Which FPU?

[MrZammler]

I'm looking for a 68882 FPU to install on the Blizzard IV 030@50Mhz card. Question: Does the FPU need to run at the same speed as the CPU? Will a PGA e.g. 33 Mhz FPU work?

Thanks.

[x56h34]

Yes, the FPU runs at 50Mhz on B1230-IV cards.
You need a 68882 PGA FPU rated at 50Mhz. Vesalia used to sell (perhaps still do) 40Mhz 68882 FPUs that were tested OK at 50Mhz. Perhaps you could give them a try.

EDIT: I'm not sure about the 33Mhz PGA 68882. That would be a large overclock. B1230-IV is hot as it is, and this would probably make it even hotter and probably the FPU would die pretty quickly.

[Lemonty]

I have the same system as you and I found the 68882@40 to be unreliable. It would work fine for maybe fifteen minutes and fail after that. Heat produced by the 68030 is the most likeable culprit. I ran it without the trapdoor lid of the 1200 but it still got too hot. I guess with the proper cooling or toweredized Amiga you are okay though.

[BoingBoss]

Hi MrZammler,

Does the FPU need to run at the same speed as the CPU?

YES!!!!  NEVER have the FPU running slower then the CPU.  It is ok to have a FASTER FPU but never slower.

Will a PGA e.g. 33 Mhz FPU work?

Only with a 33MHz or slower processor.  If you use it with a 50 MHz processor you WILL have problems.   :roll:

[Piru]

@MrZammler

Does the FPU need to run at the same speed as the CPU?

No, it doesn't. You can run FPU slower frequency than CPU.

However, with Blizzard 1230-IV you can't adjust the frequency of the FPU independent, so you need to use 50MHz part.

Blizzard 1230-III was able to run FPU on separate clock, so you could use 25, 33, 40 MHz 6888x. (However IIRC, Blizzard 1330-III manual suggests you don't run FPU at lower speed than CPU).

Will a PGA e.g. 33 Mhz FPU work?

In general, it would, if the card would be able to clock the FPU separately. But 1230-IV can't, so you need to use 50MHz part.

[BoingBoss]

Hi Piru,

YOU ARE WRONG !!!!

Do your homework, buddy.   :-)

[bloodline]

BoingBoss wrote:
Hi Piru,

YOU ARE WRONG !!!!

Do your homework, buddy.   :-)

Doomy, please don't show off your ignorance. Read what Piru wrote and understand what he was saying.

[Karlos]

Vaguely on topic...

Never really thought about it before, but on reflection, how was the coprocessor communication handled on 68020/68030+FPU ? The different speeds of CPU/FPU must require some sort of asynchronous/waitstate design, I guess...

/me goes and finds old 68K manuals

[Piru]

@DoomMaster

YOU ARE WRONG !!!!

Do your homework

Read MC68882 Technical Summary (BR509), page 5, section "BUS INTERFACE UNIT", last paragraph.

Reproduced here, for your convenience:

Since the bus is asynchronous, the MC68882 need not run at the same clock speed as the main processor. Total system performance may therefore be customized. For a given CPU performance requirement, the floating-point performance can be selected to meet particular price/performance specification, running the MC68882 at slower (or faster) clock speeds than the MPU clock.

(emphasis mine)

[Karlos]

@Piru

Asynchronous it is then :-)