The DigiBrain's Brain I think we all know a little about microprocessors and central processing units (CPUs). These gadgets run in computers and crunch numbers as well as driving monitors and writing data to our printers and so on. The difference between a microcontroller CPU and a more mainstream chip like the Pentium or 68040 is the amount of support chips needed. A microcontroller is often connected directly to LEDs and switches. It needs a minimum of external parts, as it has its own dedicated I/O ports as well as scratch RAM and a program burned into EPROM (or EEROM.) A chip like the Pentium needs outboard RAM, memory controllers, I/O buffers and gobs of "glue logic" so all these components work together properly Choosing a CPU Some people get all bent out of shape over one brand of CPU versus the other. I say, "Look at the cost and what you want to do with it!" The Motorola 68HC705J1A family, which I used, has some good feature--but than so do the Intel Zilog, Parallax and many other chips. In fact, the first version of the DigiBrain used an old Intel 8751 and it worked just fine! My final choice of the 68HC705J1A was (as usual) cost, cost, cost, availability and maybe a chance to challenge my programming skills. The version I used comes in a skinny 20-pin package with 1.2 kbytes of EPROM and a handful of RAM. The trick would be cramming everything into this tiny package. Some of you are probably giving this account the fish eye, as it seems impossible to program anything useful in 1 kbyte of RAM, right? I mean, everyone knows that Word 6.0 for Windows fits on a dozen 1.44 megabyte disks. Well, just remember that the earliest home machines, such as the Altair and Intelec8, started with 256 bytes of RAM and later moved up to a whopping 4 k. People managed to get these machines to do all sorts of clever stuff. The trick was careful coding in assembler! (Seeing how long some of today's programs take to load, you might wish that a few more people had developed that assembler programming skill Design Tools One of the things going for the Motorola CPU was its cost The one-time-programmable parts are surprisingly inexpensive. When I ordered my 68HC705J1A designer's kit, it only set me back $99.95. I looked at the development kits for other popular microprocessors and they all cost $200 on up! The Motorola kit comes with an integrated editor/assembler/debugger as well as a programmer that plugs into your PC's serial port and a sample of the erasable version of the part. The surprise was that the programmer card doubles as an in-circuit emulator. A software emulator lets you simulate a microprocessor (such as the 68HC705J1A in software on a different computer (such as your generic PC)--a very useful thing to have while testing snippets of code. Remember, though, that Motorola supplies a software and hardware emulator. This gadget allows you to plug a cable into your new design and run and simulate the action of the CPU. Thus, without programming any 68HC705J1A parts with alpha or beta versions of my program, I was able to try routines and program the Harris DDS and so on! This also allowed me to determine which CPU input was which on my bungled prototype board I think the reason this seems so fantastic to me is that the last chance I had to use such a device was when I was working on a 68000-based computer. The Hewlett-Packard in circuit emulator I used cost our company 30+ kilodollars Although it was a tad fancier than the one offered by Motorola for 99 bucks and change, the functional differences aren't worth mentioning. As I've said before, we are indeed living in a new golden era of designing and constructing ham gear at home. With tools like these, new and wonderful gadgets can be designed quite easily Making It Fit At first glance, the 68HC705J1A (what a mouthful) didn't appear to have enough I/O lines for the DigiBrain project Then I remembered that I was dealing with a real CPU. It was simply a matter of using the chip's Port A as my 8-bit data bus and turning this into two separate 8-bit ports for LCD and DDS control. This required a set of 74ALS574 latches and two bits from Port B--pretty good, synthesizing 16 bits from 10! To write the LCD, I send the byte to Port A and toggle the latch bit to the LCD 74ALS574 (U3). If I want to write to the DDS, I write my data to Port A again and toggle the latch bit to the other 74ALS574. I also decided to use the LCD module in 4-bit-data mode. (I had to do this because I needed an additional three lines to control the panel.) I suppose I could've simply added a third 74ALS574 for a third port to control eight extra output lines. But doing so would have raised the DigiBrain's cost and lost that "spare" I/O line on Port B--and I can't forget Craswell's Law: Software once written and working, of course) is cheaper than hardware.--WBOVNE
