68881 FPU and other FPU

leeeeee · Post by **leeeeee** » Fri Oct 14, 2011 3:19 am

Quote:

I think interfacing with a 65C02 would present some significant glue logic challenges.

I think this should suffice for connecting a 6502 to a 6888x

A4-A0 to A4-A0
R/W to R/W
D7-D0 to D7-D0 and D15-D8 and D23-D16 and D31-D24
Inverted phase 2 to /AS
Inverted phase 2 to /DS
Active low select to /CS
GND to /SIZE - selects 8 bit bus mode

Unless you clock the 6888x really slowly there should be no need to use the data strobe acknowledge signal from the 6888x.

Quote:

you'd have to write routines to convert to/from IEEE floating point format to use the 68881. The FPU doesn't do that for you

Actually it does. It can import and export values as byte, word and longword intgers, single, double and extended precision floating values and as a packed decimal string.

Quote:

Also, you'd need some way to add the 68881 assembly language instructions to your 65xx source code

I think you'd be better off writing a library of subroutines rather than trying to inline the 6888x opcodes. That way you only need code for instructions that make sense in a 6502 system.

Quote:

The data sheet doesn't list the equivalent machine instructions.

The 6888x user's manual describes each bit of each word for all the opcodes.

Lee.

BigDumbDinosaur · Post by **BigDumbDinosaur** » Fri Oct 14, 2011 4:00 am

leeeeee wrote:

Quote:

you'd have to write routines to convert to/from IEEE floating point format to use the 68881. The FPU doesn't do that for you

Actually it does.

What I meant was converting between ASCII and the IEEE representation. Unless I'm not seeing something in the data sheet, it only works with binary representations of various types, as well as BCD.

Dajgoro · Post by **Dajgoro** » Fri Oct 14, 2011 4:09 pm

Anyway the free shipping on ebay is not very free anymore, sow now i am considering other fpu as a possible solution, any suggestions?

ElEctric_EyE · Post by **ElEctric_EyE** » Fri Oct 14, 2011 4:30 pm

Someone here mentioned uM FPU 3.1 awhile ago...

Dajgoro · Post by **Dajgoro** » Fri Oct 14, 2011 4:43 pm

Kinda expensive, the uM-FPU v3 21$ without shipping, the ebay 68881 is still cheaper...

leeeeee · Post by **leeeeee** » Fri Oct 14, 2011 5:29 pm

Quote:

What I meant was converting between ASCII and the IEEE representation.

At some stage you're going to have to convert from/to ASCII, if you can live with scientific notation packed decimal is very handy as conversion is almost trivial. Integer types are almost as easy and I don't think there is an FPU that can directly accept or output ASCII strings.

Lee.

dclxvi · Post by **dclxvi** » Sat Oct 15, 2011 7:24 pm

Using a 68881 with a 6502 can be done, and in fact has been done. There was a floating point card for the Apple II called the Floating Point Engine that used a 68881. I don't know much about it other than that, though (I don't know how it worked). I read about it in a 1988 (or so) issue of Call-A.P.P.L.E. I still have the issue, though it's not easily accessible at the moment.

leeeeee · Post by **leeeeee** » Sat Oct 15, 2011 10:11 pm

Apple II Floating point engine.

leeeeee · Post by **leeeeee** » Sat Oct 15, 2011 10:26 pm

And a clone card.

Dajgoro · Post by **Dajgoro** » Sat Oct 15, 2011 11:17 pm

Can the 68881 fit(pins) on a regular perforated experimental board?
My ms-paint says yes(i overlapped the board images from the links above).

ElEctric_EyE · Post by **ElEctric_EyE** » Sat Oct 15, 2011 11:34 pm

The MC6888x BGA type package (i.e RC suffix) has pins that are spaced .1". That is standard spacing on alot of prototype style perforated boards, but not all... The only other thing you will need to consider are the hole sizes specified on the board you plan on buying, maybe also the thickness of the perf board...

Dajgoro · Post by **Dajgoro** » Sat Oct 15, 2011 11:46 pm

I never solder ic directly to the board, only in crazy situations(like in my digital clock project, since i hate to see the refresh of the 7 segment digits, i placed a 4026 ic(directly soldered) for every digit(14 of them)).
I plan to make myself a socket from this:
http://www.ebay.com/itm/260551794155?ss ... 1439.l2649

How do the instructions and r/w from registers work in peripheral mode?

(The 6000-post in this board)

Dajgoro · Post by **Dajgoro** » Mon Oct 17, 2011 9:26 pm

I found a nice user manual:
http://www.poeticmonkey.com/ebay/semico ... 8881um.pdf

ElEctric_EyE · Post by **ElEctric_EyE** » Mon Oct 17, 2011 11:44 pm

Dajgoro wrote:

I never solder ic directly to the board...

Neither do/did I. I used to prefer sockets that I could wire wrap to. Now in this age of 32,48,100,144,208-pin QFP, each QFP to wirewrap socket is very expensive, compared to a prefab'd board with many QFP's...

BGA will soon have to be mastered by the hobbyists...
Anyone looking to the future must realize BGA is a superior package... Unfortunately.

No socket there, but noise and speed issues are resolved with superior BGA.

Nightmaretony · Post by **Nightmaretony** » Mon Oct 17, 2011 11:48 pm

BGA has a problem of heat warpage, balls losing contact and repair capability. Notice how all the newer systems have such issues with their BGAs, creating such joys as the red ring of death....

//dip fan