a year ago •
This is something I've written about in a front page Hackaday post, but I think it's time to go over a little more of the theory of what I'm doing here. First, a video:
This is called freerunning the processor. Basically, it executes one instruction, the program counter is incremented, the address is increased by one, and the CPU just sits there, doing nothing, cycling through its address space. Attach a few LEDs to the address pins, and you have an incredibly complex binary counter, also known as blinkenlights.
That's the simple explanation. It's a fair bit more complex in practice. I need to tie a few pins to +5 volts, and ground DTACK. Oh, what about the instruction to freerun the processor? NOP, right? NOPe.
When the 68k first resets, it reads the program counter vector. The program counter vector must be an even address, and the opcode for NOP is $8E71. See that one at the end? That means NOPping the CPU from boot would create an illegal address exception. Then bad things happen.
So, I need an instruction that does nothing, and is even. Inclusive OR Immediate (ORI) does this. Specifically OR.b #0,d0. Bonus, this instruction in hex is $0000, or all zeros. All I need to do to freerun the processor is ground all the data lines.
My first go at freerunning the CPU only used one LED. This LED was tied to the A20 line through an inverter. I hate to waste the five extra inverters on that chip for a single LED, so I added another three.
Now I have status lights for the top four addresses in the computer. Since I'm putting the ROM at $FF0000, the serial port at $FE0000, the video peripherals at $FD0000, the microcontroller at $FB0000, I have a graphic representation of what the CPU is doing with all its peripherals. That's pretty cool. Useful blinkenlights.
a year ago •
Although it might make sense to start this project by building a CPU module first, I decided it would make more sense to start with the memory for this system. This serves two purposes: as an explanation of how the 68000's memory-mapped I/O works, and to have a relatively simple circuit built before embarking on the more complex that include the CPU module.
A 68000 memory access primer
As with the 6502, 6800, and 6809, memory access is controlled by the R/W line. Basically, when the R/W line is high, the 68000 reads from the data bus. When the R/W line is low, the 68000 writes to the data bus.
Unlike the older, smaller, 8-bit CPUs mentioned above, the 68000 also has additional control lines to deal with. /UDS and /LDS are the upper and lower data strobe lines. These signals indicate valid data on data lines D0-D7 (for /LDS) and data lines and D8-D15 (for /UDS).
In addition to the data strobe lines, there also exists an address strobe line, /AS. This signal indicates a valid address on the address bus.
Reset Vector Generation
Before designing our memory modules with these signals in mind, it's very important to figure out how this computer is going to boot. All computers require some amount of RAM somewhere in the address space, and at least a few instructions telling CPU what to do on a restart somewhere else. The 6502, for instance, requires an instruction in ROM at $FFFC, and a few bytes of RAM at $0000.
This isn't a problem for the designer of a 6502 computer - just put some RAM at the bottom of the address space, and your RAM at the top.
The 68000 is different. It's reset vector, or the place it looks for instructions on a reset, is at $000000. The 68000 also requires a small amount of RAM at address $000000. Let that sink in. The naive analysis of these two facts means we must store the first instruction in RAM. RAM that will be uninitialized when we boot the computer.
Fortunately, Motorola application notes give us an easy way to get around this. The solution is to deselect the RAM and select the ROM during the first four bus cycles. This can be done with a 74164 binary counter, using the /AS line as the clock input, and making a /ROMSELECT control signal with one of the outputs. This /ROMSELECT or /BOOT signal (I'm using the two interchangeably) will allow CPU to read instructions from the ROM on reset.
Above is a fairly broad overview of the ROM board's circuitry. I'm using two 32kB EEPROMs for the ROM, split between high bytes and low bytes. This gives me 64kB of ROM for this computer, more than enough to set up a few things on boot and eventually pull data off a hard drive.
There are basically three main components of the ROM module: an address decoder that enables the ROM, memory control logic that selects which chip is being read, and the ROM chips themselves.
