Hi guys!

My name is Johan Fransson, living in Sweden. I am a 35-year old engineer working with software development. Since my early teens I have always been interested in computers and also computer music. As for 8-bit technology, I have owned a Commodore 64 on which I created some programs but mostly used it to listen to SID music

Years have gone by and I have developed my musical interest, creating stuff on my own using various sequencer software. Also during the last 5-10 years I have become interested in photography (mostly people).

The 8-bit nostalgia has struck me in a hard way the last couple of years. Not only is my music getting more and more SID-like sounds, but I recently found out via a friend about the advancement in microcontroller technology and the community around it. I ordered an Arduino and Gameduino and some Sparkfun parts to create an retro arcade style game. It takes some time and a lot of programming, but the project progresses.

So I accidentally stumbled upon this forum a few months ago and saw the light. I couldn't believe it. There is actually a community about regular microprocessors, not just microcontrollers. Finally, a possibility to fulfil an old dream; build my own computer! I have lurked about for some time and realized the amount of knowledge here and for that I salute you guys! It seems it will not be impossible to build something from scratch with this kind of community.

So here I am. I have a WDC 65C02, a WDC 65C22, some RAM and EEPROM and other logic and a big breadboard on order. I am planning on building something simple, firstly with some leds and maybe a button, then maybe later on trying to use the timer in the 65C22 for sound.

Once again, thanks for a great forum, and hopefully I will be able to contribute in the future For now, I will probably just be the new guy asking questions

EDIT: It was supposed to go in the "Introduce yourself" thread. Sorry about this

Welcome! I can't believe it has already been 11 years, but I posted a lot of tips at viewtopic.php?t=342 . What made me think of it was your saying, "then maybe later on trying to use the timer in the 65C22 for sound," and that you can use one of the modes of the 65C22's synchronous serial port for a 9-level D/A converter which is definitely enough for intelligible speech, DTMF, and other sounds. If you budget the resources well, you can do a ton of things, simultaneously, with a single 65C22.

Thanks Garth, excellent tips in one thread. As a novice designer, I really don't know what hurdles I might encounter. I really liked the idea about the shift register. In terms of sound, I was reading the 6522 spec and the timer seemed like a simple way to create pulsewidthmodulated audio with almost no constraint on the CPU. For this project, in the end, I want to create a complete a computer as possible, even if it is super low res and with super simple I/O. SD cards, SPI, OLEDs etc, but those are topics for when I have been able to get a simple LED lit

The pulse-width-modulation scheme the data books talk about is for very low frequency work.

Lighting an LED will only take a few instructions after reset, something like:

if the LED goes from the port pin to ground through a resistor.

Welcome!
I would suggest copy your first post and paste it in the the Introductions section. That way your intro won't get 'buried' from all the future posts in this section.

Thanks guys. I have reposted it in the proper section. Can't wait to start assembling and then write some 6502 code

That's fine to copy your original post to the "Introduce yourself" topic, but let's keep that truly as introductions and not clutter it with discussion about things like how to accomplish projects.

Got it, let's keep that thread clean. I might as well rant some more here as I started this thread

I want to start small, then expand. I have been looking at "Adam's Great 6502 projects" http://www.reocities.com/SiliconValley/2072/6502prj1.htm and it seems like an excellent starting point and base for my project.

I have an idea how to squeeze 32 kilobyte SRAM, 16 kilobyte EEPROM (the two are actually 128 kilobyte and 32 kilobyte respectively but are not completely utilized in order to keep address decoding simple), four I/O chips using fairly simple scheme with a dual 2-4 decoder (74139) and an IC containing three 3-ANDs (7411)... But, this is all on paper (actual, physical paper), and as I am new to it all, I will probably start much smaller, i.e. first just connect the EEPROM to the 2-4 decoder to be able to boot the system up. Alternatively, I can use the simple memory decoding scheme suggested in Garth's tip of the day link posted above.

I was thinking of creating an Atmel/Arduino-based EEPROM-burner to create the BIOS. Not really feeling like manually program the EEPROM . Not sure if this is the best way to do it (or if it is overkill) but it seems rather straight forward and easy to do. Also, I am already familiar with the Atmel environment. I made a simple sketch in google docs, https://docs.google.com/drawings/d/1rSywd9sTdXWvoMCBAkXrPI95eZ-DyuT_-8-n54SWTqU/edit

This is for 8K so for 16/32K there will need to be some additions (another counter or something).

Plans for graphics output of the system are fuzzy. Do you think a reoccurring refresh interrupt on the 6502 and a couple of 6522's would be able to drive an 8x8 RGB LED-matrix, possibly with a higher colordepth than on/off (i.e. time slicing on/off the intensity level) ? I think you would need 32 pins (R,G,B times the number of rows + the number of columns), but maybe one could get away with fewer with some simple glue logic (counters/demuxers). Or will the refresh rate required simply take 100% of the CPU, leaving nothing for actual programs? Maybe a better route would be an OLED with a preattached control interface. As you can see, this part is not really thought through.

