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True PCIe Bifurcation on the Lenovo Tiny6 (M90q/P340/P350)

· 5 min read
Radu S
Radu S
Second of his name. Rattler of bits and bobs.

Recently, user xstefa12 opened an issue on the NandFarm PowerRiser GitHub, on how to bifurcate the x8 PCIe bus from the CPU.

While the PowerRiser design for the M90q/P340/P350 relies on tapping the unused PCH lanes to provide a plug-and-play NVMe slot without requiring any host board modifications, this alternative project goes straight for the throat: it forces the Intel CPU to physically bifurcate its main x8 link.

Here is a breakdown of how it works, the custom hardware involved, and how you can replicate the hardware mod on your own M90q/P340/P350 motherboard.

How to Force Bifurcation on the Tiny6​

To make this riser functional, the host motherboard must be tricked into splitting its single x8 PCIe link into a dual x4/x4 configuration. Since Lenovo's locked-down enterprise BIOS does not offer a software toggle for this, you have to force the configuration state at the hardware level by altering the CPU's CFG (Configuration) logic.

There are two documented methods to achieve this short-circuit.

Method 1: The CPU Pad Mod (Non-Invasive)​

This is the least invasive method as it doesn't require applying a soldering iron to the Lenovo motherboard. By shorting the appropriate CFG pads directly on the CPU substrate or socket, you can trick the system during the boot sequence.

Here are the most effective ways to bridge those pins without permanently altering your hardware:

  • Copper Tape & Kapton: Cut a microscopic sliver of conductive copper tape and lay it across the two target pads on the bottom of the CPU. To ensure it doesn't shift and short adjacent pads during socket insertion, secure it with a slightly larger patch of heat-resistant Kapton (polyimide) tape.
  • Conductive Silver Paint/Ink: Use a fine-tipped needle to apply a tiny, precise drop of conductive silver paint across the pads. It dries quickly, offers highly reliable continuity, and can be completely wiped away later using high-percentage isopropyl alcohol.
  • The Graphite Pencil Trick (Legacy): Using a soft graphite pencil (2B or softer), draw a heavy line between the two pads. While this is a classic overclocking trick, it is the least reliable method here, as the graphite can rub off or fail to provide low enough resistance for modern logic levels.

Locate the two specific pads on the underside of your Intel CPU as highlighted below, and short them together using your preferred method.

CPU Pad Short

If you prefer a permanent board-level modification, you can adjust the resistor configuration directly on the M90q/P340/P350 motherboard traces. The desired configuration is CFG6 = 0, CFG5 = 0, and CFG2 = 1. Lenovo left the required bifurcation components unpopulated on the motherboard.

Instead of permanently pulling CFG6 to Ground (GND) and locking the motherboard into a permanent bifurcated state, there is a "maximum effect" mod that gives you the flexibility to toggle bifurcation simply by swapping risers.

The Maximum Effect Mod​

By soldering an 0402 resistor (up to ~1kΩ) between pins 2 and 3 of the unpopulated QC4 footprint, you effectively bridge the riser's BMC_DETECT pin directly to the CFG6 pin of the CPU.

QC4 Footprint Mod

  • When using a Bifurcation Riser: Strap the BMC_DETECT pad on the riser to GND.
  • When using a Standard Riser: Leave the BMC_DETECT pad floating.

This allows you to change the CFG6 configuration state simply by changing the riser, offering maximum flexibility with only one initial motherboard modification.

Closest GND Point Closest GND Point

Alternative Solder Points​

If you find the 0402 package size of the QC4 pads too difficult to solder, the community has verified that soldering the 1kΩ resistor to the RC60 point is a viable alternative that fits much better.

RC60 Alternative Solder Point RC60 Alternative Solder Point Zoomed

Pro Tip: In electronics, it is highly recommended to never pull down a pin with a 0-ohm resistor to avoid shorting risks; use an 800Ω to 1kΩ resistor instead.

Future Revisions​

Based on community feedback, a future revision of the bifurcation board will include a physical jumper for the BMC_DETECT pin. This will allow users to safely and easily configure the BMC_DETECT state (and thus toggle the bifurcation) directly on the riser without any micro-soldering required on the expansion card itself.

Final Thoughts​

Once the hardware mod is complete, you can drop the bifurcation card into the proprietary slot. If the middle M.2 slot or the x4 PCIe edge connector fails to detect devices when you boot into the OS, it means your short circuit is incomplete and the CPU is still operating in a monolithic x8 mode.

This piece of hardware hacking provides an excellent option for users who are comfortable modifying CPU pads or micro-soldering to squeeze maximum I/O out of the Tiny6 platform. For everyone else who prefers a strictly plug-and-play solution without touching conductive paint, the standard NandFarm PowerRiser remains the easiest route.