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PC XT speed upgrade

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marcoguy

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I just purchased an NEC V20 10 MHz CPU and was wondering how to upgrade the mobo's speed to 8 or 10 MHz. I've seen it done, I just don't know how.
 
Well, the cheap and dirty way on a 5160 would be to feed a 3xclock signal in at pin 14 on the 8284, then raise pin 13 high. The 14MHz signal still appears on the bus for CGA use, but the rest of the machine runs at the new speed. 8MHz probably wouldn't hurt anything, but your time-of-day clock would run about 60 percent faster.

Some of the early "turbo" XT clones did this.

Here's another possible twist. The 8088 requires a clock with a 33% duty cycle, but the V20 doesn't care. I'm curious what would happen if a V20 were used and the PCLK output (pin 2) on the 8284 were used to feed the CLK (pin 8 ) signal instead.

That would give you a clock of 7.1MHz without upsetting the other peripheral timings.

Worth a try?
 
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Chuck, I'm a little confused. The data sheet for 8284 says that pin 2 pclk is "peripheral clock. This is a TTL level peripheral clock signal whose output frequency is 1/2 that of CLK and has a 50% duty cycle." I think you were thinking that it was 1/2 that of the oscillator frequency. Or am I missing something here?
 
Chuck(G) said:
Well, the cheap and dirty way on a 5160 would be to feed a 3xclock signal in at pin 14 on the 8284, then raise pin 13 high. The 14MHz signal still appears on the bus for CGA use, but the rest of the machine runs at the new speed. 8MHz probably wouldn't hurt anything, but your time-of-day clock would run about 60 percent faster.

Some of the early "turbo" XT clones did this.

Here's another possible twist. The 8088 requires a clock with a 33% duty cycle, but the V20 doesn't care. I'm curious what would happen if a V20 were used and the PCLK output (pin 2) on the 8284 were used to feed the CLK (pin 8 ) signal instead.

That would give you a clock of 7.1MHz without upsetting the other peripheral timings.

Worth a try?
Click to expand...

PCLK output actually gives input_frequency/6 (instead of input_frequency/2). And if I remember correctly PCLK is divided by 2 and fed to the 8254 timer.

Increasing CPU frequency in a simple way that Chuck described will also disturb the DMA controller. Normally it is rated for 5 MHz and it won't work at 8 MHz.

Anyway the simple way with a tweak that supposedly fixed DMA speed issue described here:
http://www.uncreativelabs.net/textfiles/hardware/8MEG.TXT

Another possibility is installing a special board instead of 8284, such as described here (look for PC-Bandit):
http://www.atarimagazines.com/compute/issue87/News_and_Products.php

Somewhere I had a schematics (or a PCB layout) of a similar board. I believe it used two 8284 ICs - one switchable between 4.77 MHz and 8 MHz for generating CPU clock, another for generating other system clocks (DMA, timer).
 
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You're right, PCLK is CLK/3 or OSC/6. Scratch that one.

Fortunately, you can get 8MHz and 10MHz 8237 chips now (Samsung KS82C37A, Intersil 82C37 (up to 12.5MHz), NEC µPD71037...), so the DMA isn't quite the issue it once was.

I don't like the F/C mod that's described because it also kicks up the PCLK speed and that means your timers run faster--your "beeps" sound too high and your clock runs fast (although that can be dealt with in software).

You could take the OSC output of the 8284, which is always the 14MHz crystal oscillator, regardless of the F/C pin and develop the peripheral clock from that so that your 8254 runs at the right speed. I suspect (but am not certain) that's what some of the 8MHz/10MHz (and yes, it's an honest 8MHz and 10MHz) "Turbo" boards do.
 
In the AT the 8-bit cards run with a 750ns period for compatibility with XT cards - unless ZWS is asserted (can reduce to 375ns). I'm wondering if the "turbo-XTs" simply cranked up the ISA speed to that level (or more) regardless?
 
Chuck(G) said:
Well, the cheap and dirty way on a 5160 would be to feed a 3xclock signal in at pin 14 on the 8284, then raise pin 13 high. The 14MHz signal still appears on the bus for CGA use, but the rest of the machine runs at the new speed. 8MHz probably wouldn't hurt anything, but your time-of-day clock would run about 60 percent faster.

Some of the early "turbo" XT clones did this.

Here's another possible twist. The 8088 requires a clock with a 33% duty cycle, but the V20 doesn't care. I'm curious what would happen if a V20 were used and the PCLK output (pin 2) on the 8284 were used to feed the CLK (pin 8 ) signal instead.

That would give you a clock of 7.1MHz without upsetting the other peripheral timings.

Worth a try?
Click to expand...

The Tandy 1000SX/EX/HX use an 8088 and in their fast modes (7.16MHz) the duty cycle is 50%, whereas in the slow modes (4.77MHz) its 33%. As it works just fine, I assume that the 8088, or at least the 8088-2 and more advanced parts are tolerant to 50% duty cycles. The machines use custom logic, but I wouldn't be surprised if that is almost exactly what they did.
 
Thank you all for the responses. I do have an original IBM 5160 clocked at 4.77 MHz and a 10MHz V20. I don't expect to run at 10MHz without some components failing. I just want to use a little more of the V20's power. Also, where is the 8284 located on the mobo?
 
marcoguy said:
Thank you all for the responses. I do have an original IBM 5160 clocked at 4.77 MHz and a 10MHz V20. I don't expect to run at 10MHz without some components failing. I just want to use a little more of the V20's power. Also, where is the 8284 located on the mobo?
Click to expand...
On a 5160, the 8284 is chip U1, located between the CPU and slot 8.
 
In your other thread you mentioned a Mach 20 - I thought that replaced your main CPU with a 286?
Were you considering not using it?
 
marcoguy said:
I don't have the CPU connector. I currently use it for HD FD and EMS.
Click to expand...
If you're missing the ribbon cable with the connector that goes into the CPU socket, why don't you just make your own? Jameco has the parts that you need - I ordered some awhile back to fix one of the cables for my Sota 286 accelerator that had several broken pins.

My $0.02 - you've already got the accelerator.... you might as well use it! Save the v20 for another machine, or for a different upgrade.
 
Not sure weither you would care or not, but thought of something this morning that I thought I'd mention just so you're aware of it.

Some games rely on the 4.77Mhz CPU speed because they were written when there was only one CPU speed - this is why you find a turbo button on faster XT's - so you can run at the original clock speed. Games like Calinfornia Games also use this set speed to perform certain timing tricks which allow special effects - like 6 colour CGA.

I find my XT too slow as well, but for programs that are too slow I usually just use a faster machine (like a 5170 or clone 386). But if you still want to upgrade, then the 286 option using your card would be the best. It wont be as fast as a 5170 AT, but it'll be faster than the V20 and let you run a few more programs you couldn't before.
 
lol awww well just have to buy another :p
It's all good, just wanted to make sure you were aware, if you don't use those particular titles it probably wont effect you.
 
Well, I can always switch back to the V20 if I do end up with one of those programs. I have VGA in the system, so the CGA graphics thingy shouldn't be a problem. Really, I don't game much. I program BASIC and that's about it until I get an HD for my 'mediator' p3 system to get files to the 5160.
 
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