Diving in with Calamity and Confusion

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 01/30/2015 at 07:030 Comments

On my TI-89, one of the battery contacts on the PCB was heavily corroded and it took a good deal of time and heat to reflow solder to make a new pad. I did this to the V200 as a preventative measure too, but I guess I heated up the nearby fuse to where it blew. So I used the one from my '89. That then blew today as well...

The numbers I've run didn't match the numbers and results others have posted so I needed to measure things a bit more closely. While probing pin 6 of my V200's MC68K, the shut off as I just mentioned about. As far as I can tell it's a 500 mA, 1/2 Amp, 6mm x 2.5 mm fuse. I've ordered 3 from mouser and am using a wire to short the fuse's terminnal in the interim...

The V200 is known to run at "12 MHz" and you can find this quoted in numerous places. The RC oscillator produces a frequency of ~12.7 MHz at ~13 dB as I previously measured. And, since I never measured the clock going into the MC68K, well, I though I should do that. Guess what I got? How about 13.7 MHz, which is nearly bang on with 1 / RC for 1.43 kOhm ohm and 51 pF.

Here is what that square wave looks like on pin 6 of the MC68K:

Ignore the hardware counter as the CPU is idling and the signal also looks horrible simply because I somehow missed getting ground springs for this new probe.

Since I cannot find a full datasheet on the MC68SEC000PB16, I don't know how the internals of the clock generator work so I'm somewhat confused as to how you get a 13.7 MHz signal from a 12.7 MHz oscillator.

Also, there is a program I forgot about, tibench, and it's accuracy is claimed to be pretty good, but I do not know if that has been verified by looking at the hardware with a scope. However, running the benchmark gives the output of ~13.59 MHz.

So in other words, software and hardware are close enough to know that I'm not missing anything about the calculators running faster.

With that said, I took a closer look at the RC oscillator's waveform.

Small note is that it idles at 3.3 V and then the pin is pulled low to enable the clock for anywhere from 4 cycles to 100% DC. The V200 clock is ~720 mV pp whereas the TI-89's is ~940 mV pp, but both with similar minimums of ~1.1 V. Running the numbers for the observed ~28.9 ns rise time and ~44.4 ns fall time results in values for the resistor of that RC oscillator that do not match the 1.43 kOhm value stated and measured.

In fact, they are much closer to 3.3 kOhm and 4 kOhm respectively and their difference is ~670 ohm. Effectively we're charging through 1.87 kOhm + 1.43 kOhm and then discharging through 1.87 kOhm + 670 ohm + 1.43 kOhm.

Why am I nit picking? Well, with the V200 clock running a lower peak to peak voltage and the rise and fall times are slower than what the apparent RC pair should generate so if the C9's value is decreased, the rise and fall times obviously lower, but that also means the peak to peak voltage begins to drop as well. Now we can guess how much. Just for some extra leeway, I'm going to adjust R1's value in order to try to bring the 'stock' voltage up to that of the TI-89 and that puts it at ~732 ohms ideally, though 750 ohms is what is available. To be safe though, I'm also choosing 1.02 kOhm and 1.21 kOhm resistors in case R1's values affects the clock value dramatically because something might be wrong with the assumptions. This is indicative to even if I ignore 4 pf of parasitic capacitance, even adding half of 670 ohms drops the RC oscillator frequency to 11.1 MHz, well below 12.7 MHz. Raising the peak to peak voltage level will also increase signal quality if this can be achieved.

Things I'm considering with the overclocking:

Anyhow, as a result I'm re-doing my capacitor selection. Also, since quarters are of no object, i.e. since the difference between $0.14 capacitors and $1 capacitors is less than lunch money, I'm going to splurge even more though I'm going to remain specific. The RC oscillator source had a frequency output about 2.4 dB worse on the V200 than the TI-89. I noted in the mouser search parameters a "High Q" MLCC selection. A higher Q in filtering means that the quality of a resonant circuit is increased and thus I hope they can improve that signal quality regardless of the OC. Finally, with the resistor spread and the near guarantee of being able to get 2x performance at 22 pF w/o an SRAM upgrade, I'm choosing the following 4 capacitor values: 16 pF, 18 pF, 24 pF, 33 pF

Anyhow, the results of the time I've spent on this today are the following

Frequencies by Resistor, Low = +8 pF parasitic, +4 pF parasitic, High = +0 pF parasitic:

Frequency Stepping, Low = +8 pF parasitic, +4 pF parasitic, High = 0 pF parasitic: