Replacement memory cartridge for the MK-90 microcomputer
Hardware
The circuit was built with a small microcontroller and serial FRAM
nonvolatile memories.
The FRAM chips cannot be replaced with EEPROMs.
The series resistors on the bus and the diode (preferable Shottky) are
for protection purpose only, and could be omitted.
Software
mk90_010.zip - AVR source code
The "Read Postdecrement" function (not used by the MK-90)
isn't implemented in current software version.
Usage hints
- The cartridge can be inserted into the slot only when batteries
are in place.
Otherwise the calculator won't turn on, and both the cartridge(s) and
the batteries have to be removed for a while to bring it back to life.
It's an expected behaviour pointed out in the manual, even if it
doesn't seem to appear with original cartridges.
- Both the original and cloned cartridge can be used simultaneously,
they don't interfere with each other.
- The MK-90 manual (BASIC V1.0) indirectly acknowledges
existence of a serious bug in the file system.
It probably only concerns saving programs of size being multiple of
512 bytes, that have been run at least once.
See the errata at the end of the manual for suggested workarounds.
The bug may be fixed in later BASIC versions.
- The replacement cartridge contains 16kB of memory (while the
original one only 10kB), therefore 6kB of memory remains unused.
However, the entire memory can be made avilable to the MK-90 by
formatting it with INIT, then writing value of 0x1C to the
address 0x0412 with the programmer.
The MK-90 should display 28 free blocks now.
