Clock generator

[mkemp]

The "underclock" thread prompted me to ask: is there an inexpensive thing that could substitute for the clock crystal of a PC, XT or 286 and vary the freq, say from 2 to 30mHz? My thought would be to set it to match the original then nudge it up to the point of instability. Back it off a couple of notches for testing, then install the selected freq crystal.

 

[Chuck(G)]

They existed, back in the 286/386 days. XcelX was one such brand, offering continuously-variable clock frequency. Here's an InfoWorld article on it

You might want to investigate doing your own PLL design. It should be comparatively simple.

 

[james1095]

The "underclock" thread prompted me to ask: is there an inexpensive thing that could substitute for the clock crystal of a PC, XT or 286 and vary the freq, say from 2 to 30mHz? My thought would be to set it to match the original then nudge it up to the point of instability. Back it off a couple of notches for testing, then install the selected freq crystal.

Sure, you could remove the crystal and feed the output of a function generator in there, although units that go higher than a few MHz are rather specialized. There also exist a lot of different PLL chips that will synthesize any frequency you want within a range.

 

[mkemp]

Sure, you could remove the crystal and feed the output of a function generator in there, although units that go higher than a few MHz are rather specialized. There also exist a lot of different PLL chips that will synthesize any frequency you want within a range.

I found this: http://www.electronic-circuits-diagrams.com/oscillatorsimages/oscillatorsckt1.shtml
I'll keep looking.

 

[Chuck(G)]

The problem with simple RC oscillators is that they suffer from jitter and stability. Your 286 has fairly tight restrictions on the quality of the input clock signal. And the NE555 won't get you to the frequencies you need.

depending on the range and granularity of the variation, there are a couple of digital options.

A frequency divider is easiest--just take a high-frequency crystal oscillator (say, 30 MHz) and divide it by varying quotients using either a counter or shift register.

The widest range of frequencies and finest granularity are realized with phase-locked-loop designs. Here's an example. Simpler designs are possible, depending on the number of bells and whistles you'd like.

 

[per]

I just want to note that some PCs and XTs uses only one crystal for the entire system. Those systems should not have it's crystal replaced since that would cause issues with ISA cards as well as other functions like time-of-day, DMA/memory refreshes, the CGA color composite output and the pitch of the beeper.

 

[Chuck(G)]

Those are mostly PC-XT class systems, but even then, there were "turbo" systems that split the peripheral and CPU clocks.