I have been very busy lately, developing a complete replacement main board that will fit into the empty shell of a TI-84+. This will enable you to build your completely own, Linux powered, TI-84+
You almost can't differenciate the original from a modded one.
It features an ATmega328P-AU Arduino Uno compatible keyboard processor and power management controller and real time clock (to keep the time running, while the Pi is turned off).
This will be a two board solution. There will be an upper and a lower board.
On the upper board, there is space for fitting a Raspberry Pi Zero (W) and a 128x64 monochrome/grayscale LCD that I would provide. The bottom half of the board (including keypad matrix, RTC, Uno and the Adafruit PowerBoost500 for LiPo operation) will be attachable to the upper board through a few solder links. Therefore you can decide whether you want to provide your own single board computer or if you want to go for a Pi Zero and my preferred, matching LCD.
The headphone/Link jack will either expose the Pi Zero's analog video signal plus one GPIO pin or the I2C bus, depending on solder jumper configuration. The Uno is connected to the Pi via SPI and UART. You should be able to reflash the Uno from the Pi, if you disable the Linux console on the serial port.
Otherwise, the serial console will be displayed on boot in a serial terminal mode of the Uno.
The Uno is also connected to the 128x64 screen and will go into high impendance mode to let the Pi access the screen, once the Pi is booted and ready.
I guess I can already warrant a sneak peak at the keypad matrix
I hope there is demand, and if there is I may offer a kit or the bare PCBs.
If someone does not like to solder SMD, I could also offer individual assembly of the boards.
There is also need for a good name. Currently I have settled for "Pi84--", but feel free to post any suggestions if don't like the name.
From a price perspective, the bottom board with all components (except for the Adafruit PowerBoost 500) may end up costing around 12€. The power boost is available for about 15€ and a matching battery will cost under 10€.
The top board may probably cost around 18€ (not including the Pi Zero (W), but including the screen).
Dead TI-84+'es are often available at your school/uni for free or just a tiny fee, if there is a renting program available. On eBay, used TI-84+'s with a broken screen can be bought for as little as 10€.
So, in total, you will probably be paying about 65€ for your own, Linux enabled, fully hackable, state of the art, calculator.
And of course could you run one of the TI-84+ emulators on there if you wanted to
Note, that this is the actual, project releated topic, while my original post was posted in another topic here.