Yeah, I wrote these two in reverse order because one idea is easy to follow through one, even though it was not the original cause behind all of this. Despite it's slow AMS, the V200 is very dependable and I use it from unit conversions and calculations all of the way to the occasional derivative or integral. That said, I spend a lot of my free time awake in the not so bright time of day. Though night is more peaceful, it is considerably darker. As such, I've wanted to see how difficult it would be to make a custom backlight for my V200 so I took it apart.
As try as I might, the SOT-223 LDO that powers the whole calculator is one that I cannot identify. As such, I have to be careful with the extra power I'm drawing and make sure I don't affect the other IC's on the board. Because I couldn't spec it and from more than a decade of use, I know how this calculator responds when it is browning out. Adding an additional load to the LDO will greatly decrease it's efficiency and drain the 4 AAA batteries oh so much faster.
Now, as much as I'd like to talk about the backlight setup, that modification runs a much greater risk of damaging that calculator that will force me to buy a new one due to breaking one of 2 ribbon cables or snapping the glass of the LCD. I've not begun planning for this process yet, so I'll touch on the more immediate current draw problem I have: the overclocking. Reason stands that the SRAM speed is the limit to the overclock. Despite the additional current draw of the ASIC and MCU from the OC, the SRAM needs to be faster, which could mean more current. As such, I've established a limit of such that the 20 mA max current draw for the current SRAM chips I plan on upgrading is something I cannot overshoot, over better yet, improve upon.
After plugging in the details, I have 2 options for upgrades:
A "16 mA", 45 ns variant of the original, i.e. CY62128EV30LL-45ZXI
or a "2.5 mA", 45 ns ISSI chip, the IS62WV1288DBLL45TLI
Their pinouts and high/low enable pins are the same. Looking through the datasheets, the Cypress SRAM has an operating current of 11-16 mA and the ISSI one is 8-15 mA. The supply currents top out at 1.3-2 mA and 2.5-3 mA respectively. And finally the standby currents both top out at 4 uA. Both of these chips beat out the original with ratings of 20 mA for operating current and 12 uA for standby.
There is spec I'm happy to have noted as well, input and output capacitance. The original Cypress chip is rated for 6 pF and 8 pF respectively. The ISSI chip matches this spec, but the newer Cypress chip does not with a rating of 10 pF for both. When looking at the timing table for both the ISSI is a bit better in this regard as well, which makes complete sense.
Yeah, it's a bit of effort to find the right RAM when many have OC'd their calculator without upgrading it, but I rather have this upgrade be reliable and just as dependable. That and if I choose not to OC the calculator, the already 3-6 month lifetime of 4 NiMH AAA's should grow a bit more, no?