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So when I build the program counter, I'm gonna lay it out physically the same way it is in this diagram, and I'm gonna put it just above the register, so I'm gonna take my register.
This is a register, of course.
The ale, you and the B register.
So I'm gonna take another bread board, um, and connected here above the register.
So take off that strip and connect these here like that and just above the average Cheryl, build our program counter.
Start with 74 less 1 61 which is our four bit counter.
And then we also have the 74 or less to 45 which were is our tri State buffers.
And so, you see, that's the same same chip we're using for the register for the ale, you and the B register.
So very similar approach to connecting to the bus there.
We'll start by hooking up the power for each of these chips.
I'll start with the count enable.
That's ah pretty straightforward because 74 less 1 61 has these enable signals are these in these enable pins, so connect pin seven and 10 together and that'll serve as our enable input.
So this will serve is our is our counter enable signal which all tie too high for now just so that it's enabled.
So next I'll hook up the outputs of the 74 l s 1 61 which are pins 11 through 14.
And those outputs we want to go through the 74 l s 2 45 that connects the outputs of the 74 less 1 61 counter to our tri state buffers.
So it's connecting.
It depends 11 through 14 here.
And so that allows us to to implement our program counter out signal our counter out signal here, which controls whether the content of the counter going out on the bus.
And to do that, we use our enable signal here.
So this enable basically is our program counter outs up in 19.
So if I hook pin 19 up here, that's our That's our counter out signal and I'll tie that.
Hi, this is ah, inverted signal.
So that's now currently disabled.
And of course, we can also each of these each of these ah pairs of pins, you know, this'll buffer from 11 to 9.
There's also a buffer from 9 to 11.
We can set the direction of this of this chip using pin one.
It's in this case because we're connecting the counter chip, though the 1 61 to the to the top side here.
The bottom side is with this side will be connecting to the bus.
And so for output, we're gonna be going in the down direction here.
So from from B to A and so from be today, we want our direction input to be low.
So we want pin one to be low.
So I'll just go ahead and tie that low.
Just connecting pin one to ground like this.
And of course, we definitely want to be able to see what's going on in in here.
So you can never have too many ladies, eh?
So we'll hook.
Some led us up to this so we can see the contents of the program counter whenever we want on.
So this is gonna be looking at the you know, this is this is basically tied right to the output of the 74 less 1 61 will be able to see what those four bits are so this point if I connected to power to do something and, well, it starts at all zeros.
Let me try connecting a clock to it.
They connect the power for my clock.
There's the clock circuit does connect the power here.
And then the clock itself goes into, uh, pin to on the 74 less 1 61 So I can at the clock to pin, too.
It looks like we're counting.
So we've got a counter.
That's great.
And so if we take our counter enable and we take that low, the counter stops counting.
So our clock is still going, but the counter has stopped.
And then if we bring that high, the counter counts again.
We go low.
It stops so we can control whether the counter is actually counting.
So that seems to be working.
Let's try the counter out.
So this is what takes the contents of the counter and puts it out on the bus.
So the bus is gonna be connected down here on the bottom side of the 74 less to 45.
So let's connect a couple of ladies up just to see what this is outputting and right now it's disabled, so it shouldn't be out putting anything.
And it looks like that is the case.
I'll just connect these four ladies.
But if we take our counter output enable and we bring that low, we see that the contents of the counter are being put out on the bus.
And if we take our counter output enable and take it high, then it's no longer putting that out on the bus.
So looks like the counter out is working and the counter enable are both working.
So the only piece we have left is the jump and the jump basically is a program counter in.
So it should take the contents of the bus and loaded into the program counter.
And this should be pretty straightforward as well, because the 74 less 1 61 1 of the reasons we chose to use it is it has these data inputs.
So has this a BCD input and it has a load signal over here.
So the load signal is gonna be our jump, so I could just connect over here.
This is the that load pin that's our That's our jump signal, and it's ah inverted input.
I can see it's got the little bubble.
There s O Normally, it would be high tide high and then if we wanted to load a value in, we'd bring it low.
So if I turn off the counter enable and just leave it there, then if I bring my my jump, If they take that low, that'll activate the jump and it should load whatever's coming in here.
Of course, these aren't connected, so the default high and it looks like it's just loading.
It's loading that you know that the default of high for this four, there's four inputs that aren't connected.
So it looks like that jump is working.
Or maybe so.
Really, What we just need to do is connect these these four inputs for for that jump over to our bus and so I'll just connect them over, too.
The outputs of the 74 hours to 45 which are also just gonna be connected to the bus.
So they're the outputs of 74 less to 45 but they're also connected to the inputs of the 74 less 1 61 So I think they're pretty much does it for the program counter, though I think there's one more thing I'll do, which is, you know, because this is Ah, eight bit bus and we're only using four bits, you know, the least.
Taking the four least significant bits for the program counter.
We've got these these other four bits that it doesn't really matter what they are when we're out putting.
But I think it be nice just to tide of all to the ground s.
So I'm just gonna I'm just gonna do that something I could detect these top four bits all to ground.
And so now when we're out putting our program counter, whatever it is, if we enable the counting when we all put this program counter and of course, we're out putting the lower four bits.
But then these upper four bits we'll be out putting zeros rather than, you know, probably ones or something else Doesn't really matter.


Program counter build

林宜悉 2020 年 3 月 28 日 に公開
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