68008 SRAM Microcomputer – Main Circuit v0.4

The design phase is coming to an end now, if no problems show up during verification phase later, this will be the main circuit:

K1-Bus 68008 CPU board with SRAM – Main Circuit

Connection of the static Ram and Eprom are straight forward. Ram is mapped starting at address 0x00000 and the Eprom is mapped starting at 0x40000 by use of the 2-to-4 line decoder IC2A.

After power-up the Eprom is forced into the address range of the Ram so that the reset vectors can be read from Rom. This is done by diode D2 which pulls the input A0 of IC2A high while the two-NAND-gate flip flop is in the power-up state.

The address decoder ICB2 either activates !DTACK or !VPA to terminate a bus cycle. !VPA is activated for addresses with A19=1 and A14=1 which will perform a slow 6800-style memory cycle, for all other addresses !DTACK is activated. K1-Bus peripherals may add wait states by pulling !K1_WAIT low. By choice of A14 to discriminate between fast and slow I/O all peripherals can be accessed using short addressing with A15 to A31 all '1'.

The reset circuit has been described in all details in blog post 68008 SRAM Microcomputer – Reset circuit.

Multiple bus masters and single-stepping using the !HALT input are not supported. Bus errors are not detected. All bus cycles will terminate, unless someone pulls !K1_WAIT low. The CPU board does not have it's own timer interrupt and no own I/O pins. This must be provided by at least one attached K1-Bus peripheral board.

For PCB images and Eagle CAD schematics you can view the project page at k1.spdns.de/Develop/….