VIC ultra minimum system

Modding and Technical Issues

Moderator: Moderators

User avatar
lance.ewing
Vic 20 Enthusiast
Posts: 187
Joined: Sat Nov 10, 2012 3:19 pm

VIC ultra minimum system

Postby lance.ewing » Fri Jun 24, 2016 3:25 pm

On page 18 of the 6560/6561 datasheet, it shows a diagram for the VIC minimum system. I've often stared at this and wondered about building something as simple as the diagram.

Recently I've been thinking solely about the video side of things. What would the minimum VIC system be if you were only interested in generating a fixed video output? We could ignore the 6512 (i.e. CPU), the 6520 (i.e input), 4052, and sound amp.

What I'm thinking about is a breadboard set-up. Could we use a VIC chip, an EPROM chip (for some pre-configured memory), the necessary clock generation, and s-video output circuit, to generate a fixed video output (for testing purposes)? If so, what would that minimum VIC system circuit look like?

eslapion
ultimate expander
Posts: 4162
Joined: Fri Jun 23, 2006 7:50 pm
Location: Canada
Occupation: 8bit addict

Re: VIC ultra minimum system

Postby eslapion » Fri Jun 24, 2016 8:12 pm

Well, I guess you could get away with RAM, a 12bit counter and an RS-232 comm chip and get them to just poke to ram whatever comes in through the serial port.

An alternative is to have a ROM chip, perhaps a 27c256 in which the last 16 bytes correspond to values you want to attribute to the 6560/61 registers. Have a 4bit counter to copy the register values which sets the 6560/61 to scan the content of the ROM for display which is then completely fixed.
No one can do everything. Everyone can do something.

User avatar
lance.ewing
Vic 20 Enthusiast
Posts: 187
Joined: Sat Nov 10, 2012 3:19 pm

Re: VIC ultra minimum system

Postby lance.ewing » Sat Jun 25, 2016 2:28 am

I have a few 28c17 EPROM chips needing a use, but for an ultra minimum system, it would be nicer to have a single larger chip rather than using multiple of these. Given it would be a testing set up, what you describe is the general idea. The RAM approach would certainly be more flexible though. Yeah, the more I think about it, the more a dynamic approach with a comms chip sounds like a better way to go.

User avatar
lance.ewing
Vic 20 Enthusiast
Posts: 187
Joined: Sat Nov 10, 2012 3:19 pm

Re: VIC ultra minimum system

Postby lance.ewing » Fri Jul 01, 2016 2:33 pm

I already have some 74LS161 4-bit counter chips as well; at least 5 I think. I only need one for this job though. And given I already have the EPROM chips (and a programmer), I'll probably give the simpler design a go before moving on to a design that includes a comms chip and RAM. I'm seriously thinking about doing this at some point over the next few months. I'll need to order a 6561-101 first though. There's a place in London that can source one for me, so I'll give that a go. I trust them more than some of these other web sites that sell them, as I've ordered from them before.

What I will probably do is have the 4-bit counter on an endless loop, setting the 16 control registers. This will be completely redundant, but I'm assuming it will be a slightly simpler circuit. What I want to try to do is fit everything on the one breadboard if I can. Might be a bit crowded, but I'm hoping I can come up with the absolute minimum circuit to display an image on a TV screen.

So the first thing is to source the 6561-101, and then next comes the input clock generator circuit. It needs to be a two-phase non-overlapping clock at the PAL sub-carrier frequency. I'll need to source a crystal for that. I see there's an appropriate one on ebay for just over a pound. I guess I should be using the same clock generation circuit from a PAL VIC 20 schematic. There's a few variations on that. Some have the PAL sub-carrier frequency for the crystal, and others have two times that frequency for the crystal (and I assume they divide that down to half).

eslapion
ultimate expander
Posts: 4162
Joined: Fri Jun 23, 2006 7:50 pm
Location: Canada
Occupation: 8bit addict

Re: VIC ultra minimum system

Postby eslapion » Sat Jul 02, 2016 12:20 pm

On the VIC-20, a set of 74LS245 buffers are required to isolate the CPU from the 6560/61. However if you replace the 6502 with a 6510 then this barrier is no longer required and you could make a minimum computer with a 8k x 8 SRAM, a VIC-I, a 6510 and a 2114 color RAM, a 4066 to control color RAM access and a ROM.

That's truly minimalist. Call the the VIC-Minimum...
No one can do everything. Everyone can do something.

User avatar
Kakemoms
Vic 20 Afficionado
Posts: 465
Joined: Sun Feb 15, 2015 8:45 am

Re: VIC ultra minimum system

Postby Kakemoms » Mon Jul 04, 2016 1:11 am

You could try to use a WDC 65C134 which has ROM and a few bytes of RAM built-in. Not sure if its enough to get the 6561 to do something, but it has a serial bus built-in so you may be able to control it from a PC or alike. I was actually planning to use the 65C134 myself, but have started with the 65C02 + 65C22 + SRAM to get some experience before I try to cut things down (=make them smaller).

