[Flashforth-devel] FlashForth on the PIC18F26K42

[Jordan N. <jni...@gm...>]

Hi everyone,
I have been working on getting FlashForth running on the PIC18F26K42 as
part of my final year engineering project.
I thought people might be interested in my work to that end, which can be
seen at
https://github.com/iamjpn/flashforth on the k42 branch.

The 26K42 has quite a few differences from the other PIC18s.
* The BSR is now 6 bits rather than 4 bits, and data memory ranges from
$0000-$3FFF.
However, for the 26K42, banks 16 - 55 are not implemented and banks 56 - 63
are SFR.
This made me decide to keep the same memory mapping as the other PIC18s,
with the RAM starting at $F000.
I further split the memory, to keep all the SFRs available:
$F000-$F7FF in FlashForth = $0000-$07FF in hardware
$F800-$FFFF in FlashForth = $3800-$3FFF in hardware
All this requires a lot of bit masking, makes storing more complicated and
makes half the RAM unavailable.
It would be a better idea to just RAM at  $C000 on the 26K42, which is what
I'm now working on.

The larger memory size also means that movff does not have enough reach,
movffl needs to be used.
You need to be using the latest MPASM or movffl will not work.
( https://www.microchip.com/forums/FindPost/1064775 )

* Many SFRs are no longer in the Access Bank.
A lot more bank selecting needs to be done.

* No more USART.
The 26K42 has a new, different UART module, different code needed for this.

* Instruction Memory
The 26K42 uses a block size of 128 bytes for writing and erasing program
memory.
64 bytes was previously assumed throughout FF.

* Timers
Slightly different set up

* EEPROM
Begins at address $310000 instead of $F00000

* Naming
A few names used in FlashForth conflicted with names used by the 26K42, so
had to be renamed.

I have not yet touched multitasking, HW flow control, cpu load, cpu idle
mode or USB serial emulation, will work on these next.
I have not used my changes in anger yet, but it seems to be functional.

The changes I have made make FlashForth no longer compatible with the other
PIC18s. After it is fully working on the 26K42 it would be nice to
integrate the changes back in.
I'm a little unsure what is the best way to go about this.
It would need quite a lot ifdefs if used in a similiar way to how currently
differences in devices are handled, but I'm not sure what a better approach
would be.

I hope this is interesting to people,
Jordan Niethe.