Somewhere someone knows something I do not. It was more than a decade ago that overclocking these calculators was common knowledge. Half of the sources have disappeared over time and the other half tell you nothing more than the instructions and the maximum speed the calculator will run at and why. The 6 overclocks I've done support this information, i.e. 22 pF w/ 1.43 kOhm resistor being the fastest OC and sometime unstable with stock SRAM, but I'm still at a loss to quantify the circuity of the clock generator which is why I wanted to do this mod in particular.
- This isn't a simple, bare bones RC oscillator
- The frequency of the RC pair is slower than the output clock for the MCU/CPU
- I'm still scratching my head over this detail
- Maybe its the capacitive reactance of my probe?
- But its a 6.5 pF tip. With the ground lead, it's higher capacitance, no?
- The frequency was the same with just a ground spring though
- This isn't a phase-shift RC oscillator
- The resistor is in parallel to the ASIC and the capacitor
- After a few hours of looking over schematics, I cannot find a reasonable account for the fundamental design of this clock generator
- Plotting the data I have, albeit 6 samples with frequencies sensitive to temperature, shows that a natural log function fits the data with the x-axis being expected frequency and y-axis being the resulting frequency
- Once the oscillator's behavior can be characterized, a function can be generated to plot predictable frequencies.
Photo of the V200 with the resistor and capacitor removed
Maybe I should poke around a bit more with my scope. I got similar frequencies on more than one other pins on the ASIC. TICT's documents on TI's 68K calculators is nice, but it's hardware is written from a software perspective and gives little insight into the functionality of the hardware. Anyhow, I'd like to make a correction of a spec I overlooked: the NOR FLASH can run at 10 MHz, not the previously stated 5 MHz.
This plainly leaves the SRAM as the limiting hardware if the MC68K MCU can safely be run at faster speeds. based on the numbers I saw, I'd estimate that the ASIC would have to produce a ~60 MHz clock for the MC68K in order to clock the NOR FLASH that high. Running at 16 MHz chip at 3.75x its rated speed. However, with the SRAM having timings pushed right up to it's minimum spec, I'd have to upgrade it once more. Something more along the lines of Alliance's AS7C31024B-20TCN seems to be necessary and we're stating to see some power hungry components come into play.
Using the limited dataset of 4 OC's with the 4 caps, a logistical regression suggests that a 30 MHz speed would require a 1/RC frequency of ~80 MHz:
Methinks though that this isn't true based on the limited dataset...
The LCD screen already dims a touch when the CPU is fully engaged. There are 2 SOT-223-3 IC's on the PCB labeled "DRN.3" that are right next to the ASIC that appear to be 3.3 V LDOs and you can see one of them in that photo above, U14. There is another, though, right next to the battery contacts labeled "KFP.3" that also appears to be a 3.3V LDO. LDOs are great for low power, low noise uses, but their efficiency tanks when current draw becomes too much higher.
The screen also gets very dark after soldering around the area indicating some contrast control setup by either of these LDOs methinks. I need a better DMM to make some measurements. I do know that the quad op amp, U15, is applying the LCD bias with voltages up to ~14 V and after soldering the new SRAM, particularly U7, the LCD was also fairly dark for a spell.
I have some LCD polarizer around and I'll be swapping that out to see if I can improve it's contrast ratio before digging into the circuitry. I do have one other sample I wish to purchase to see if it'd help too.
Anyhow, this calculator's functionality is currently more significant than my desire to poke at it with an oscilloscope and DMM so back together it goes.