exxos's Atari, Amiga & retro forum

Evening all.

I'm looking at the FPU portion of my board at the moment, and already found one bodgaeable tidbit (I wired A0 when I shouldn't have done -- derp).

So here's the schematic:-


Which I hope represents the 32bit port version of the diagram on the data sheet:-

.


But I have a couple of questions to those more learned in the ways of the floating point unit.

I'm currently supplying the motherboard clock as FPUCLK on the basis of being scared about not having the clock available during CPLD flash. This works well when the processor is also at the same clock (both 16MHz), but fails if the CPU has a faster clock. Logically, I think this is probably expected (DSACKx will still be asserted when the CPU has moved on?). Can anyone confirm that in the event of asynchronous clocks the FPU's must be the faster?

Secondly, this is a 32 bit port -- I have both DSACK[0] and [1] connected, but I only ever see activity on DSACK[1]. This is not what I was expecting -- I was rather expecting both to activate at the same time, but this is *not* an asynchronous bus access, this is a co-processor access, so perhaps it's different. Can anyone confirm what's meant to be going on here?

Thanks in advance!

BW.

So things I've found out so far:-

• The FPU seems to work at 50MHz. Huzzah.
• It works very well when the FPU clock is faster than the CPU clock, but as they start to get closer to each other my open-drain DSACK[x] lines can't recover high fast enough so need to be driven both high and low unless the FPU chip select is active.
• The FPU can be happily speed tested despite no DSACK0 activation.
• It seems pretty much every test run fails -- the wrong results are coming back. This is probably because DSACK0 is not being driven.
• My PLCC68 socket is sponsored by bacofoil.

Take a goosey at those pins!

Hopefully this terrible socket is all the problem is -- I'll try to sort out those pins later, but otherwise might need a new socket.

@exxos, I seem to recall you had problems with PLCC pins not quite fitting some sockets with your ROM chips. Do you recall which make was OK? RS do Preci-Dip and ASSMANN (gotta love that). I bought MultiComp from Farnell last time (avoid those).

BW

Regarding DSACK0 not being asserted, I suppose you read this part of the FPU User Manual?


What this is saying: If the FPU is configured for a 32 bit bus and the CPU accesses one of the FPU registers for which A4=1 (i.e, Operand, Register Select, Instruction Address, or Operand Address), then DSACK0 is asserted.

czietz wrote: 30 Sep 2021 17:48 Regarding DSACK0 not being asserted, I suppose you read this part of the FPU User Manual?

dsack.PNG

What this is saying: If the FPU is configured for a 32 bit bus and the CPU accesses one of the FPU registers for which A4=1 (i.e, Operand, Register Select, Instruction Address, or Operand Address), then DSACK0 is asserted.

Hi Christain,

D'oh. Yes: sorry.

I had read back during the board design then completely blanked it since! I can trigger on ( A1 & ~FPUCS ) to check output.

I think I know the answer already, alas: My FPU seems fried. It won't pass the tests when in the stock Falcon configuration now & anything genuinely FPU-intense crashes. I didn't see a single DSACK0 assertion (which could be the socket, or it could be the FPU). It's time for a new one.

Which solves another problem: perusing Exxos' store for a new FPU, I realise he also sells through-hole PLCC68 sockets. So I'll have some of those whilst I'm at it. :-)

Thanks, @czietz.

BW

Badwolf wrote: 30 Sep 2021 15:01 @exxos, I seem to recall you had problems with PLCC pins not quite fitting some sockets with your ROM chips. Do you recall which make was OK? RS do Preci-Dip and ASSMANN (gotta love that). I bought MultiComp from Farnell last time (avoid those).

Its rather complicated...

https://www.exxosforum.co.uk/forum/viewt ... ?f=23&t=74

TL;DR
Just use 3M sockets and just don't bother with any other fullstop .I've stopped buying other brands of sockets. The ones in my store used to be cheaper ones, but I've been putting 3M ones in my store for a while now anyway.

Multicomp isn't a brand either. Its "multiple companies" . Really a Argos outlet of crap :lol: not saying its all bad. I wouldn't have a problem buying multicomp 0805 stuff for example.

exxos wrote: 01 Oct 2021 01:12 Just use 3M sockets and just don't bother with any other fullstop .I've stopped buying other brands of sockets. The ones in my store used to be cheaper ones, but I've been putting 3M ones in my store for a while now anyway.

Thanks Exxos. Good rule of thumb. I hadn't spotted you had PLCC68 sockets on your store until last night, else that'd have solved that problem at the outset!

Mulicomp isn't a brand either. Its "multiple companies" .

Oh. Hahaha! I'm such a noob. :lol:

BW