CPU speed problem in Acer Aspire 3627WXMi

A first look

This Acer Aspire 3627WXMi (Model: MS2180) was purchased from Laptops Plus in Sydney, Australia. There has always been a problem with the laptop causing it not to run at its maximal performance. First of all, let’s take a glance at the specifications of this laptop:

• Intel Pentium M 740 (1.73GHz, FSB 533MHz)
• 14.1″ WXGA CrystalBrite TFT LCD
• Kingston 1GB DDR2-533 RAM
• 60GB hard disk
• Dual layer DVD-RW Writer
• Atheros 802.11b/g

The memory was upgraded from the stock 256MB by the dealer. As with any other laptop, there is a white label on the cardboard packaging that outlines the machine type, model, serial number and parts specifications. The interesting thing is, the label says that the CPU should have been a Pentium M 725A (1.6GHz, FSB 400MHz). However, the dealer remarks that the label was incorrent, and should have been the PM 740 instead. I thought, “Nice. Free CPU upgrade!”

1.7GHz CPU running at 1.3GHz

After a few days, the problem emerged. Even though the CPU is rated at 1.73GHz, the maximum speed attainable from this laptop was only 1.3GHz even when on AC power. Puzzled, I started to play around with the power management settings. In Acer ePower Management, CPU speed is already set to maximum, but the computer still runs at 1.3GHz max.

CPU-Z indicated that the system is only running at 100MHz FSB. So when this PM740 runs at its maximum clock multiplier of x13, the resultant speed is 1.3GHz. I calculated that had the FSB been 133MHz, a multiplier of x13 would set the resultant frequency to the nominal 1.73GHz. But what was causing this problem? The only thing that was a custom upgrade was the memory.

Memory problem?

So I inspected the memory. Kingston PC2-4200. DDR2 533. These memory modules are definitely capable of running at an FSB of 133 plus quad pump to 533MHz. CPU-Z reads that the SPD had an entry for running at 533MHz. To make sure that memory wasn’t the cause of this problem, I’ve replaced them with tried and tested DDR-667 Memory from my Lenovo 3000 V100. It did not make any difference.

Pin modding…

My next attempt was to force the chipset to run the CPU with FSB of 133MHz. Using the wonderful guide at Laptop Logic, I tried pin modding, which involves shorting one of the clock selection input pins to ground, forcing the chipset to register this bit as low even if the CPU wants it at logic high. And I started to take the laptop apart.

The CPU and heatsink were exposed. The next step was to remove the CPU and insert the bridging wire.

Overclocking the CPU – or not

Once that was in place, I put the CPU back into its socket and everything else back together. I started up Windows, and to my surprise, it didn’t work. Thinking that it might be a CPU fault, I tried this again, this time using a Pentium M 715 (1.5GHz, FSB 400MHz) from my old BenQ Joybook 5000U.

This time, since I am pin modding to overclock rather than to fix a broken CPU, it should work. Starting Windows up again, it turns out that it is running happily at its native frequency of 1.5GHz at FSB 400MHz. So it didn’t work, and I wanted to find out why.

The answer is in the chipset

I had always known that the laptop ran on the Intel 910/915 chipset, according to CPU-Z. Searching on the internet, it is clear that the Intel 915 is designed for the Pentium M, and is capable of running at an FSB of 533MHz. So why is it that the system will not run a PM 740 at its native FSB, AND will not overclock the PM 715? There can only be one answer: the chipset is incapable of running FSB 400MHz.

But it is an Intel 910/915, so why can’t it? Looking through the photos I took, the following photo had proven my theory and gave the answer.

The laptop’s chipset is, specifically, Intel 910GML. According to the Intel website, the 910GML chipset only supports up to 400MHz FSB. So to the friends out there on the internet who also want to try the pin mod on your Pentium M, rest assured that pin modding on an incompatible chipset won’t kill your processor or main board.

Having a chipset incapable of 533MHz FSB means that it was almost impossible to use the PM 740 CPU at 1.73GHz, since the clock multipliers are locked to a maximum of 13. Perhaps there are ways to unlock the clock multiplier. But I could imagine that much more must be done than just putting a piece of wire between two holes.

Eventually, I decided to “upgrade” the laptop from a Pentium M 1.73GHz (that can only run at 1.3GHz) to a Pentium M 1.5GHz. Since having the bridging wire in the socket was ineffective, it’s better just to remove it before it can cause any damage.

Conclusion: Questionable Acer design

The answer is evident. The processor was operating at a reduced speed because the chipset was incapable of operating at the higher speed. The question that remains is, why was the processor there in the first place?

The label on the packaging clearly says Pentium M 725A 1.6GHz. Why would the actual product differ from what was written? If the laptop dealer had known that the laptop was using the newer, incompatible processor, why would they continue to sell it rather than return it to the manufacturer?

Nonetheless, there is an implication of Acer’s questionable hardware design. Even if the laptop came with the Pentium M 725A (1.6GHz, FSB 400MHz) instead, only the Celeron M processor series are supported on the 910GML. it is understandable that there is a market for low-end chipsets, but why would Acer design the product that uses an unsupported CPU?