DIY Fpu Unit for STFM

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tcat
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DIY Fpu Unit for STFM

Postby tcat » Wed Mar 07, 2018 11:33 am

Hi,

I plan to build a simple FPU board for MC68000, based on a DIY design. I need to decode many address lines. Am I safe to replace original 74HCxx chips with 74LSxx ones, as these seem easier to get here?

I cannot tell any difference between LS and MC technology, and when to use one or the other. The FPU is MC68881@16MHz, and the FPU board should piggy back on standard CPU MC68000@8MHz using standard ATARI STFM power supply.

Many thanks.

Tomas
Last edited by tcat on Sun Sep 29, 2019 9:38 am, edited 3 times in total.


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Pr0f
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Re: 74HCxx vs 74LSxx

Postby Pr0f » Wed Mar 07, 2018 11:52 am

The main things to watch for are slew rate, propagaton delay and fanout

The LS devices will drive more devices than the standard HC, but there are a raft of variants. It does depend a little on what you are driving, and what if any are your speed restrictions. There is a Motorola tech arcticle for using the FPM on a 68000 / 68008 as a peripheral chip.

http://www.google.co.uk/url?sa=t&rct=j& ... roV5VKxQM7


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Re: 74HCxx vs 74LSxx

Postby tcat » Wed Mar 07, 2018 5:24 pm

Hi PrOf,

Perfect, the document even mentions some macro definitions, that could help to provide FPU library. 74HCxx are used to drive chip select, would it fail to function with 74LSxx?

CIRCUIT.PNG
FPU-CPU peripheral wiring
CIRCUIT.PNG (8.54 KiB) Viewed 1984 times

Many thanks.

Tomas


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Pr0f
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Re: 74HCxx vs 74LSxx

Postby Pr0f » Wed Mar 07, 2018 10:36 pm

I can't see why LS wouldn't work in that design - all those logic gates are only driving one load each, so LS will easily handle that. Speed should be ok at that clock speed. If you use sockets, it would be easy enough to swap out chips if you needed to.


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Re: 74HCxx vs 74LSxx

Postby Dave » Thu Mar 08, 2018 2:43 am

74HCT would seem to be the easiest fit for this.

Be aware that with the data bus size difference, you’ll probably want to latch half the data bus to present 16 bits to the copro. And the same in reverse. A good look at the Qubide design will show you one way to do it.


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Re: 74HCxx vs 74LSxx

Postby Pr0f » Thu Mar 08, 2018 7:13 am

That's not needed on the 68881/2 as these chips can accept 8 and 16 bit data too, it's just a question of putting the correct signals in place on the chip so it knows how you are talking to it.

It was quite clever of Motorola to allow the FPU to be used as a peripheral as well, as it opens it's use up to the other family members and even other processors.

It made me wonder if it would be possible to come up with some 'graphics' instructions and potentially interface a graphics co processor to the 68K (020 and above) - it's a lot of work though.


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Re: 74HCxx vs 74LSxx

Postby tcat » Mon Aug 12, 2019 10:02 am

Hi,

An old topic, I know. I've had some time and designed a PCB prototype based on the schematics above. As I have no SMD skills, I just stick to standard TTL in DIP sockets. Exception being PLCC68 for the FPU. I tried to make the board as small as possible, and this is best I can do at the moment.

sfp004-pcb.png
DIY FPU unit for STFM - SFP004

Seeking help...
[a] how can I plate through the pads under PLCC68 socket in my DIY board make
[b] how can I fit the board inside STFM machine, the idea was to hook to CPU pins with a ribbon cable (using Floppy straps from an old PC)

P.S. my STFM model has the CPU fitted close to the mainboard edge, it is situated roughly under the keyboard spacebar.

Many thanks
Tom


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Re: 74HCxx vs 74LSxx

Postby Derek_Stewart » Mon Aug 12, 2019 8:08 pm

Hi Tom,

The through plating could done with hollow PCB plating rivets.

I use them to repair double sided PCBs, where the existing through plating has been damaged or pulled out of the PCB.

The copper rivet is inserted into the PCB hole and using a special tool to compress the end onto the PCB and solder the ends of the rivet to the tracks.

See this article on Hackaday:

https://hackaday.com/2015/02/25/diy-through-hole-plating-like-a-boss/

I would guess your PCB would have to connect the the CPU socket.

The Atari STFM CPU socket has not much head room due to the keyboard.

I had an accelerator CPU T32 inserted in place of the 68000. I had to angle the board to the same orientation as the keyboard. By using SIL sockt strip on one side.


Regards,

Derek
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Re: 74HCxx vs 74LSxx

Postby tcat » Tue Aug 13, 2019 8:38 am

Hi Derek,

Ah, `plating rivets', then. Looked for them on the net, they sell in a thousand pack, I have counted I just need 33 holes to plate through, standard 0.8mm drill, these are on the PCB front. The rest of PLCC68 holes are routed on the rear, no plating needed.

Thank you
Tomas


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Re: 74HCxx vs 74LSxx

Postby Derek_Stewart » Tue Aug 13, 2019 8:46 am

tcat wrote:Hi Derek,

Ah, `plating rivets', then. Looked for them on the net, they sell in a thousand pack, I have counted I just need 33 holes to plate through, standard 0.8mm drill, these are on the PCB front. The rest of PLCC68 holes are routed on the rear, no plating needed.

Thank you
Tomas

Hi Thomas,

I have some spares, but I think think they are smaller than you need (0.8mm OD/0.6mm ID). I was going get some larger rivets (1mm/0.8mm)

I got them from Ebay, someone in Greece, who allowed smaller quanities, I will look for the Ebay ID for him.

If you like I can buy 100 and send you some.


Regards,

Derek

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