New Project: 68008 Microcomputer


one of my weird ideas is to build a microcomputer with every CPU i own. Ok, maybe not really *every*, but some of them i memorize with nostalgia. These are:
  • Z80 with SRAM
  • Z80 with DRAM and paged memory
  • 68008, one with SRAM and one with ~ 2MB DRAM SIMMs
  • 68000, let's see
  • 68020, eventually with FPU
  • 68040, with ~ 64MB DRAM PS2-SIMMs probably
All of them will connect to the K1 Peripheral Bus, so that half of the work is already done. ;-) Beyond that, no I/O will be implemented on these boards.

You see, in essence these are two processors. I started with a ZX Spectrum and proceeded with an Atari ST before i entered the world of Apple, Linux and not Windows.

68008 SRAM Microcomputer

Let's start with the 68008. I have both, the DIL and the PLCC variant. I'll use the DIL version for the SRAM board and the PLCC version for the DRAM board, because it can address more memory: 4MB instead of 1 MB only.

The 68008-SRAM  board will the half sized – 79x100mm – pretty tight, but it will fit. The 68k CPUs are a little bit nasty to integrate into a system, because they have quite a lot of requirements, especially the 68008 which implements an asynchronous bus model. But i'll use any trick, cheat and simplification i can find to make it fit. :-)

Basic Requirements

  • 68008 PDIP CPU
  • Eprom 32 .. 256 kByte
  • SRAM 128 or 256 kByte
  • K1-Bus half-sized card

Project Page

The Board

Layout v0.1 2014-01-14
To the left is a first layout of the board with all components required for the current circuit.

The top row ICs are glue logics.
To the left is the K1-bus connector.
The circuits to the left of the CPU connect the CPU to the bus.
The two 74HC573 registers expand the 8 bit bus of the 68008 to the 8 or 16 bit K1-bus.
The 74HC367 hex driver plus some resistors implements the i2c interface.
The 74HC138 decodes the strobe signals for the K1-bus.
In the center of the board is the CPU
and right of it the SRAM and the Eprom.

Requirements and Simplifications

Memory Map. Memory is divided into 4 sections: RAM, ROM, fast I/O and slow I/O each of which is 256 kByte in size. This limits the size of the RAM and Eprom.

CPU Clock. This is generated by a 10 MHz clock module. I avoid the hassle of generating the clock signal "by hand" with a quartz and inverters.

CPU Reset. The 68000 family needs 0.1s low on Reset and on Halt for power-up initialization. This is currently done with some R and C and a Schmitt Trigger inverter.

CPU DTACK. The 68008 needs an acknowledge for every memory read or write access. By delaying this signal you can add wait cycles. By never asserting this signal you can make the CPU hang for ever. This signal will be handled in the most simple way possible: It is tied to ground and this way always asserted. According to some other 68008 projects, where they do this for the test run on a bread board, this should work. Drawback: No wait cycles possible. I'll need a reasonably fast Eprom and SRAM. This also applies to the fast I/O.

Bus Error. DTACK is always asserted and BERR is tied to Vcc. There will never be a bus error.

Bus arbitration. The K1-bus does not support multiple bus masters and so does this board: bus request BR and bus grant BG are not used.

Interrupt control. The 68008 PDIP has two interrupt input lines which can encode 4 states: no interrupt, 2 normal, prioritized interrupts and a non maskable interrupt. The only source for interrupts on this board is the K1-bus and so i need only one normal interrupt. The K1-bus supports prioritized interrupts by enabling/disabling interrupts directly on the attached extension cards.
Now the nasty thing: Devices must provide an interrupt vector during the interrupt acknowledge cycle. An automatic vector can be requested by asserting VPA and so we need to know when a bus cycle is an interrupt acknowledge cycle. For this we must decode the Function Code outputs FC0, 1 and 2 which are all '1' during an interrupt acknowledge bus cycle.

K1-bus access. As said above there are two address ranges for the K1-bus: fast and slow.
The idea is to use a standard memory cycle for fast I/O where wait cycles are not supported because DTACK is permanently asserted. This is suitable for very fast peripheral cards and for switching interrupts and i2c on the K1-bus.
For slow devices i want to use 6800 peripheral I/O cycles by asserting VPA (valid peripheral address) in this address range. This will do a bus cycle synchronized with the free-running E output of the CPU (which has a fixed period of 10 CPU clock cycles) with at least 11 and at most 18 CPU clock cycles due to synchronizing; and no wait states because of the fixed alignment to the E signal. Eventually i'll come up with something better here.

CPU VPA. This input was already discussed in two requirements above: Interrupt control and K1-bus slow access. During an interrupt acknowledge cycle it is pulled low to request an automatic vector (interrupt routine start address) and in slow I/O it is pulled low to request a slow 6800 peripheral bus cycle.

K1-bus 16-bit I/O. Peripherals on the K1-bus may use 16 bit I/O. The 68008 has only an 8 bit data bus. There are two possibilities: I use it 'as is' and attach only 8-bit extension cards. Or i add 2 latches to store and receive the upper byte during a 16 bit I/O. I have some K1-bus cards which use the 16 bit bus, most namely the IDE board because IDE is 16 bit wide, and so i'll invest in two '573 data latches.

K1-bus i2c. Peripheral cards on the K1-bus can have i2c EEproms to signal presence of and identify the card and to provide a universal byte-coded drivers. This costs one '367 hex driver IC plus some resistors. This makes it possible to add arbitrary cards to the microcomputer.

ROM and RAM. The EPROM and the SRAM may be up to 256 kByte in size each. Due to space constraints only one SRAM IC is possible. Memory access cycles of the CPU are without wait states (see DTACK above) and therefore the memory ICs must be fast enough: Scrutinizing the bus cycle timing charts i expect that 150 ns access time will do it; eventually up to 200 ns will work.

System Timer. There is no system timer on the board. Instead it is expected that one of the K1-bus cards supplies one. This is fairly easy, because my serial cards with one (or more) 88C192 dual UARTs can supply this.

Serial and Parallel Ports. Any connection to the outer world requires a K1-bus extension card.

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