The Designing and Building of an 8-Bit Computer: A Personal Story Written by Mark Balch Co

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The Designing and Building of an 8-Bit Computer: A Personal Story Written by Mark Balch Copyright 1990 by Mark Balch, All Rights Reserved. You may freely copy and distribute this text file provided that: it is not sold for profit nor used in a commercial environment; that it is distributed in its absolutely original, unmodified form; and that it bears the original copyright notice seen above. These past few weeks have been very exciting for me because I have been working with and programming my first home-brew 8-bit computer. This article is not, by far, the last word on building computers but is intended to serve as an eye-opener for those interested in the inner workings of computers. I am not going to explain every facet about 8- bit computers because such a project could easily fill several reference volumes and would require the authorship of a true professional. I am simply going to describe my own little creation: the preparation leading to its design and construction, the actual processes of designing and building my computer, the long process of finding and correcting my initial hardware problems ("bugs") and my current, on-going exploration of the computer's capabilities from the programming side of things. I first started researching the low level intricacies of 8-bit computers about three years ago by reading Steve Ciarcia's monthly column in BYTE magazine called "The Circuit Cellar." (I had first seen this column as early as 1985 when I glanced through old issues of Byte out of curiosity. Unfortunately, Steve no longer writes for Byte magazine due to what are, in my opinion, questionable attitudes of the management. But, lucky for his readers, Steve founded an excellent magazine along with fellow electrical engineers devoted to hardware "hacking" called Circuit Cellar INK. In addition to this, beginners like myself and industry pro's can communicate with each other by calling the CCI BBS which will be mentioned later in more detail.) In this monthly column, Steve would present working projects along with schematics and thorough explanations (schematic diagrams are the detailed plans or blue-prints for an electronic circuit). These projects included 8-bit multi-purpose control computers called micro- controllers, a "smart" printer spooler, a logic IC (Integrated Circuit) tester and many other interesting topics. His last three articles even detailed a multi-processing super computer! I never built any of these projects, but each month they taught me more and more about computer design and gave me ideas for my own projects. At around the same time, I purchased a book dealing with the Motorola 6800 microprocessor family which explained, in great detail, the workings of 8-bit microprocessor systems. I also sent letters to semiconductor companies such as Motorola, Rockwell and Texas Instruments asking for information on their microprocessor, memory and peripheral products. These companies were wonderful with the myriad of free reference manuals that they sent me. Over a one year period of time, I probably mailed more than 20 letters, mostly to Motorola and a good deal to Texas Instruments, and in reply received at least 2 bookshelves worth of valuable information in the form of their product fliers, reference manuals and application notes (application notes are usually sample designs incorporating a certain product for a specific task). These companies were so obliging because the more engineers that use their semiconductor products, the more chips they sell and the more money they make. Nevertheless, all of this free information supplied me with weeks worth of reading material from which I learned a great deal. About a year later, I decided to design a robot based on an article that I found in a 1986 issue of Byte. I carefully drew the schematic diagrams for my simple circuit by hand on three sheets of graph paper. The microprocessor that I chose for the robot was the 6808 which I had learned about from that book. Anyway, this was my first attempt and to say "things got messed up" is an understatement. I ordered parts, some expensive pieces of equipment to "burn" EPROM chips (more on this later) and realized that I didn't know how to begin. Worse yet, the science fair was about a month away. The details are very cloudy in my head for some reason, probably because I had no idea about what I was doing. The project was scrapped and I think I continued to read more about electronics. The year after (two years ago) I decided to go ahead and build a normal computer board, no wheels and no robot. I was able to get my parents to buy me an expensive schematic drawing program called Schema for my IBM clone computer to enable me to draw better diagrams. I designed the computer around the 6808 microprocessor (MPU) again and ordered some parts. But wouldn't ya know social life got in the way when I joined my school's production of the musical Cabaret. I didn't start building the project till February 1st and with the science fair a month away things went hay wire. I cut corners to build it quickly and made a mess for the second year in a row. Thinking back on that project, the design was a pretty good one with only a few flaws. Had I given myself enough time I might have gotten it to work, but who knows. Okay. I had enough. This was crazy. I made up my mind to build a computer, and really build one this time. Summer vacation was over and school had started. (We're back in September of 1989 in case