Name: 世界最差顯卡？激動人心的結論 (World's worst video card? The exciting conclusion)
Uploaded: 2021-01-14T10:13:36.000Z
Duration: 24 min 23 s
Description: 【看影片學英語】數萬部 YouTube 影片，搭配英漢字典即點即查，輕鬆掌握單字發音與用法，長久累積看電影不必再看字幕。

In a previous video we built this circuit that generates a valid VGA signal, and when we plugged a monitor in

we saw that the monitor recognized that it was in 800 by 600 mode.

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And that's because we're sending at the right horizontal and vertical sync pulses for that mode.

We're sending a 3.2 µs horizontal sync pulse 38-whatever thousand times per second and

60 times per second. And the way this all works is we have a binary counter here that's counting from 0 to 264

you know over and over again, and it's counting at exactly 10 million times per second.

So we can look for these particular values to get the pulse width and the pulse frequency

to match exactly; and then same thing in the vertical direction, you know

we've got another counter here that counts each line on the screen from 0 to 628

we measured all the timing and everything in the last video to make sure it matched the specs and it did and

so we saw the monitor recognize it as a valid signal. So if you want to know more about what's going on here,

I definitely recommend you check out the first video, but now you know

we probably want to actually display something on the screen.

And to do that, the VGA interface has a few more signals for red, green and blue. and

Because we're using the sync signals here to stay synchronized with the monitor

we can use our horizontal and vertical counters to keep track of what part of the screen the monitor is currently

"painting" at the moment. And so the 0 to 200 tells us where it is from left to right, and then the 0 to 600

Will tell us where it is from top to bottom and so as the monitor paints the screen

You know, left to right, top to bottom: the same way you'd read a page in a book.

These red green and blue signals is how we tell the monitor what colour pixel we want at that particular location.

In other words: because we're synchronized, we can use our counters to know exactly what pixel the monitor is.painting at the moment

So for example if I take just one bit from one of these counters

(so this is the the vertical counter) if I take those, you know 1 2 4 8 16s place

Let's say this 16 s placed bit right here

you know as we count down the screen from 0 to 600 down the screen this bit is going to flip on or off every

So if I take that bit and hook it up to the the green signal and I'm hooked it up through a

resistor (for reasons that I'll get into here in a minute)

You can see every 16 lines the green flips on and off and that's because as our article counter counts from 0 to 600

16 places bit flips on and off because that's how binary counters work

Now if I hooked the red up here to the eights place in the same way

and now you now you can see the green and the red as well as

Yellow when the two mix and then of course, you know, I can also hook up blue

So, let me just do that for the sake of completeness

And now you can see the three primary colors mixed to give us actually 16 distinct colors

but you know as much fun as colorful stripes are what I really want to do is display a more complex picture and

We need the pixel data stored in some memory somewhere

Normally in a computer the data for whatever's on the screen is stored in a part of RAM

that way the software running on the computer could just write new data to the same Ram location and the image on the screen will

Change right away, but I'm not gonna hook this to a computer at least not yet

So I'm just gonna store the image on an EEPROM and this is a 28 C

256 EEPROM so it'll hold 32 k of data which should be enough for an image of some sort

and the way this will work is we already have our horizontal and vertical counters giving us an x and a y

position and if we feed all of those signals into the addresses of the

Then the EEPROM will give us a byte stored at that address and that byte could be the color of the pixel for that particular

So that's how we'll store the image in the EEPROM

But there's one weird issue which is because we went with the 10 megahertz pixel clock instead of 40 megahertz

we only have 200 pixels by 600 instead of 800 by 600 and

The pixels would be kind of stretched out because really what we're doing is we're repeating each pixel four times as we go across

So it'd be nice to maybe slow our vertical counter down. So we repeat each line four times as well

And it's actually pretty easy to do if we have our counter counting in binary like this

You know here we're just counting 0 1 2 3 all the way up to 16, you know, just counting in binary

2 2 and so forth up to 8 and if we lop off another bit

Then it repeat each number four times and only counts up to 4 instead of counting to 16

So if we just ignore the bottom 2 bits of our Y counter that all divided the counter by 4

We actually still have another problem which is the EEPROM. I have the

22:56 it actually only has 15 address lines so we couldn't fit all these address lines here

Even if we wanted to so we've actually got to get it rid of at least three address lines in order

So given what we've got with this EEPROM and to keep the right

proportions end up having to drop three bits from our Y counter and one bit from the X counter and that leaves us with a

Final image resolution of 100 pixels by 75 pixels which you know, I don't know it's maybe not the most impressive resolution

But I mean, what do you want for me? I'm trying to build a video card on bread boards

So 100 by 75 is is what we get. So let's hook the EEPROM up like this

I'll start by adding another breadboard and connecting power and ground to the EEPROM

Then I'll tie the right enable pin high since it will only be reading and it's active low and

Output enable and chip enable are both active low as well

So I'll tie them both low, so everything's enabled for output

Then the first seven address lines go down to our horizontal counter

We're skipping the first bit and the next seven address lines go to the vertical counter and this time skipping the first three bits

So that's 14 address lines 7 for X 7 for Y. There is a 15th address line, which we're not going to use

So now we've got all of our address lines connected like this

So for each of our 100 by 75 pixels, we should be getting a byte of data out here

That'll presumably tell us what color that pixel should be

But how are we going to turn this 8-bit data into the signal that the VGA monitor expects?

the VGA interface has three pins red green and blue and each expects a voltage between zero and point 7 volts and

Whether it's closer to 0 volts are closer to 0.7 volts

Determines how much of each color is mixed together to determine the color of the pixel?

But we've got eight bits here of data that are either 0 volts or 5 volts. How do we get something that's point 7 volts

well is the case where we have one voltage and we need a lower voltage and so we can use a voltage divider and

A voltage divider is just two resistors like this. So relative to ground down here

We've got 0 volts and up at the top. We've got 5 volts and in the middle here

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世界最差顯卡？激動人心的結論 (World's worst video card? The exciting conclusion)

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