For input I would ultimately want a keyboard. If it is supposed to be a computer, I want it to be one

Found my notes on address decoding:

cpu A14 -> demux1 A0
cpu A15 -> demux1 A1
cpu A12 -> demux2 A0
cpu A13 -> demux2 A1
demux1 /Y0 -> and1(1)
demux1 /Y1 -> and1(2)
demux1 /Y2 -> demux2 /CS
demux1 /Y3 -> and2(1)
+5V -> and1(3)
+5V -> and2(2)
+5V -> and2(3)
and1 -> SRAM /CS
and2 -> EEPROM /CS
demux2 /y0 -> ??? /CS
demux2 /y1 -> ??? /CS
demux2 /y2 -> ??? /CS
demux2 /y3 -> VIA1 /CS

All the +5V and ANDs might seem strange. I thought it would be a good idea to have similar number of components in the paths to get the same propagation delays for the EEPROM/SRAM/I/O, but maybe that is not important? In that case, it can be somewhat optimized.

If I have calculated correctly, this would give me:

RAM $0000-$7FFF
I/O1 $8000-$8FFF
I/O2 $9000-$9FFF
I/O3 $A000-$AFFF
I/O4 $B000-$BFFF
ROM $C000-$FFFF

But again, it seems fairly simple on paper. Maybe too complex in reality?

Exactly why I want to start with this, thanks. Being a novice at connecting actual things together, this would be a simple way to prove that everything works, then go from there with more software and more sophisticated I/O.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

I glanced at the specifications of the decoder and it didn't seem that much worse than the NAND/AND. But granted, I am new to reading specs so I'll take your word for it.

I see it now. Your scheme is kind of brilliant, and much better than what I had in mind!

Sorry if I skip ahead, I have no one at work to talk to about my ideas about so I get overexcited I will take some time to read your thread thoroughly.

Well i am building a 6502 sbc for the first time also. Your plans are similar to mine. I've based my board on the SBC-2 from 8BIT, it is a simple design to start with. For the keyboard, lcd, rtc, mouse i am using a pic16f877, i connected it using the PSP port, which allows to use it as a regular peripheral. For now i managed to get the keyboard working, and a 4 line lcd with buzzing character 7. I still have to find out how to get the i2c bus working for the rtc. Now i started to build a graphic board using a MC6847 CRTC chip, it has color(requires MC1372), and it is kinda simple to use, and available on ebay!!! I would like to be able to read sd cards and connect to the Ethernet, but i still haven't figured out how to do this...
There are lots of topics in the hardware thread on acia, crtc, ps/2, ect that i opened for my sbc...

It depends. A 74F138 has a typical prop delay of 6ns, according to the TI data sheet. 74ABT gates are down to 2-3ns in most cases. If your address decoding logic isn't too complicated you'll get performance well above what we would normally require in our home-brew circuits. I can run the POC unit at 12.5 MHz, and it has 74AC logic.


Yeppers. Get that single engine plane flying before you start designing the four engine jumbo jet.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

I have read the tip of the day thread now, some excellent tips in there. Somewhere in there was a tip about breaking it down to smaller pieces and solving one problem at a time, and I think this is probably the best advice anyone can give a novice like me Still, I like plans of grand designs in theory. It's fun theorizing. But my head is far ahead of my actual design skills so what you guys are saying regarding a simple addressing scheme etc makes perfect sense to me.

I made a quick attempt at modifying the simple addressing scheme to support 32KB RAM and 16 KB ROM (just for the fun of it; for starters, it doesn't really matter which is 16 KB and which is 32 KB). The Alliance AS6C1008-55PCN 128 KB SRAM has one active-high CE2 and one active-low /CE:

Modified version of Garth's Address decoding:
/A15 = NOT A15
/Phi2 = NOT Phi2
SRAM CS2 = A15
SRAM /CS = /Phi2
SRAM /OE = GND
EEPROM /CS = A15 NAND A14
I/Ox /CS2 = /A15 NAND A14
I/Ox CS1 = Ax+3, x = (1..10)


Oh, and one question. While lurking a few weeks ago I found a post recommending what chip technology to use for new designs. It said something about "rather than LS or HC, choose... in that order". I have searched but I can't seem to find it. As I ordered the glue logic, I vaguely recalled that "ACT" was the technology of choice, so that's what I chose in my order. But now I'm thinking I might have remembered it wrong. Do you guys remember which thread I mean or maybe can point me in the right direction here. Or maybe it doesn't matter much, as long as we're talking < 4 MHz systems.... ? As you can see, still behind on my digital technology skills, but here to learn right?

Thanks again guys.