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Overclocking a TI-92 Plus Print E-mail
Jul 05, 2012 at 01:06 AM

Well, this is a nice one. I really enjoy to use my Texas Instruments TI-92 Plus, it helps me with nearly any of my numerical calculations, it's just great for that. But it it's also pretty old and sometimes I have to wait a few more moments more than I expected to be necessary for this result. So I googled a bit, as I knew it was probably possible to overclock this thing a bit, but I only found out I have a very strange revision like a TI92-Plus Mark II version before they finally released the Voyage 200. All the hints and pictures didn't really apply to my model, obviously the component side of the PCB seemed to have had some redesigns. Not good, at least that's what came to my mind first, I didn't want to go search the clock timing capacitor myself, for at least two reasons.
First, I couldn't get the damn thing running instantely with the case off. I always had to mount the enclosure at least with a few screws to get it to boot, which unfortunely blocked access to the interesting side of the PCB. It could be some some useless kind of tamper detection, but I didn't really care.
Second, I don't really trust my oscilloscope probes too much as I did some stress testing on a RS485 bus networked system I built for my father and the probes dropped out quite often, felt like dodgy contacts.
Then, just out of curiosity, I had a look at the hints for the Voyage 200. Et voilĂ , the 0603 component arrangement close to the TI ASIC looked very familiar. So they must have changed this before doing the bigger jump to the Voyage 200, my luck. Well, it wasn't absolutely identical, but for me it was close enough, even the component designators remained the same. This is a picture of the relevant capacitor C9:

Since I don't have a possibility to measure capacitance this low good enough, I am still not 100 percent sure this is the right one :) The original C9 ceramic capacitor is said to be a 47pF type. The highest speed modifications I found used a 8pF, which would round about double the speed of your system.

My first experiments with two 20pF ceramic caps in series (this should make the total capacitance eqal to 10pF) made the boot process abort with an address error. In my case, most likely due to self-induced timing issues, it was simply running too fast for this particular hardware. So I altered my speed bump down to one 20pF cap, unfortunately, I had no 0603 20pF here, so I had to use a larger (0805) one. I just took one cap off and botched down the second one from the 10pF series triangle. This may admittingly be the worst solder work I have done in the last weeks, I just wanted to finish the thing, there was still some serious work to do and i needed my calculator for this.

For now, I believe this thing is faster now, the menues feel quite snappier and it still works a treat. Yet, I haven't measured or done anything that would really prove it's clocking higher now, but anyway, I know myself too well. I won't let that solder joint on the 0805 capacitor that ugly, especially when I frequently rely on a few of the calculations done with this tool.

So I guess there could be a follow up switching back to the original timing cap and doing a longer calculation for reference. Then I could change to a smaller cap (this time more with care) and - if it still works - do the reference calculation again to see whether there was a measureable increase in speed 8)

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