Adding a *very* simple coprocessor to a PICAXE ?

Buzby

Buzby

Senior Member
Hi all,

I'm in a portakabin in a Norwegian dockyard, working on a boring documentation project. During a lull between coffees I was looking idly round the Net, when I came across a website devoted to the Motorola MC14500. This reminded me, I've got one somewhere.

For those of you who think an 8-bit processor is low-end, the MC14500 is a 1-bit Processor. It's got no RAM, no ROM, no program counter, and only 16 instructions, two of which are NOPs.

Has the panel any ideas what could I do with this in conjunction with a PICAXE ?.

Cheers,

Buzby
 
D

Dippy

Moderator
No I can't. Get on with your documentation! :)
Just kidding! I'm sure someone here used one in 1996.;)

I'm sure if you saved us all time and gave a link to a Data Sheet then someone with some time could think of something.
...or find a more interesting chip maybe?
 
Buzby

Buzby

Senior Member
Code: 
...or find a more interesting chip maybe?
Somewhere in my dark crevices I've got a three chip set from Signetics from the same era. These made a programmable graphics generator, with layers and sprites. I interfaced them to a SC/MP in 1979, so I'm sure a PICAXE could use them as well.
 
premelec

premelec

Senior Member
I recall using one of these one bit units in 1978 - their target market was stuff like traffic lights - if you want some challenge just go only back to the PICAXE 08 - that's enough fun and challenge... [The Motorola reduced number of instructions was something I could remember in 1978 though excessive in the context of Turings 6... ]
 
hippy

hippy

Technical Support
Staff member
Buzby said:
I'm in a portakabin in a Norwegian dockyard, working on a boring documentation project.
Click to expand...
I expected "I'm in a portakabin in a Norwegian dockyard; help! I can't get out! HELP!", so I'm bitterly disappointed ;-)

I was always impressed by the MC14500, but not sure how you'd really use it to good effect with a PICAXE though. It was really an early "bit slice" ALU element and run by microcode If you look at the datasheet figure 2 you'll get an idea of how it fitted into that sort of system ...

http://www.brouhaha.com/~eric/retrocomputing/motorola/mc14500b/mc14500brev3.pdf

The handbook explains a lot more ...

http://www.brouhaha.com/~eric/retrocomputing/motorola/mc14500b/mc14500b_icu_handbook.pdf

The extra you need is really hardware, as the MC14500 isn't really anything without its I/O ports. The best I can see the PICAXE being used for is being the EPROM driving it and the microcode address counter.

I've got a box of Am2901 4-bit-slice ALU's I''d hoped to put to good use but never have.
 
Buzby

Buzby

Senior Member
Hi Hippy,

I was impressed by the thing, that's why I bought one, 30 odd years ago. ( I think I saw it in Elektor )

But it really is a one-trick pony, I can't see any way of making it a useful add-on to anything.

I could easily make a PICAXE emulate the RAM, ROM, PC, and I/O that it needs, but what could I do with it then ?.

It definitely could be used to build a stand-alone device of some kind, but I cba thinking of one just yet !.

It shall stay in it's dark crevice a little longer.

Cheers,

Buzby
 
hippy

hippy

Technical Support
Staff member
Buzby said:
But it really is a one-trick pony, I can't see any way of making it a useful add-on to anything.
Click to expand...
I think it's the nature of being so cutting-edge for its time that makes for its limitations now. It arrived just before micros were becoming more integrated and began putting everything on one chip, it is from the days of being just one small cog in a much bigger wheel - so now it's a bit like taking a NAND gate and wondering how to use it with a desktop PC.

There were some pretty bizarre system architectures around in the 1970's as semiconductor companies worked out how to do things. I always wanted an SC/MP, then an Intersil 6100 ( a DEC PDP-8 minicomputer on a chip ) but it was all far too expensive for a young lad. Now we've got PICAXE. Alas, they're no longer ceramic with real gold legs.
 
darb1972

darb1972

Senior Member
If you ever happen to dive back into your dark crevice another 30 years from now maybe you could donate it to the museum (the chip, not your crevice). :)
 
hippy

hippy

Technical Support
Staff member
Buzby said:
It shall stay in it's dark crevice a little longer.
Click to expand...
I've found a reason it could see the light of day. Not a brilliant one but perhaps it appeals.

You could feed a button into Data ( using a largish in-series R because that's a bidirectional pin ) and connect a LED to Result Register (RR) output; you can draw 10mA from that pin but I'd say aim lower.

Then by driving Reset (RST) first then the Clock Input (X2) and providing an instruction (I0-I3) from PICAXE output pins you can instruct the MC14500 to set / clear / toggle the LED depending on button state.

Going onwards, you could drive ( and read ) the Data (D) pin from a PICAXE pin, select 1-of-N PICAXE inputs to pass through to D before executing an instruction, and you can even take the Write signal back into the PICAXE (interrupt?) to set 1-of-N PICAXE outputs high or low.

The neat thing is that you are not doing any processing, just instructing the MC14500 to do that processing which is the essence of a microcoded, bit-slice architecture. The PICAXE does some moving data round but that's just to save on adding hardware so isn't really cheating.

If you put your instructions into EEPROM in the format %iiiibbbb where iiii is an instruction and bbbb is a bit (0-15) to read/write you've created a genuine interpreter system. I'd find it quite fun to do but as to useful application other than learning a lot and being able to put on your CV that you "built a CPU" I'm not sure.
 
hippy

hippy

Technical Support
Staff member
A sideways challenge : Code a PICAXE so it functions as an MC14500. You'll need 6 inputs ( RST, X2, I0-I3 ), 6 outputs ( Write, RR, JMP, RTN, FLGO, FLGF ) and 1 bi-directional Data (D).

You don't need to match it's speed nor its timings.
 
M

Marcwolf

Senior Member
Commerative PicAce

Thats an Idea

How a bout a range of commerative PicAxe's with classic white ceramic bodies, gold chip covers and gold legs. Something that would REALLY look impressive in a project


Dave. Who still laments the lost of front panels with all the blinking lights
 
G

geoff07

Senior Member
I wonder if there is a word for that sad redundancy of perfectly good kit well within its failure lifetime that is just plain obsolete because something else ate its lunch?
 
E

eclectic

Moderator
geoff07 said:
I wonder if there is a word for that sad redundancy of perfectly good kit well within its failure lifetime that is just plain obsolete because something else ate its lunch?
Click to expand...
Several:

Progress!
Marketing.
Floccipaucinihilipilification.

e
 
J

John West

Senior Member
Also, a two word phrase: "my projects."

Examples:
A. The WWVB receiver I built with discrete transistors and a good week's worth of effort, right down to fabbing my own circuit bds, that was replaced within a few years by a $12 "atomic time" wall clock that also included the temperature reading.

B. My real-world interface for the C-64, that likely still works just fine if I ever dig it out from wherever I chucked it after Commodore was crushed by Apple and IBM.
-

Such obsolescence of high quality electronics is something I run into almost every day in the surplus electronics biz. Beautifully built, (and still within original spec) old equipment that's undesirable now (worthless) because something that's smaller, cheaper and easier to use has entirely taken over. I sometimes take a piece of such gear home just to give it a good home where it will be properly appreciated - but I still use the smaller, cheaper, easier to use gear whenever I'm working on something.
 
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