you lost track of my excellent time-keeping!) I went back to my computer and spent some time with Schema. What a terrific program: expensive but definitely worth ever penny to an electrical engineer. I came up with a straight forward design for an 8-bit computer. It was again built around the 1 MHz 6808 MPU and was to have an expansion bus capable of addressing 32K of off-board input/output or memory, 2K or RAM, 8K of EPROM, an LCD, a serial port capable of operation up to 19.2K baud, 8 LEDs, 4 push buttons and an 8 position DIP switch. Now let me do some explaining in case some of the above terms aren't too clear to you. The 6808 that I used has an operating or clock frequency of 1 megahertz (MHz). All computers have a certain clock frequency which dictates how fast instructions are fetched from memory and executed. Everything that the MPU does is synchronized by a constant pulse train called the clock. A 1 megahertz clock speed means that the MPU receives 1 million clock cycles per second (mega meaning million and hertz meaning a cycle of repetition. You have probably seen clock speeds of 10, 12, 16, 20 MHz and higher in computer catalogs. Yes, a 30 MHz 80386 MPU receives 30 million clock pulses per second!). Each instruction that the MPU performs is broken down into parts and each part is executed in one clock cycle. Different instructions have different execution times. On the 6808, the average instruction execution time is about 4 or 5 clock cycles, ranging from 2 cycles all the way up to 12 cycles. The faster the clock frequency, the faster an MPU can execute instructions. Why didn't I use a 20 MHz clock frequency? Well first of all, the highest possible operating frequency of the 6800 family is 2 MHz. Second, the faster your MPU operates, the more expensive and harder to build your computer gets. At 20 MHz, you need very fast memory which ain't cheap, and you have to be careful about other technical problems arising from radio interference in the computer's signal lines. The 2K of RAM means 2 kilobytes of Random Access Memory. Thinking back on things, I should have designed 8K of RAM...oh well. Random Access Memory is the term given to memory that can be written to and read from. RAM is known as volatile memory because it cannot retain its information when power is removed from the chip. Here's something that most people don't realize about memory quantities: the 2 kilobyte number is a round number. The RAM chip actually has 2,048 bytes of RAM but 2,000 is selected because it is easier to keep track of. If you're wondering why the RAM chip is made to hold 2,048 bytes (2,048 isn't exactly a round number), it's because computers operate on the binary, or base 2, number system at their lowest level. All numbers in computers are powers of 2. This is because a signal wire can either be ON or OFF. So you have two possible numbers: 1 and 0. 2,048 is 2 raised to the 11th power. This means that 11 address signal lines (numbers) are needed to access all of the 2K of RAM. The 8K of EPROM means 8 kilobytes of Erasable Programmable Read Only Memory. This memory is non-volatile, meaning that it retains its information even while the power is off. Under normal operation, this memory cannot be written to, so it is called Read Only Memory. But as the name implies, under certain conditions the chip can be programmed and also erased. A special device known as an EPROM programmer is needed to "burn" the information into the chip with high voltage electrical pulses. Another device known as an EPROM eraser is needed to erase the information in the chip before it can be re-programmed. This erasure process is accomplished by a high powered ultra-violet light source. Now here's the question: why use an EPROM if you have RAM? Well, the MPU by itself is a dumb piece of silicon wafer. Without a program it won't do a thing so you must always have some program immediately available for it when the computer is powered on. The EPROM retains its data almost indefinitely, so even after power has been turned off, it can supply the MPU with instructions as soon as the computer is turned on. Okay great, but why do you have to make it erasable? Once you have a perfect program developed, you don't have to make an erasable PROM chip, but during the initial stages of development, you must be able to correct your mistakes. If the EPROM was not erasable, an engineer would waste dozens or even hundreds of PROM chips before he got his program or design correct. And believe me, no matter how perfect you think your program is the first time, it's bound to be buggy. As you gain experience, the number of bugs of course decreases, but they are always popping up when you least expect it. The LCD is an acronym for Liquid Crystal Display. You've seen them lots of times on calculators, watches and maybe some machines. They can be used to display letters, numbers and graphics. The one that I used is a 16 character display. They're not cheap though. Buying them from retailers can set you back about $30 for the one I used and up to $80 for a 40 character 2 line display. The best place to buy them is from odd-lot clearing houses who buy over-stocked items from manufacturers and sell them at great prices. I got three of them for $30, a pretty good deal. The serial port is actually composed of two chips, a connector and some assorted electronic components. The first chip is known as either a UART (Universal Asynchronus Receiver/Transmitter) or an ACIA (Asynchronus Communications Interface Adapter). I prefer UART because it has only two syllables! This chip is the heart of the serial