User avatar
lance.ewing
Vic 20 Enthusiast
Posts: 187
Joined: Sat Nov 10, 2012 3:19 pm

Re: VIC ultra minimum system

Postby lance.ewing » Thu Jul 07, 2016 12:21 pm

I've had a look at those a few times in the past, and been tempted. Might be worth trying. My original goal was to use the minimum components needed on a single breadboard to get the VIC chip to produce video output, but I guess if one of those components happened to do a lot, it isn't a bad thing. It depends on your few of minimal. I was thinking I could do it without a CPU, and that the 4-bit counter would be all I need initially to load the control registers, but when I move on to needing to re-configure the VIC registers and memory in real time, then I'd need the comms, and so something like the 65C134 would serve the purpose.

User avatar
lance.ewing
Vic 20 Enthusiast
Posts: 187
Joined: Sat Nov 10, 2012 3:19 pm

Re: VIC ultra minimum system

Postby lance.ewing » Sat Jul 16, 2016 2:56 pm

In Friday's post (as in the mail), I received five 6561-101 chips. I thought I should get a few spares, just in case I blow one or two up during my experiments. I'm not planning to connect anything up to them for quite a while though. Firstly I need to work out and test an appropriate input clock generation circuit. I have a good idea of what I need to do there, but to properly test it, what I really need is an oscilloscope, which I haven't had up until now. But as of tomorrow evening, I should be in possession of a shiny new Picoscope 2205A USB oscilloscope. I already have a few crystals that oscillate at the PAL sub-carrier frequency, which is what the 6561 needs for input. I'll use that in conjunction with a 74LS02, some resistors and a capacitor to generate the two-phase non-overlapping clock inputs that I'll need. Hopefully the Picoscope will let me test that it looks as it should.

That's just step one though. I'll also need to work out the s-video connection (which looks to be quite straight forward), and the whole memory access and control register side of things. One step at a time though, learning as we go.

User avatar
lance.ewing
Vic 20 Enthusiast
Posts: 187
Joined: Sat Nov 10, 2012 3:19 pm

Re: VIC ultra minimum system

Postby lance.ewing » Sun Jul 09, 2017 10:27 am

It is nearly a year on since I received those 6561-101 chips but only my first report back on progress. To be honest, I got distracted by a few other things along the way, both things in life, and by other hobbies and interests. Over the past few months, my VIC interest has surged back to the top again and I've been playing around on a large breadboard trying to get one of these 6561 chips to do something, with more success than I was expecting actually.

What I've currently got set up is the ability to set the values of the VIC control registers by using a combination of DIP switches and a push button switch that controls the WRITE signal. I've also got an 8-bit latch with some LEDs on the other side that in combination with another 4 switch DIP switch (i.e. the address) let's me see what value is stored in each control register. My longer term plan is to use an Arduino Nano to automatically set the control registers to some initial values (rather than me having to go through the routine of using the DIP switches each time, which I'm getting quite quick at doing now but would rather avoid). So I've obviously got the clock circuit working, the phase 1 and phase 2 outputs controlling things, such as the 74LS245 chips, and the video circuit is partially working. It is only black and white though. This is where I'm hoping someone can help me.

I started out with the circuit from the s-video mod here: http://sleepingelephant.com/denial/wiki ... deo_output

And as I say, I plugged it in to a TV's s-video input and all I got was the luminance; no colour. The picture seems fairly stable though, so I guess the syncs are all at the right frequency. I can set up a border and background so that it looks like the normal startup screen, but its all shades of grey.

Since then I've been reading through various past discussions on this forum about the s-video mod and variations of it, but I'm a little bit out of my depth here. Electronics is something I've only been dabbling with as a hobby. One of the reasons for building this breadboard circuit is to learn from doing it. But I seem to be stuck. I can't get the colour to come through. I've had my Picoscope on it looking at what is going on, so I can provide a bit of that info.

For the chrominance pin, I've got the 0.1 uF cap and the 75 ohm resistor, and then a pin soldered to the chrominance wire in my s-video cable. That's all there is to the chrominance circuit at present. With the cable and screen disconnected, the voltage level on the cable side of the 75 ohm resistor is oscillating about 380mV either side of 0V (This is when I've got the whole VIC screen set to RED). I understand that this is about the right level for a chrominance signal; at least as far as I can tell from the searching I've been doing on line. However, as soon as I connect up the cable and screen, the voltage level drops so that it is oscillating only 175mV either side of 0V. The waveform also doesn't look as well formed as it did prior to plugging in the cable and screen. The first question I have then is whether this is normal. What should I be seeing between the 75 ohm resistor and the chrominance wire in the cable?

