Introduction

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.

Background

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.

Introduction

ThinkPad X61 is a classical machine, along with T61 they are the last 4:3 ThinkPads made by Lenovo. I bought my X61s used in 2010 (I simply couldn't afford it back in 2007 when it first came out) and keeps using it as my main laptop (I have a much more powerful desktop PC for most of the stuff though). Many of the components have been upgraded since then, 2GB DDR2 is upgraded to 6GB, 160GB spin drive has been replaced by a 512GB SSD, and the TN screen has been replaced with an AFFS (IPS) screen. The thing that never changed is the processor: the 65nm Core 2 Duo L7500.

X61 comes with various CPU options available when buying new:

X61:

• Core 2 Duo T7100, 1.8 GHz, 2 MB L2, FSB 800, 65 nm, 35 W
• Core 2 Duo T7250, 2.0 GHz, 2 MB L2, FSB 800, 65 nm, 35 W
• Core 2 Duo T7300, 2.0 GHz, 4 MB L2, FSB 800, 65 nm, 35 W
• Core 2 Duo T8100, 2.1 GHz, 3 MB L2, FSB 800, 45 nm, 35 W
• Core 2 Duo T8300, 2.4 GHz, 3 MB L2, FSB 800, 45 nm, 35 W
• Core 2 Duo T9300, 2.5 GHz, 6 MB L2, FSB 800, 45 nm, 35 W

X61s:

• Core 2 Duo L7300, 1.4 GHz, 4 MB L2, FSB 800, 65 nm, 17 W
• Core 2 Duo L7500, 1.6 GHz, 4 MB L2, FSB 800, 65 nm, 17 W
• Core 2 Duo L7700, 1.8 GHz, 4 MB L2, FSB 800, 65 nm, 17 W

However, when buying used, especially in 2020, it is really hard to pick one specific processor. Most of the listings online come with a T7100 or T7300 processor, built with a 65 nm process. If the machine is an X61s, then the 65 nm processor is pretty much the only choice.

Unless the processor can be replaced or upgraded.

It is quite common to replace the processor in T61s, just buy a new processor, install that on to the motherboard, done. Or if one wants to use newer FSB 1066 processors or even quad-core processor, BIOS mod and motherboard mod are available to allow the processor to run on the T61 motherboard.

The X61, on the other hand, has the processor soldered down on to the board, making it harder to replace the processor. And it is much harder to buy BGA processors than just normal PGA processors: pretty much no one would take out the BGA processor from their laptop and just sell that, and it is pretty hard to buy new old stock BGA processors. But the good thing is if the CPU can be indeed bought and installed, all the mods available to the T61 are also available to the X61.

This post is a comprehensive note about how to install probably any Core 2 Duo mobile processors onto X61 if the power supply and cooling allow. And my method uses a PGA processor rather than a BGA processor so the processor is still easily purchasable on eBay or other platforms. Quad-core processors run much hotter and require further ACPI mod which is not described in this post.

I understand that these kinds of mod have been done on X61 many times before. But I just want to document comprehensively how I did it and what each step is.

I am also aware of the existence of the X62/X63 motherboard project. This project only focuses on upgrading the original motherboard.

I used P8600 as an example in this post, but other processors should also work.

All information is provided "as is". I will not be responsible for any damage due to the use or misuse of the information provided. Modding BIOS and hardware is risky and likely to permanently brick the machine if not done properly. Do this at your own risk.