The HM-68 Homemade 6802 Computer
"Hassler's Monster," with a nod to Mary Shelley.
Constructed by Dave Hassler, October 2023.

Based (mainly) on a 6800 design by Tetsuya Suzuki.
(Creative Commons 3.0)
[see schematics for other credits]

Specifications:
MC6802 MPU at 4 MHz (1 MHz on the bus)
MC6850 ACIA (19.2 to 2.4 kbps)
28C256 32K EEPROM
62256  32K RAM ($0000-$7FFF)

Logic, decoding, timing:
74LS00, 74LS04, 74LS138, 74LS393

ACIA mapped to $8018 (ctrl) and $8019 (data) --
feel free to make it a more "traditional" $8004/5,
which will save some software mod headaches later.

ROM banked into two 16K areas, both mapped to $C000.

Address $A000 on the bus for future expansion
(6821? video? disk/SD card? RAM? ALL???).

For interface to a modern serial terminal emulator,
a TTL-USB serial cable is required.  I use a
PL-2303 that can handle 5 vdc.  As a Linux user,
minicom serves well as a terminal.  I have the
baud switches of the ACIA clock set to 9600 bps,
and I have a character delay of 5 msec and a line
delay of 30 msec on the terminal.

The monitor program is a lightly modified version of
SWTBUG, with the scratchpad RAM moved from the 
traditional $A000 (absent on my machine) to $7F00.
Almost all I/O addresses in SWTBUG are the same as
in MIKBUG; see the included documentation and source.
This should be burned into an E/EPROM beginning at
$2000 and $6000 in a 32K chip's memory.  Tiny BASIC
and VTL-2 are included in this package, to get one 
started -- use a character delay of 10 msec and a 
line delay of 150 for BASIC, 15/300 for VTL.

From there, the world's your oyster...

Good luck and best regards, Dave Hassler  
