8284A in PC-Sprint to clock an NEC V20 @10MHZ?

Hardware questions and modifications

8284A in PC-Sprint to clock an NEC V20 @10MHZ?

Postby Eric » Fri Jan 10, 2014 9:24 pm

This is something I've been wondering for awhile…

You know how you can clock a CPU in the PCjr @8Mhz by piggybacking an additional D8284 timer IC to the existing IC and adding a dedicated 24Mhz crystal, right?
(Edit: I'm talking about PC-Sprint. You all kbow what that is, but somehow I forgot to mention it. Late night, I guess… :P)



The installed D8284 supports up to 8mhz, but its variant of the D8284A, can go up to 10Mhz. Is it possible to get a 10Mhz 8088 or V20 running at full speed if you replaced the timer ICs with a D8284A and used a 30MHZ crystal?

If it is possible, I'm almost sure this configuration would lead to instability if it works with the assumption that no syncing issues or some crap like incompatibility would result, which leads me to my next question…

Would replacing the system's RAM with faster RAM (and possibly S-RAM) address the instabilities this may cause?

Please enlighten me! I'd love to know if anyone has tried this and what the results were, if it won't work and the reasons why or if anyone has ever even tried it at all.

I may have overlooked something, but if I didn't, I'm still a noob and feel I need to be encouraged or discouraged before diving into a project that involves finding, buying and potentially wasting vintage chips and components. :p

/Discuss
Last edited by Eric on Tue Jan 14, 2014 12:17 am, edited 3 times in total.
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Re: D8284A in a PCjr to clock an NEC V20 @10MHZ?

Postby Brutman » Sat Jan 11, 2014 1:16 pm

The original modification you are referring to has been done. The original version is called PC Sprint. Quite a few years ago Matt C did a PCjr specific version. Matt - are you listening?

My memory is fuzzy but this worked by using one clock and having the two different 8284s operate at a multiple of that frequency. The original 8284 has to stay at the original clock frequency because the entire rest of the system (CGA, video, serial, etc.) requires it. The second 8284 drives the CPU at 2x.

I think that if you stayed at an even multiplier (1, 2, 3, 4x) it would work. I don't think you can have a non-integer multiplier. That would require the second 8284 to be running out of phase with the first one. Hilarity will ensue.

The other issue is diminishing returns. You can stuff a Core 2 in the CPU socket and clock it at 2 Ghz if you want to. But the system memory and the rest of the system still operate at 4.77Mhz. (Actually the Core 2 example is bad because the caches on it are so large, but you get the idea.) The NEC V20 is a better chip to do this with than the 8088 because it has a larger instruction prefetch buffer, so it can go a little longer between memory accesses for instructions than the 8088 can.

Would any of the real EEs care to correct me? I'm just a humble software engineer ...
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Postby Eric » Sat Jan 11, 2014 4:20 pm

I guess the short version of the question would be

Can 10Mhz be acheieved on a V20-10 by using a 30MHZ crystal with a 8284A instead of a 8284 with PC-Sprint on a JR?


EDIT: I did intend to mention PC-Sprint in the original thread, but posting late at night without proofreading really screwed with the clarity. Oh well, I'll just edit the post.
Last edited by Eric on Tue Jan 14, 2014 12:18 am, edited 1 time in total.
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Re: D8284A in a PCjr to clock an NEC V20 @10MHZ?

Postby Eric » Mon Jan 13, 2014 4:28 pm

I just remembered that the JrExcellerator achieves 9.45mhz with a V20.

I'm not sure of the black magic on that board, but I'm thinking I can do the same if I do the PC-Sprint with a 10MHZ V20, a 8284A piggybacked on the D8284 and a 30mhz crystal soldered to the 8284A and achieve 9.45Mhz.

I really think I might give this a try.
Should I or is there some black magic involved with the JrExcellerator that goes beyond this?


--
BTW, 've edited the title of this thread to clarify my intentions.
Last edited by Eric on Tue Jan 14, 2014 12:19 am, edited 1 time in total.
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Re: D8284A in PC-Sprint to clock an NEC V20 @10MHZ?

Postby Brutman » Mon Jan 13, 2014 10:05 pm

4.77 * 2 = 9.54 ...

It's just clock doubled. Did the original PC Sprint require the crystal to be changed?
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Postby Eric » Mon Jan 13, 2014 11:51 pm

The clock generator does require a crystal that is three times of what you want to set the cpu to. The switch allows you to swap between stock speds and the speed you've assigned with the crystal.

PC-Sprint usually uses a 24mhz crystal, a 8284 and an 8MHZ CPU. The problem is that the 8284 only goes up to 8MHZ at the maximum, but the 8284A can do 10mhz.





I did try to use a 30mhz crystal on the stock 8284 with PC-Sprint, but it didn't work out. It just drove the cpu at the stock frequency and I figured it was due to the 8284 being limited to 8mhz like it says in the datasheet.

According to the datasheet, the 8284A does 10MHZ. (I'm not sure it it does 5, which will be a problem if it doesn't)
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Re: 8284A in PC-Sprint to clock an NEC V20 @10MHZ?

Postby Eric » Tue Jan 14, 2014 12:36 am

Just did a little digging with google and I guess this post on VC confirms that you can achieve 10mhz with a 8284A and a 30Mhz crystal.
It also says it can lead to instability.

It's unclear if you can switch the speed back to 4.77, though. Anyone know if it can?
If it can, I'd definitely give this a try.

I also wouldn't mind replacing the RAM in the Jr with some faster ram to improve stability… if that would even work…
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Re: 8284A in PC-Sprint to clock an NEC V20 @10MHZ?

Postby Eric » Fri Jan 17, 2014 11:39 pm

Now about the other thing I was wondering about, but don't want to make a thread for:

Has anyone replaced the internal RAM with faster RAM or even S-RAM? Would you notice a difference with more intense software and games?


I'm not really looking to expand the amount of internal RAM, just replace it with faster RAM and use the JrIDE and its awesome meg of SRAM to get the system to 736k.
Last edited by Eric on Sat Jan 18, 2014 1:37 pm, edited 1 time in total.
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Re: 8284A in PC-Sprint to clock an NEC V20 @10MHZ?

Postby alanh » Sat Jan 18, 2014 12:38 am

I'm about to do that tomorrow. I'll let you know how it turns out.
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