Teardown: TI-89 Titanium

[NiHaoMike]

I have owned a TI-89 Titanium for a few years now, and one of its main problems is lack of support for recharging its batteries from USB. (And removing the batteries to charge them resets the time/date, quite an annoyance if you use that part.) Another problem is that it's a little slow for graphing.

So now I did a teardown, hopefully to try to solve the first and maybe even the second problem.

Observations:
* There seems to be a special TI ASIC along with a Freescale microcontroller and a 4MB Sharp Flash memory. Not sure why the Freescale chip is there. It only runs at 16MHz, so that explains the slow speed. The clock looks like it's coming from a PLL inside the ASIC, so overclocking would be difficult without breaking features like USB.
* The USB port supports host mode (able to supply power). That, along with the use of 4 AAA batteries, will greatly complicate the process of designing a charger circuit.
* The design makes it very difficult to probe the board and operate the buttons at the same time.

[metalphreak]

48 MHz crystal on the left there (Black KDS Taiwan). Looks like it feeds the main TI chip. This will be the one running the USB interface for sure as the USB PHY needs a stable 48MHz clock source. You can see the traces go directly to it.

The freescale chip might be some doing some kind of co-processing? Product Brief: http://www.freescale.com/files/32bit/doc/prod_brief/MC68SEC000.pdf

[johnmx]

The main processor is the Motorola 68000 32-bit processor.
You can overclock the normal TI-89, but I’m not sure if it’s possible on the TI-89 Titanium.
I did it on my TI-92 calculators:
https://www.eevblog.com/forum/index.php?topic=4293.msg56928#msg56928

[NiHaoMike]

I actually overclocked my TI-83. To do that, I just rubbed a moist finger over the CPU area to find the RC circuit. Then I tried bridging various resistors with a 10k or so to find the exact resistor, then I replaced it with a potentiometer. (I believe there's a second, low frequency oscillator since the blinking cursor did not change speed.) The limit seems to be the LCD controller - the graphics would glitch long before the processor actually crashes.

The TI-89 (and probably the TI-84 as well) appears to be PLL driven, which means the timing is much more accurate, but it's difficult or impossible to overclock. I did manage the USB charging mod though - just a LT1054 in doubler topology with an undersized cap to limit current (to about 30mA) and the shutdown pin tied to the host enable pin so it automatically disables in USB host mode.

But considering that even the TI-89 uses really underwhelming technology, maybe we should try to design an open source graphing calculator? Some of the cheap STM32s run at 168MHz, an order of magnitude faster than the 89. I'm sure we can design a calculator for less than $50 (in parts) that can really outperform the 89.

[alm]

The mechanical design and keyboard are going to be challenging in a DIY design without access to custom molds. And these are about the only thing modern day calculators have going for them compared to say a smart phone with a decent calculator application. A smart phone will have an even faster CPU, and the incremental costs are fairly minor for most people. So unless you come up with a design with an excellent mechanical keyboard (HP calculators before they were outsourced), very sturdy case (how often are high school students dropping those calculators), and a battery that lasts forever, I don't see the point.

[NiHaoMike]

I actually have downloaded some Android calculator apps, but they just don't feel the same without an actual keypad. And none I have seen have the capabilities of an 89, although that really should be quite easy to fix.

What might be interesting would be to use an eink or other static display and have the calculator always be "on". When the user presses a button, the processor would wake up to update the display, then fall back into standby after a short period of inactivity. Some newer eink displays are plenty fast for this application and offer the best contrast for outdoor use. The power usage would likely be low enough that a few small solar cells can keep it charged, with USB charge and sync as a backup.

[Semantics]

I actually overclocked my TI-83. To do that, I just rubbed a moist finger over the CPU area to find the RC circuit. Then I tried bridging various resistors with a 10k or so to find the exact resistor, then I replaced it with a potentiometer.

Could you elaborate on this moist finger technique in general terms? Bolting pots to all sorts of devices for frequency changes sounds like fun.

[NiHaoMike]

The idea is to alter the frequency of that oscillator so you can find it.