A project log for Voyage 200 Upgrades

Following up with nearly a decade old thread about overclocking TI's V200 with a bit of a twist

michael-obrienMichael O'Brien 01/23/2015 at 09:250 Comments

For some reason the 47 pF value for the caps is sitting in my mind. Either after 'calibration' the LC meter is so far off that it read both my TI-89's and my V200's caps 51 pf, both ~8% high, or both of them are within their 10% tolerance and reading 8% high, or the LC meter is right. I have a bunch of uF caps, but no spare pF caps so I'll defer to Occam's razor and say that both caps are 51 pf.

With that said, I know the proposed speeds my HW1 TI-89 and my V200 are supposed to be running at: 10 MHz and 12 MHz respectively. I was happy to see sleeping occurring on both calculators, but experience has shown that this was expected. I loaded up a quick looping "benchmark" to be able to pull the clock on full time and went at it with my scope. First up, my TI-89:

-12.8 dB for the frequency output in case anyone was wondering. This was computed with a Hanning windowing.

Next up is my V200:

About a 2.4 dB loss in the signal quality which is perceptable by the fuzziness of the sample in the first screen shot compared to the 89's.

Anyhow, since I've not worked with RC oscillators yet, a quick google search revealed the relatively simple formula of 1/(RC) to determine the frequency, though that gives rise to a small problem.

The RC pair on the TI-89 HW 1 is a 1.12 Kohm resistor and a 51 pF cap. This yeilds a frequency of ~17.5 MHz. Ummm, yeah, that isn't anywhere near 10.5 MHz so there must be some stray capacitance in the board or silicon, et al. Well, in order to bring the frequency down to ~10.5 MHz, we have to add in a series capacitance of ~34 pF and that's no small chunk of the original capacitor. More on this after we move over to the V200, the project child.

The V200's RC pair is made of a 1.43 Kohm resistor and a similar, though one size smaller, 51 pF cap. Plugging these in we get an oscillating frequency of ~13.7 MHz. Not bad, but still a bit off. Given that there is 1 hardware generation and about 5 years between them, it's good to see that this stray capacitance now works out to ~4 pF, an order of magnitude better.

Anyhow, with that said, if I were to OC my TI-89, I'd need much larger jumps in order to create the same performance. And at the same time, the stray 4 pF of capacitance in the V200 wasn't taken into account on my original estimated frequencies:

Aside from the Flash ROM being a dead end for upgrading it, the details regarding the ASIC's operating frequencies are also unknown.This means that either of these components can limit the max speed I may be able to reliably achieve with this endeavour, but I won't let that stop me. That 24 pF cap might just be a sweet spot that I can hit and maybe one of the other two will be a surprise.