port. It receives data from the MPU and converts it into a serial format and then sends it to the second chip. In reverse, it takes serial data from the other chip and converts it into 8-bit bytes for use by the MPU. I used the Rockwell 6551 which is a very easy to use but complete chip. Many engineers had recommended it to me. Motorola makes the 6850 which is also a UART, but I don't like it. It requires an external clock source and is not as flexible. Heck, they made a great MPU, so who am I to complain!?! The second chip is known as a level converter. Standard logic levels are from 0 to +5 volts. This is known as TTL logic (TTL means Transistor-Transistor Logic. They are the low level basic logic circuits from which all other circuits are built). But the RS-232 serial communications standard uses -12 to +12 volt logic levels. Somehow, the computer industry got stuck with this "great" idea, so we all have to design around it. The level converter simply converts the RS-232 signals to TTL logic signals and vice-versa. The chip I used is the MAX232 which is a great product from Maxim Semiconductor. Unlike the standard chips used for signal conversion, the 1488 and 1489 which require external +12 and -12 voltage sources, the MAX232 converts +5V to +10V and -10V without any external hardware. This minimizes circuit complexity, but increases the cost a little. If you've forgotten by now, I am explaining all of the parts of my computer design! Last but certainly not least is some input/output (I/O) circuitry that I incorporated into my design to allow status signaling to the user through 8 LEDs and user input through a keyboard consisting of 4 buttons and 8 switches. By the way, LED is an acronym for Light Emitting Diode. These guys are available in several colors and they do what their name implies: light up! All of this miscellaneous I/O hardware is tied together for easy communication with the MPU by the 6821 PIA. This is another Motorola product designed to complement its 6800 family of MPUs. PIA stands for Peripheral Interface Adapter. It has 2 8 bit I/O ports and 4 controllable signal lines. I connected the 8 LEDs to one port, the DIP switch to the other and the four buttons to the signal lines. Once I figured out how to work it, the PIA performed very nicely and allowed very flexible control of my I/O devices. I now take you back to the story of building the beast. We were up to the point where I designed the computer with Schema. Okay. During the design process, I communicated with engineers on the CCI BBS and asked little questions here and there. They offered much help and I soon had three nice looking pages of schematics printed out. I was ready to build the computer, so I ordered all of the parts from mail- order houses in silicon valley. The technique that I used is called wire wrapping. There are two basic methods for prototyping circuits (prototyping being the first time you are building the circuit) along with some others: soldering and wire wrapping. Some people like soldering. Steve Ciarcia solders all of his projects, but as someone said on the CCI BBS, Steve was destined to be a brain surgeon. The underside of his boards really look like art. Most of us are not so talented and soldering can become a great mess. Melting through insulation, twisting, turning, burning! Eech. For my previous project (the one that I built, but didn't work) I had called the CCI BBS and asked about prototyping methods. The general consensus was to wire wrap. So, I bought some tools and went ahead with it. The wire wrapping part of my previous project was certainly a success. The process is very easy to learn. This year, I ordered some more wire and assorted materials necessary to build the project. It was a hefty bill, but I paid some and my parents paid some. I started building it, I think, in October or November. In any case, I finished in a few weeks and was ready for the initial test by December. Everything looked great, but I was scared as hell to try the thing out. I soon got up the nerve to program an EPROM with a simple program and plugged it into its socket. I turned the power on with a rapid heart beat and relaxed. The damn thing didn't even cough! It just sat there with the power on. No smoking, no explosions, no LEDs going on, no words on the LCD. I was disappointed, but did I really expect it to work? No, I really looked at it as something that I couldn't possibly get to work. I build a computer? Come on, a computer is a computer, not some lego set or something like that. I gave it a few more tries and still nothing happened. I checked my design and everything seemed okay. I checked the wiring and everything seemed okay. Okay. I decided to check the power rails. I took out my multi- meter which I bought at Radio Shack on sale for $9.95. A multi-meter is an instrument that can measure several electrical characteristics of wires. The most basic multi-meter (what I had) can measure voltage and resistance. I selected voltage mode and put the probes to my power rails. Hey! I found something. The voltage read almost 0 which meant that either my power supply was not functioning or there was a short. I checked the power supply and it was fine, so there must be a short... but where? I called the CCI BBS once again and asked for some help. About two weeks later, I found the short with a technique that an engineer had explained to me. Okay. I'm ready. Again I plugged in the EPROM and again nothing happened. After a few more desperate tries, I gave up. It was now the beginning of January. I decided to stop work until February. About two weeks