According to the memory map I have in my notebook, the 64kB of ROM will be decoded at $500000 through $50FFFF. This means the ROM is selected whenever address lines 22 and 20 are high, and lines 23 and 21 are low. A three-input NAND gate (74ls10) and a few inverters are all that are needed to enable the RAM.
Of course, I'll also need to include the /AS line and the /ROMSELECT line. Easily done.
ROM Memory Control
The ROM memory control is used to toggle the output enable pins on the EEPROMs. The Motorola 68000 user manual has a table going over when valid data should be on the data bus according to the /UDS, /LDS, and R/W lines. Long story short, the above circuit will work just fine for enabling either EEPROM.
Yes, this is much more complex than the ROM module. This is a product of the 68000's huge address space, and my desire to have a ludicrous amount...
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a year ago •
Compared to the 8080, the Z80, the 6809, 6502, and all the other 8-bit microprocessors used in boxxen of yore, the CPU I’m using for this project - the Motorola 68000 is both extremely powerful and extraordinarily complex. The power comes from a huge address space and some neat features like a divide instruction. The complexity comes from it’s asynchronous nature.
A single-board computer using the 8-bit 6502 processor is very simple compared to a 68k computer. Conjuring up a simple 8-bit computer is as simple as getting a RAM and ROM chip, connecting all the data and address lines together, and throwing together a little logic glue to get the whole thing working. The 68k is another story entirely. Thanks to its asynchronous nature, you have to deal with something called the DTACK, or Data Transfer Acknowledge. This is an input pin on the processor that indicates the the data transfer from RAM or ROM is completed. If this isn’t low at the right time, the entire system just stops.
Making sure the data gets from the memory to the CPU isn’t enough? There’s 64 pins on the 68000, and there’s more than a few more useless pins for my project.
The bus arbitration pins - /BR, /BG. and /BGACK control which device in the system controls the data and address busses. It’s great for DMA operations, crazy video schemes, and shoving data from a cassette port directly to memory without going through the processor. DMA would require a good bit of circuitry, though, and I won’t be using it anyway.
Oh. There’s also processor status pins on the 68k. These are output pins that tell the system if the current cycle is being used for user data, user program, supervisor data, supervisor program, or an interrupt. Very cool, and a good example of how the 68000 was inspired by the minicomputers of the 70s, but utterly useless for a small box that will sit on my desk, tweet, and play Breakout.
Complex, yes, but I don’t actually need to use all those pins. Those processor status pins can be easily ignored. I won’t be doing any cool DMA stuff with this computer, so I can just tie the /BR, /BG, and /BGACK pins to +5 Volts. Pins /IPL0, /IPL1, and /IPL2 only indicate the priority level of an interrupt, and I can’t imagine designing hardware in response to an interrupt in this system.
With all those useless pins out of the way, what am I left with? I have 24 address lines, 16 data lines, a reset pin and four pins for controlling memory access:
/AS - Address strobe
Indicates there is valid data on the address bus
R/W - Read / Write
Defines whether the data bus is being used for reading or writing
/UDS and /LDS - Upper and Lower Data Strobe
Indicates the presence of valid data on the data bus. When the CPU is reading the data bus, if /UDS is high, data bits 0-7 are vaild. If /LDS is high, data bits 8-15 are valid.
E, /VPA, and /VMA - 6800 Peripheral control
These lines are used for interfacing 6800-series chips with the CPU. Since I’ll be using a 6850 ACIA, these pins are necessary
/DTACK - Data Transfer Acknowlegdge
This is a line going into the 68000 to tell the CPU a device has received data on the data bus. If I were not using 6800-compatable parts or the 6800 peripheral control pins, I could simply tie /DTACK to ground and hope my memory is fast enough. That’s the easy way out, and I’d really like to do this project right. Generation of the /DTACK signal is easy enough.
And that’s it. There are a ton of pins on the 68000, but if you want to build a simple computer you can ignore everything except the data and address pins and four bus control pins. The amazingly complex 68k then turns into a very very simple synchronous CPU just like the 6802 and Z80. Really, other than the fact a 16-bit homebrew computer requires twice as many RAM chips (although you could always use a 16-bit...
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