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Old 19 April 2011, 15:20   #1
hall
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FPU and overclocking advice

I wanted to be able to run some demos that require FPU. I did some reading up on the 68882 and found the general opinion to be that regardless of what is printed on it, they are most likely “all the same”. So I went ahead and got a 25 MHz MC68882RC25A to be fitted in my M1230XA @ 50 MHz.

Fast forward a couple of days: With the FPU fitted and running off of the same 50 MHz crystal as the 030, I'm greeted with the yellow screen upon boot. Oops... Everything working fine with the FPU removed though.

Is this likely because the FPU doesn't want to play at 50 MHz, or is it more likely that the FPU is dead/damaged? Would it be worth the trouble to get some slower crystals to try out?

Any help appreciated
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Old 19 April 2011, 18:20   #2
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Question

According to the M1230XA user manual jumper 1 selects either the CPU clock or a seperate FPU clock which requires a second oscillator to be installed.

You also need to configure the EEPROM with the supplied software to a 68882.

Have you done this correctly?
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Old 20 April 2011, 05:22   #3
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I've played around with many a fpu...

In PGA format:
MC68882RC25A 36mhz
MC68882RC20A 33mhz
XC68882RC40A 55mhz

In plcc format:
MC68882FN33A 60mhz
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Old 21 April 2011, 00:07   #4
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@SpeedGeek
Thanks for your input! Yes, I'm sure I've done everything correctly. The jumper is in the correct position. Come to think of it though, I only configured the EEPROM with 68882 @ 50 MHz. Perhaps I should try a slower clock speed as well, even though the current oscillator still is 50 MHz of course.

@joeksterz
OK, thanks for the info! Are you saying it should play nice at 50 MHz or am I pushing it?
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Old 21 April 2011, 01:06   #5
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@hall
Was this 68882 a "working pull" when you received it? Are you sure it is installed correctly, has no bent pins and seated well in the socket?

I don't know why there are EEPROM settings for different FPU speeds but it's worth a try. You could also disable the fast memory and see if it makes a difference.
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Old 21 April 2011, 16:25   #6
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My fpu with the same markings as yours, tops out at 36mhz. I would try a lower speed first -say 33mhz...
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Old 25 April 2011, 14:14   #7
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Sorry for the late reply guys!

@SpeedGeek
I can of course not be 100% sure, but for what it is worth it was bought as such and from a company rather than a private seller. Cosmetically it looks almost like new and is well seated in the socket. I've tried all your suggestions with the same result. Anyway, thanks for your input - much appreciated!

@joeksterz
Thanks for the info! That is probably it then. I'll get an oscillator below 36 MHz and report back.
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Old 26 April 2011, 00:37   #8
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@hall
If thats that case you must have one of the few MC68882 chips which have a very limited overclocking range. I overclocked two 25 Mhz MC68882's to 50 Mhz. No problems at all!

Last edited by SpeedGeek; 26 April 2011 at 16:45.
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Old 15 May 2011, 23:27   #9
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Update for the sake of completeness:

I have now tried a 33 MHz oscillator, and everything appears to be working just fine. Indeed it seems this particular MC68882 has a limited overclocking range, as pointed out earlier.

Thanks to joeksterz and SpeedGeek for pointing me in the right direction.
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