later, I logged onto the CCI BBS as I always did weekly and found a message waiting for me. It was from an engineer in silicon valley who had started working with 8-bit MPUs when he too was a sophomore in high school. I was then a Junior, but that's beside the point. He gave me encouragement and some hints on how to find my short. I wrote back thanking him for the encouragement but telling him that I found my short and that the computer still didn't work. I was confident of my design because of those I had seen in Steve's column and in the new magazine Circuit Cellar INK. He soon replied with some trouble-shooting techniques. The first one was a simple test to determine if the MPU was actually executing instructions. It consisted of a single jump command that keep jumping back to itself. I tried it and it worked. This was amazing. For the first time, I actually had some assurance that my design had some life in it. Knowing this, I decided to scrap the fancy demo program I wrote and go for a simple LED flasher. I wrote the program and burned it into an EPROM. But it didn't work. This was very disappointing, but I didn't give up this time, because the first test had worked. After a few hours of playing around, I found the problem! Three address lines were wired backwards to the address decoder chip. Time for some background: most 8 bit microprocessors can address 64 kilobytes. This actually equals 65,536 bytes because there are 16 address lines. Since the computer does not consist of one big 64K RAM or EPROM chip but rather several smaller (smaller in terms of the number of bytes they occupy in the address space) peripherals, this large address space must be divided. A common denominator is 8 8K chunks of memory. The top three address lines (A15 through A13) are connected to an address decoder which determines which memory block is currently active according to these three address lines. Because we're working with the binary number system, three binary digits can represent 8 combinations. The decoder chip, in this case a 74LS138 decoder chip, activates one of eight enable signals depending on the content of the three address signals that it receives. Now, how does this represent a problem to me if the three lines are wired backwards? Well let's take the example that I had. My LED flasher program needed to address the 6821 PIA so that it could turn the LEDs on and off. This PIA is connected to the second 8K memory block enable line coming from the 74LS138 decoder. To select this second memory block, the MPU had to place the digits 001 on the lines A15-A13. But if the three address lines were wired backwards, the decoder would see 100 instead of 001 and would select the wrong memory chunk. I discovered this problem by monitoring the decoder's memory enable lines with a logic probe. The logic probe simply tells me whether the signal line is high (1) or low (0). For some strange reason, the second memory block was never being accessed, but the fourth was. After some thought, I realized that the binary number representation for 4 was the exact opposite for that of 2. I ran upstairs to get my reference book and sure enough, the decoder was getting the signals in reverse order. I went back upstairs and fixed the problem. One very big advantage of wire wrapping is that mistakes can easily be fixed. All I did was unwrap the wire from the three signal lines and I rewrapped them. It took about 5 minutes to do. I plugged the EPROM back into its socket and poof! My LEDs were flashing! True, my complicated computer didn't look like much more than a christmas tree, but I knew that it was working. The LEDs were not flashing on their own: the MPU was telling them to flash. Now this was a victory! Once I got to this point, things started falling into place. Within a week and a half I got everything working perfectly. Sure, there were troubles but I got them resolved. The only modification I still have to make is an enable circuit for my expansion bus. I forgot to design one in and because of that the bus cannot be used. But that's okay for now because I am not yet building peripheral cards for my computer. I am now working on a terminal program that will enable me to download programs to the computer's RAM from my communications program on my IBM clone. It should be working within a week. Some miscellaneous problems that I encountered and fixed include: two signals on the serial port were wired backwards, one wire on my LCD was broken, the expansion bus mystery mentioned above, some pull-down resistors that should have originally been pull-up resistors and connecting the LEDs to the other I/O port instead of the one it was originally connected to. It was like unravelling a mystery plot, almost every day after I got the LEDs flashing, something new fell into place and started working. From here on in, its just a matter of software. The hardware is all done (whew!) and now I just have to deal with it from the programmer's point of view. The programming isn't bad, but now and then you get a little bug that drives you crazy until its found and eliminated. The entire project was and continues to be a great learning experience and usually alot of fun. Maybe one day, you'll see my name on a super computer?! Who knows. I got the baby purring and from here the sky's the limit. My next project is going to be a faster computer with a monitor, full keyboard and disk drives. I probably won't get it right on the first power up, but now that I know my computer works, every other problem will be a minor inconvenience! 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