PCJr PC-Sprint / PCXT PC-Sprint

Hardware questions and modifications

Re: PCJr PC-Sprint / PCXT PC-Sprint

Postby ZiggyTheHamster » Sun Sep 29, 2019 6:01 pm

Weird. I've got two uPB8284AD chips (good for 24MHz) on order and will see if that helps. Maybe the base RAM is defective and I'm only noticing it at 8MHz? The corruption is infrequent, but some games which do a lot of drawing like the Carmen Sandiego games do this often enough to be annoying.
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Re: PCJr PC-Sprint / PCXT PC-Sprint

Postby GHiero » Mon Jun 21, 2021 12:39 pm

GHiero wrote:Topbench results with the PC-Sprint mod and NEC V20 @ 10MHz :

Score: 8
Memory: 1537
Effective addressing: 1122
Opcode exercise: 926
Vid adapter speed: 1451
3DGame distribution: 855


I did not fully understand the PC Sprint mod when I first got mine. I originally thought that the PC-Sprint module I installed in my PCjr. was clocking the V20 CPU at 10MHz, but when I actually looked at the board, the crystal on it was 22.11MHz, which gives the underlying 8088 or V20 a speed of 7.37MHz. The PC Sprint works by substituting the main Intel 8284 clock generator, which generates almost all clocks in an 8088-class machine, with one devoted solely to handing the CPU's clock. The 8284 divides the clock input by 3 to get the CPU clock. Has anyone had full success at 8MHz or higher on a PCjr (or generally)? I think that floppy drive access may not work fully at speeds higher than 8MHz.
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Re: PCJr PC-Sprint / PCXT PC-Sprint

Postby bagelswitch » Thu Jul 08, 2021 3:03 pm

GHiero wrote:I did not fully understand the PC Sprint mod when I first got mine. I originally thought that the PC-Sprint module I installed in my PCjr. was clocking the V20 CPU at 10MHz, but when I actually looked at the board, the crystal on it was 22.11MHz, which gives the underlying 8088 or V20 a speed of 7.37MHz. The PC Sprint works by substituting the main Intel 8284 clock generator, which generates almost all clocks in an 8088-class machine, with one devoted solely to handing the CPU's clock. The 8284 divides the clock input by 3 to get the CPU clock. Has anyone had full success at 8MHz or higher on a PCjr (or generally)? I think that floppy drive access may not work fully at speeds higher than 8MHz.


bagelswitch wrote:YMMV of course, but with my boards I haven't had any success above 8MHz (using a 16MHz-capable V20).

With any crystal I tried faster than 24MHz, the system will cold boot, and even run topbench, but anything that involves much disk or memory access results in a hang.

w/ a 24MHz crystal, things are rock-solid.


You will need to use the 8284A1 for anything above 8MHz. I have a latching switch and a relay attached to the slow/fast jumper on my boards. Normally I boot at 4.77MHz and then switch to "turbo" speed afterwards with the switch, otherwise the system often hangs somewhere in the boot process, such as the memory test, at higher speeds. When I made my board I had 10 made, so I built them with a variety of different crystals for experimentation - as I mentioned above, I can run topbench at higher speeds (up to 11MHz) but never got anything above 8MHz truly stable (and it isn't necessary for any practical purpose (of mine) anyway).
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