S7-vrm

Necroware's S7-VRM

This project is a Voltage Regulator Module for the Socket 7 mainboards as defined by Intel in it's Pentium® Processor Flexible Motherboard Design Guidelines. You can upgrade VRM capable Socket 7 mainboards with this module to be able to use dual-voltage CPUs like Intel Pentium MMX, AMD K6, AMD K6-2 etc.

Disclaimer: this VRM can damage your mainboard, CPU or both. Please use at your own risk.

WARNING: inserting the module the wrong way around would put +12V on all critical lanes. It would destroy the mainboard, the CPU, the memory and all expansion cards. This can't happen on mainboards with VRM socket, because there it is not possible to insert it in reverse. On free standing VRM pin header, it is probably a good idea to cut +12V pin 5 and the unused pin 22 to use them as a key.

Youtube Videos:

• Part 1: https://youtu.be/CMiGVQbMC5U
• Part 2: https://youtu.be/J0NLGfocviU
• Part 3: https://youtu.be/kBPp9EAIC8I
• Part 4: https://youtu.be/XV0b5Tvf5gY

CPU Voltage Type Selection (J1)

There are two kind of socket 7 CPUs, single-voltage and dual-voltage CPUs. Dual-voltage CPUs are designed to use different voltages for the internal core and for the I/O. The I/O voltage is specified at 3.3-3.5V, but the core voltage can be much lower. For the very power efficient AMD K6-III+EE the core voltage is just 1.6V.

From revision 0.6 and higher the S7-VRM supports both types of CPUs, where for the single-voltage CPUs the voltage is completely generated on the module and for dual-voltage CPUs the I/O voltage is generated by a linear voltage regulator on the mainboard, where core voltage is generated on the VRM module.

To make this work you have to set the jumper J1 accordingly. Please read which kind of CPU you have before using this module.

CPU Voltage Selection (SW1)

WARNING: Single voltage CPUs are designed to run at 3.3-3.5V everything below that value could potentially make your system unstable. Everything above that can damage your mainboard or CPU or both. Dual-voltage CPUs usually need core voltages far below 3.0V, AMD K6-2 for example expects only 2.2V core voltage and everything above that can potentially damage the CPU. Always pay attention to the CPU type you use and apply voltages out of spec only if you really know what you are doing.

WARNING: different revisions can have different voltage settings, please always pay attention and look into the proper documentation for the revision you have.

Various voltages can be set using the SW1 switch on the module (0=off, 1=on):

Important remarks

The transistor Q1 with integrated pull-up resistor is optional. It is used for enable/disable signal and is unused on most mainbards.

The PCB is made for a through hole inductor, but if you have only SMD it is also possible to solder that instead. Keep in mind that the inductor has to stand the required current. Also slightly different inductors are allowed, everything between 2µH and 4,7µH should work as well. With the higher inductance you get less current ripple, also voltage ripple can look better, but the maximum possible current will decrease. Playing with different inductors will also influence the compensation network.

Capacitors C7 and C8 are used in so called compensation network and are responsible for DC-DC converter activation and stability. Those parts are very delicate and dependent on the ESR and values of the output capacitors C9-C12, and the inductor L1. If your regulator shows stability issues or not starting at all, try to remove the capacitor C8. If you change any other parts C7, C8 and R1 have to be adapted accordingly.

Bill of materials

The inductor doesn't need to be very exact, anything between 2.5µH and 4.7µH should work, but the sweet point is at around 3.3µH. The inductor can be self made by using a T50 ferrite toroid. For example T50-26 with permeability 75µ wrapped in 10 loops of 1.3 mm coper.

Supported mainboards

This module should run with all mainboards, which provide the VRM module header as specified in Intel Pentium Mainboard Design Guidelines. Most of such boards were based on Intel Triton (i430FX) and VIA Apolo Master (MV series) chipsets, but there were also quite a lot of later boards with newer chipsets (f.e. i430VX), which supported such an external VRM as well.

Most of the mainboards which were equipped with a VRM option, were produced before Intel Pentium MMX, AMD K6 and other dual-voltage CPUs were officially available. However those CPUs have changed the multiplier selection behavior of CPU pins BF0/1. Not only a new pin was introduced for higher multipliers, but also pin BF0 was not pulled up internally anymore as it has been done on single-voltage CPUs before. This ended up in wrong multiplier detection on newer CPUs like Pentium MMX 200, which would be suddenly detected as 166MHz one. This can be fixed by adding a 10k pull-up resistor to the CPU between pin BF0 I/O VCC.

Faster Super Socket 7 CPUs, like AMD K6-2 and newer added another multiplier selection pin BF2, which is not supported on those old mainboards at all. It can be added using another mod, but is actually not necessary. If the CPU multiplier is set to 2x, those newer CPUs would interpret it as 6x and for 66MHz FSB go directly to 400MHz. However to be able to detect the CPU properly you would need to mod the BIOS. This is however optional, the system should work also with outdated BIOS. In such a case the CPU just would not be reported properly.

For the list of tests mainboards see: https://github.com/necroware/s7-vrm/wiki

License

This work is licensed under the Creative Commons Attribution-ShareAlike 4.0 International License.

Links

• Intel Pentium Mainboard Design Guidelines