Connecting To The AMD On-Die Thermal Diode

Mod for direct die temp readings – Willy Lorenzo

NOTE: The modification described below will void all warranties for affected devices – neither the author or Overclockers.com will be held liable for any direct or consequent damages of anyone electing to try this modification.

The best way to monitor the temperature of AMD CPUs is to read the on-die thermal diode. This diode is far more accurate than a thermistor on a motherboard, which can be off by 10ºC or more. If your motherboard can’t read the on-die diode, there are systems available from Maxim and Analog Devices that can read the diode, and there are articles on the Internet describing how you can build your own simple diode reader.

But none of these readers will work if you can’t connect to the CPU’s diode pins. The following article presents a method of accessing these pins. It may seem a bit drastic, but it’s within the abilities of the typical computer enthusiast.

Connecting to the CPU diode is a challenge because of size and accessibility. The CPU pins are tiny, hidden in the ZIF socket and are not readily accessible. Most people first think to solder leads to the appropriate pins on the bottom of the motherboard. This sometimes works, but there are two problems with this approach:

The first problem is that, on some boards, one or both of the diode pins may be grounded or connected to some circuitry. On such boards it is impossible to electrically isolate the pins. You might be able to cut the traces to the pins…but not if the trace is on an internal layer.

The second problem is more basic and applies to all motherboards. Even if you solder to the bottom of the motherboard, you still don’t have a soldered connection to the diode pins. The actual CPU pins are being pushed against the ZIF pins by the ZIF clamping system. With no true solder joint, there is a possibility of inaccurate readings due to weak contacts.

The solution to these problems is to electrically isolate the diode pins and then solder leads directly to the pins. To accomplish this I

1. Removed the ZIF socket top plate
2. Made cut-outs for leads and for access to the diode pins
3. Bent the diode pins
4. Soldered leads to the diode pins, and
5. Reassembled the socket

This sounds daunting and, to be sure, it is not for the faint of heart. However, neither special skills nor special tools are required to accomplish this task. You probably already have everything you need.
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Here is a list of tools used:

• X-acto blade # 24
• Bosch metal cutting jigsaw blade mounted in large X-acto handle
• Small flathead screwdriver
• Weller WM120 pencil type soldering iron

As you’ll see below, the #24 blade is used to remove the ZIF socket, but any blade that fits the ZIF socket slits will work. An X-acto #227 serrated blade would’ve been perfect for making the necessary cuts in the socket, but I didn’t have one, so I used a jigsaw blade I found in the toolbox. See…you can make anything work. A fine tipped soldering iron is really the only necessary item on the list.

Removing the ZIF socket top plate is probably the trickiest part of this process. Not all ZIFs are alike, but they should all have two or three thin slits on both sides. If you take a look at the ZIF pictured above, you’ll see it has two slits per side and two larger slots along the lever pivot.

Insert an X-acto blade into the slit and, while gently lifting from the bottom with a screwdriver, lever the side of the ZIF outward. When the internal lip is clear, the ZIF will pop up slightly. Repeat for all four slits, and then use a small screwdriver for the rear slots. A previous socket that I had modified didn’t have these rear slots. You just have to examine your particular socket and determine what’s going on. What you don’t want to do is to start prying around any ol’ place with a screwdriver. The plastic used for the socket is somewhat brittle. It will flex some, but will crack if stressed too much.

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There sure are a lot of holes in that socket, eh? The first thing I do is scribe lines on the plate showing where I’m going to cut.

To find the diode holes, orient the plate as in the picture below. Starting at the topmost row and from the left, count 9 columns. Count only the top row columns…don’t think about the second row. The ninth and tenth columns are the diode columns. The bottom holes of both columns are the holes you’re looking for. You know you got it right when, immediately to the right in the eleventh column, there is no hole.

Once I found the holes, I used a plain X-acto knife to scribe lines outlining the area that will be cut away. In the next picture, it’s hard to discern the area of the diode holes that is being outlined – just look at the picture that shows the finished cuts to get a better idea.

The scribe marks for the leads are much easier to see in the picture. The lead wires will run between the diagonal lines of pins. A perfect cut will cut the holes in half. Once the lines are scribed, I stopped and double-checked to make sure I’ve scribed the correct places.

Turn the processor upside down and test fit the top plate. Use an X-acto knife to clean up any leftover shavings that might prevent the CPU from seating properly.

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Bend the diode pins toward the center of the CPU – a small Phillips head screwdriver works well for this. Try to keep the pins centered in the cuts while bending, and bend till the pins just touch the CPU.

Find red and black wires to use as leads. Use 26 or 24 gauge wire; whichever best matches the thickness of the top plate. I used wire from a broken heatsink fan. Bend the wires as shown, with the wires sitting on top of the diode pins. This is the time to be meticulous.

Once the wires are fitted, remove the top plate and solder the wires in place. Make sure you tin the wires first. A steady hand is helpful and it helps to have something holding the wires in place. After soldering, check your joint with a magnifying glass to insure that the joints are properly wetted.

Once the wires are soldered, test fit the top place once more. You may need to shave some plastic to make room for the solder joints.

Now to put it back together. Lay the top place in place over the ZIF socket…

…and press down evenly to snap into place.

Finally, reinstall the CPU:

You now have two leads that can be soldered to any appropriate sensor device. You want to keep these leads short…long leads can give inaccurate readings. I try to go as short as I possibly can, leaving just enough length to clear whatever cooling device I’m installing.

The red lead connects to pin S7, also known as THDA, the anode(+) end of the thermal diode. The black lead connects to pin U7, also known as THDC, the cathode(-) end of the thermal diode. Depending on your diode reader, the red lead goes to connections with names such as D+ or DXP. The black lead goes to D- or DXN.

Like I said, this mod a bit drastic. However, I think the results are worth it. As you can see, all that is really necessary to complete this mod is the willingness to hack up your ZIF socket and your CPU. Once you get past that, the rest is easy!

Willy – aka “Graystar”