be a control signal That will hook up to our control logic later on but for now if it’s low Then that means that this clock pulse will never Change and so the output will will stay whatever it is and then the other input will be the actual system clock and so then if this goes high then whenever the system clock pulses then this Register will read in the content from the bus and the output will [change] so we have this control line that will control whether or Not we want the output to change so the other input to our and gate [have] got to go up to our System clock and so I’ll connect this here And if we come up here, you can see the system clock is used in this register here, and I can Connect that there too to the system clock up here get this led back in place Until then yeah this system clock will come down and connect to the other input of our end gate So there we go We’ve got our clock coming in we’ve [got] our control signal coming in and then the output of that goes into the actual clock input of our 74 LS 273 and so the output that we’re actually displaying over here will only change if the If this if the output register is enabled and the clock pulse comes So it’s actually about it for the output register Although of course we need to connect it to our bus And I think now is as [good] a time as any to actually connect everything to the bus because at this point We’ve got pretty much everything done except for the control logic now to make the bus I’m going to use these power rails that we’ve pulled out from between these boards as we’ve connected them together And so I’m going to take four of these and connect them together because these these actually connect together very nicely like like this And this will make up our bus because this then gives us eight Signals that go up and down like this And we can connect all of the different components of our computer to those same eight signals and actually to go the whole Height of our computer I’m going to use two [of] these connected together like this, but because we’re going to sandwich The bus in between kind of the two halves of our computer. We do have these little Connector tabs on the on the one side that I want to get rid of so that everything just kind of fits snugly together So to get rid [of] those you can just use some Wire cutters to try to snap them off and they snap off pretty well So now this should sit together a little bit better. So we’ll start by [joining] the two pieces [of] the bus There we go, and then I’m going to set my bus up so that it lines up at the top And I’ll go ahead and connect the new register that we just built to the bus. So there we go. I think we’ve got D0 through D7 Hooked up appropriate to the appropriate places on our register. So that should do do it for our register, but of course We might as well go ahead and cook everything else up to the bus as well so just above our output we’ve got our b register and our sum and Our a register and our program counter so I can start hooking these things up to the bus as well Since the program counter and of course the program counter is only four bits So we only need to hook the first four bits up to our bus And of course those will go to the the four least significant bits on the right and then the top bits will just all be Zero so I actually have to tie everything on our bus to ground through through pulldown resistors at some point next We’ve got the a register here, and so that’s just a full 8-bit Register that we can hook right to our bus. So there’s the a register all eight bits of that hooked up to the bus We can do the exact same thing with the D register So that’s the B register So now we’ve got our a register [and] our b register both hook up to the bus So I got the a register [and] B register feed into the some register which is either going to add or subtract the contents of the a and B registers and That can output its contents to the bus as well So let’s look the some register up to the bus now So there’s the output of the some register or the alu going to the bus as well and the other side of our computer We can bring that over here and connect that up as well So [of] course this is our clock up here the next we’ve got the memory and there’s a memory address register And then of course [the] memory contents, so the memory address register. We can hook that up now first It looks like I need to move these Connections from the Haver [adjuster] down just a little bit I’ve got room so our memory addresses are only four bits So we only need to hook up the lower four bits of our bus Because any time we’re going to have a memory address on the bus that’s going to be between 0 and 15 So we just hooked those up, and that should be it [for] a memory address Next is the memory content so once we’ve put an address here We can either write or read contents from them or you know it’s going to be 8 bits So we need to get these 8 bits here connected to our bus So it looks like maybe I want to move some of these things up a little bit to make room [I] had all these jumper wires measured out ahead of time, but I necessarily know where to put them exactly [okay], so move those up a little bit And I think that’ll give us room to hook up our 8 bits from our memory and there we go We got it to fit. So that’s our memory contents coming onto our bus and then I Think the last thing we have is the instruction register So there’s the [instruction] register, so that’s all of our registers so far. We’ve got our output register down here. We’ve got our a register B register and Some output or difference or alu arithmetic logic unit. We’ve got our program counter up here. Which is just [four] bits We’ve got our memory address register. Which is just four bits, and then we’ve got the contents of our memory and our Construction register now something else. We need to do is because we have some of these Registers hooked to our bus only on the lower four bits We’re going to make sure that the top floor bits Default to zero and actually what I’m going to do is I’m going to tie all of these or pull all of these to ground with a with a pulldown resistor so these are just 10k resistors [that] go from ground to each line on the bus and so if none of the Modules on the computer [our] asserting either a positive or or ground signal onto a bus line then there They’re going to be sort of pulled to ground by default so we should have all zeros on the bus if nothing on the bus If none of the modules on the bus are actually asserting any any particular value I’d also like to be [able] to see what value is on the bus So I hooked up eight leds one to each bus line by bending the leads Just right so that they they all line up very nicely at the top of the bus and [I] connected one of the cathodes over to an adjacent board that I Put next to it so I could get the cathode lined up so that [it] connects to one of our ground points then it bent the the rest of the cathodes over in a line and One by one soldered each of the the cathodes to each other so [that] all the cathodes are soldered together And I used a little mini grabber to hold everything in place Which which worked out really nicely because it doesn’t have a lot of thermal Mass So it didn’t suck all the heat away when I was trying to solder so the soldering went pretty quickly And of course trimming off the little bit of each lead as I went and so here’s the finished led indicator [saying] that I can hook up here to the Bus so it slides in very nicely right at the top of The bus, just like that and the cathodes are all connected to ground over here So I should let us see what values on our bus at any point in time Now another thing we want to make sure we take care of is connecting power and ground across the board as well And actually rather than just having that one cross connection for power and ground I’m going to connect a power down here on the bottom as well Just to make [sure] we don’t have any problems of power distribution on the board Because you know these bread boards aren’t perfect obviously And you can get a fair amount of resistance if you know let’s say power is coming in over here For it to get all the way around down to here you know there’s a fair amount of resistance and impedance and things that can happen there, so adding more of these power cross connects and just to make [sure] you’re not getting a you know a Big voltage drop as you as as you’re distributing power to the throughout your entire set of bread boards So we’ll go with that for now. We’ve got a pretty decent power distribution The other thing I want to do before we power this up and start playing with it is I just want to make [sure] the clock is also connected across because we’ve got our clock signal We’ve got our clock module up here in the in the top left and we’ve got the signal getting distributed over here But well are there’s a few pieces that still aren’t connected so I’ll connect the clock input for the program counter here To the clock for the a register Which is already connected down to the b [register] and then on down to the output register? and then from there, we want to connect it over to the clock module so we’ll connect right across and Over to the clock and this is their clock output up here and so now that clock signal is getting distributed all the way down the right-hand side of the board and it’s already being distributed here along the left side so it’s coming down to our memory address register and then from there down to This is actually our memory for when we write And then we just need to connect it down To the construction register, so we’ve got our instruction register clock signal here So we just need to connect that up to the rest of the clock signals and there we [are] so now this clock for [our] Instruction register is tied [into] the rest of our clock for everything else So now we’ve got our clock signal distributed to all of the different modules in the computer that require the clock As well as our power So I think the only other thing I’m going to do right now is we’ve got these sort of empty spaces here And it seems to make sense to add some additional [breadboards] here Which is where [our] control logic is going to go eventually you know connect up all the power to each of these And then we’ll add another connection for power Just down here on the bottom again Just to make sure we don’t have any problems with [power] [distribution] and getting enough power to all of the different parts of the board And speaking of power let’s try connecting power to our computer and see what happens of course we’ve got everything here, but the control logic So each of the modules individually should work just fine, and so we see the program counter is counting away here And we can turn off counter enable and it stops counting but each [of] the modules we should be able to put its contents onto the bus so the counter has these three bits set so If we change our counter out We see those three bits go on the bus. So that looks good We should also be able to load something from the bus actually what I want to do is let’s run this counter See if I can stop it on one There we go Because what I want to do is I want to put a one into the b register so I’ve got a one on the bus now from the counter and if I go to my d in signal which is Somewhere be out. Let’s [begin], so [let’s] begin and if we take that low Then the B register reads that one from the bus into the b register and so now I’ll take that high so we’re not doing that anymore and Now I don’t have to put this one on the register anymore, so the counter out Turn that off. So now there’s nothing on the bus but if I take the sum Which is the sum of the a and the b which right now is zero in a plus [one] and b is a one So I can put that on the bus so I can say some out Now that one is on the bus and if I read that into the a register, we’re going to stop the clock if I read into the a register, so if I say a in It’s going to take the one from the bus and put it in the a register on the next clock pulse So [let] me advance the clock And you see that happens the one goes on in the a register but now my some register is one plus one and so that’s two which is going on the bus and So if I do another clock pulse, it’ll read that into the a [register], and there’s a two and if I run the clock You’ll see it’s now counting because it’s adding the [1] and the b register each time so now If we have our output register Inputting from the bus as well We should see that count showing up on [our] output [register] and there it is and if we want we can switch to the two’s complement mode for our output and now it’s Counting down to zero and then it should start counting up to 127 And so I’ll leave it there for now, but in the next video. We’ll take a closer Look at exactly what’s going on here, and what we’ve built, so far So that we can better understand what it is exactly [that] the control logic needs to do in order to turn this into a fully functional programmable computer

About the Author: Michael Flood

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