In case you don't know, nspire CX II is a calculator made by Texas Instruments released in 2018. It has an ARM926EJ-s processor running at 396MHz, 64MB of LPDDR memory, and 128MB of SPI nand flash. Though this is probably already an overkill for a calculator, but, can we get even faster on this calculator? Of course, it would really mean anything. I don't need that additional performance, overclocking a calculator is just like overclocking anything else, it is just purely for fun. If I do need more performance, I would go and pick up a laptop. Let's get started.
To overclock any modern processor, there are two key parts: clock synthesizer, memory controller. The first one boosts up the clock frequency, and the second controls the memory timing to make sure the memory runs at higher frequency (if you are unable to decouple the memory frequency with core frequency). The TI nspire CX II uses a custom SoC which I don't have any documents. The operating system on the nspire CX II is also locked down to disallow any unsigned binary code execution. So to overclock it, there are three steps, 1. Get the "root priviledge" on the nspire OS, 2. Understand how clock generation works on nspire, 3. Write a program to overclock it.
Luckily, the first part has been done by the community. There is a tool called Ndless which exploits some vulnerabilities in the nspire OS to allow unsigned code execution. The version released in September 2019 introduced initial support for nspire CX II. Huge thanks to the ndless team who made this all possible.
The nspire SoC has an internal clock synthesizer (PLL). Since there is no datasheet or manual for the SoC, I have to reverse engineer it to find out how it sets the clock. Luckily, there is a community developed emulator for the nspire CX II, also based on reverse engineering efforts obviously. (Note: CX II suppoort was developed by the same guy who implemented CXII supported for ndless, Vogtinato.)