The 6502 40th Anniversary Computer Badge


Build a 6502 Computer Badge
at the Vintage Computer Festival Midwest 12
to celebrate the 40th Birthday of the
Apple II, Commodore PET, and Atari VCS


1977 was a momentous year for the home computer industry. The Apple II, Commodore PET, and Atari VCS were all introduced. These classic machines became the founding fathers of three revolutionary families of microcomputers that changed the world. Interestingly, the "brain" inside each of them was the MOS Technology 6502 microprocessor.

The 6502 was designed by Chuck Peddle of MOS Technology in 1975, at the dawn of the microcomputer revolution. It was an elegant design; simple, yet powerful. Its simplicity made it the cheapest 8-bit microprocessor on the market, endearing it to bean counters everywhere. And its power quickly captured the hearts of design engineers. By 1997, Steve Wozniak designed it into the Apple II. Nolan Bushnell had it designed into his Atari VCS video computer system. And Jack Tramiel had it designed into the Commodore PET.

It has become a common practice to have some kind of "badge" for computer shows and hacker conventions. Since this is the 40th anniversary of these incredible machines, we thought it would be appropriate to make a badge using a vintage microcomputer. So, we built a 6502 computer on a Badge for this years' Vintage Computer Festival Midwest in Chicago IL on September 9-10, 2017!

The badge is a complete working 6502 computer, with RAM, ROM, LED display, serial port, and on-board BASIC. Cost will be about $20; batteries included. We'll even include an inexpensive USB-serial converter in case your computer doesn't have a serial port. Connect it to a terminal or computer, load a message, and the badge scrolls it across the LED display. Or, show off your BASIC or machine language expertise by writing something truly spectacular!



The Plan

It was 40 years ago today
That computers taught the kids to play
With chips to make a new kind of game
And the world has never been the same
So let me introduce to you
The micro called the 6502!


Links

Parts

We worked hard to keep the cost low, so as many people as possible can build a badge and get to know this classic CPU. Each of us raided his "junk box" to donate parts and software. The remaining parts rely on surplus dealers, fleabay, and (if all else fails) regular electronics distributors. Here's the Badge parts list, including sources, in case you want to build one for yourself from scratch.

VCFMW 6502 Badge Computer Parts List
QTY
ID#
Description
2 C1,C3 22uF 16vdc electrolytic capacitor (mouser.com 667-ECE-A1CKA220)
3 C2,C4,C50.1uF 50v X7R ceramic capacitor (jameco.com 332672)
1 C6 3900pF 50v ceramic capacitor (mouser.com 80-C320C392K5R)
1 D1 7-seg 7-digit LED display (mpja.com 19391-OP)
1 D2 1N4001 50v 1a diode (jameco.com 35975)
3 D3,D4 1N5818 schottky diode (jameco.com 177957)
1 R1 10K x 4 8-pin SIP resistor network, isolated (mouser.com 268-10K-RC)
1 R2 220K 5% 1/4w carbon film resistor (jameco.com 691420)
1 R3 2K x 4 8-pin SIP resistor network, isolated (mouser.com 268-2.0K-RC)
1 R4 68 5% 1/4w carbon film resistor (jameco.com 690582)
1 S1 switch SPDT micro slide (mpja.com 18453-SW)
1 U1 74HC139 dual 1-of-4 decoder (mouser.com 595-SN74HC139N)
1 U2 R65C02 microprocessor (jameco.com 43166)
1 U3 27C256 32k EPROM (jameco.com 39731)
1 U3a 28-pin IC socket, 0.6" wide (jameco.com 40328)
1 U4 RAM, 0.3" wide 2k CXK5814 or 32k CY7C199 (jameco.com 242376)
2 U5,U6 74HC273 octal latch (jameco.com 45743)
1 Y1 2 MHz ceramic resonator with capacitors (mouser.com 520-ZTT200MG)
1 PCB (get it from us, of course :-)
1 holder battery holder for 3-AAA cells (jameco.com 216303)

It works!

June 8, 2017: [Lee] The prototype PCBs arrived, and of course I had to build one up immediately to test. Everything fit together perfectly. If you use a screw-machined socket for the EPROM, the 0.3" RAM hides under it just fine. NOTE: Be sure to install the RAM and EPROM backwards from the other chips!

When "off", the RAM is powered, and battery current is so low it's hard to measure (on the order of 5-6 microamps). When switched "on", battery current is on the order of 8-10ma. The 2MHz clock is running, and the data and address buses show the 6502 is executing code. But there was no display activity, and no response to serial inputs.

June 10, 2017: [Daryl] Oops! The ROM code I sent was for my simulator, which had the I/O at a different address. I fixed it, and sent Lee a new copy.

June 12, 2017: [Lee] Burned another EPROM, and ran it. IT WORKS! The default "Welcome to VCFMW 12" message scrolls across the LED display! You can see a couple photos of the Badge in operation at the top of this page, and a few videos of it here of it . Hoo Hah!

Power consumption is pretty good. It draws about 25ma average; 10ma min when most LEDs are off, and 42ma peak with many segments on. The nimh AAA cells I'm using claim they are 1800maH (though I doubt it). If true, they could run the badge for a few days. :-)

But there are a couple problems to resolve. First, my last-minute fix for the pullup resistor just about eliminated the hysteresis in the reset circuit, and moved the low-battery threshold too high. Second, once turned on, I can't turn it off! I open S1, and it just keeps going. The 6502 stops during reset with A14=0, A15=0, and R/W=1. This tells the RAM to read. If any data bit from the RAM is 0, it pulls GND low (via the input protection diodes in the 6502 and 74HC273), defeating the open switch.

June 28, 2017: [Lee] Fiddling with the SIP resistor values fixes the reset threshold and hysteresis; but I haven't fit them into two SIPs (yet). I may have to add another discrete resistor (somewhere). The failure to turn off problem has an easy fix: The "off" contact of the switch grounds the clock input to the 6502. This makes O1OUT high, disabling the address decoder to turn all memory off.

July 4, 2017: [Daryl] Here is what I call my "final" version of the software. It fits nicely into a 16K 27C128. You can also burn it into a 32K 27C256 if you prefer. Features:

LED display: Monitor commands: EhBASIC

I will start documenting Monitor commands and the LED control software. There is an EhBASIC manual of sorts that I will also provide. In the meantime, an EhBASIC manual can be found here.


Last update 7/6/2017.
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