On the luminance side, I've got the peak to peak from sync to white set at 1 V. The sync level doesn't go any lower than about +220mV though, and depending on what colours I've set the border and background to, it shifts around and can be quite a bit higher than that, i.e. the blanking level isn't fixed at a given level. I've read that the sync should normally be down at about -300mV and blanking at 0V. In my circuit I've never seen it like that. My luminance circuit has changed a bit over the past week or so though. Yesterday I tried the version with the diode and second pot. The first pot controls the peak to peak. The second pot shifts the voltage level up and down, but the sync won't go lower than about +220mV. The second question I have then is whether it is important to get that sync level down to -300mV and the blanking to 0V, or whether it doesn't matter?

Back to the chrominance. Some of the discussion I've read in this forum from years past mentions a more complicated circuit for the chrominance. Given I'm doing this on a breadboard, I have no concerns over damaging a real machine, or restrictions on space or anything like this. What I'm really after then is the ideal circuit for the chrominance if there were no restrictions. Does anyone have some thoughts on that?

eslapion
ultimate expander
Posts: 4162
Joined: Fri Jun 23, 2006 7:50 pm
Location: Canada
Occupation: 8bit addict

Re: VIC ultra minimum system

Postby eslapion » Tue Jul 11, 2017 9:39 pm

lance.ewing wrote:... What I'm really after then is the ideal circuit for the chrominance if there were no restrictions. Does anyone have some thoughts on that?

As suggested by e5frog about 6 months ago.

https://www.digikey.ca/product-detail/e ... ND/4744313
No one can do everything. Everyone can do something.

User avatar
lance.ewing
Vic 20 Enthusiast
Posts: 187
Joined: Sat Nov 10, 2012 3:19 pm

Re: VIC ultra minimum system

Postby lance.ewing » Fri Jul 14, 2017 2:27 pm

Thanks for the tip. Order placed with Digikey and package dispatched. Will be interesting to see how it goes.

Since my previous post, I've realised that it doesn't seem to matter what level the sync and blanking are at, as long as the gap between them is about 300 mV and the sync to white about 1 V. I connected my Picoscope to the composite pin of a cable coming from a real VIC and can see that the sync level is just above 1 V with the white just above 2 V. So all the info I read about the sync ideally being at -300 mV for PAL seems not to apply here and my efforts to get the luminance level lower were pointless.

I also connected up my breadboard VIC to an s-video to composite adaptor and measured the composite signal with the Picoscope. Comparing this with the measurement from the real VIC for the same colour, everything seems aligned apart from the strength of the chrominance being less than half. So this is possibly my main problem then.

I also noticed that I'm borderline with the input voltage to the 6561. The spec says a minimum of 4.75 V and I've currently got 4.8V on that pin. I guess its within the spec.

eslapion
ultimate expander
Posts: 4162
Joined: Fri Jun 23, 2006 7:50 pm
Location: Canada
Occupation: 8bit addict

Re: VIC ultra minimum system

Postby eslapion » Fri Jul 14, 2017 3:43 pm

lance.ewing wrote:... I connected my Picoscope to the composite pin of a cable coming from a real VIC and can see that the sync level is just above 1 V with the white just above 2 V.

These numbers here are relative to the ground level of your VIC-20. All video signals should be considered as AC or floating. This means there is a delta of 1V between the sync and the white and that's exactly what it should be.

So all the info I read about the sync ideally being at -300 mV for PAL seems not to apply here and my efforts to get the luminance level lower were pointless.

Wrong! This -300mV is relative to black, NOT relative to ground because, once again, all video signals should be considered AC or floating.

If you use your Picoscope with a VIC-20, have the VIC display a white border and a black background. You'll see the sync is 300mV lower than black and white is 700mV higher than black or 1V above the sync. Exactly how black, sync or white is above the ground is completely unimportant. What matters is how they are relative to one another.
No one can do everything. Everyone can do something.

User avatar
lance.ewing
Vic 20 Enthusiast
Posts: 187
Joined: Sat Nov 10, 2012 3:19 pm

Re: VIC ultra minimum system

Postby lance.ewing » Tue Jul 18, 2017 12:21 pm

Thanks for clarifying that. I was struggling to find something online that would confirm one way or the other. Part of me suspected it was relative, but then I kept finding documents that appeared to imply that blanking was actually at 0 volts. It may be that there is a bit of misinformation out there on the net, as there is with every topic.

I suspect my main problem at the moment is noise added by the fact that its all wired up on a breadboard. There seems to be quite a bit of it just before the colour burst, which isn't going to help a lot. I've now got a separate buffer circuit for the chrominance, so the signal is stronger and about the amplitude that it should be. So everything looks to be the right shape when compared with a real VIC, apart from the noise.


Return to “Hardware and Tech”

Who is online

Users browsing this forum: No registered users and 2 guests