Pentium 4 mystery gizmos explained

Register readers come up trumps again

In our Pentium 4 picture special yesterday, we wondered aloud about what a collection of small devices attached to the bottom of the chip were.

Click to view image at full size

Michael suggests:

"The gizmos on the bottom appear to be GTL+ 50 ohm terminators.

"Basically all Intel Cpus post Pentium Pro have a bus unit based on an open standard for interfacing called GTL. GTL is an 'open drain' interface which means that there is only 1 transistor that switches on the low side to ground. Thus each pin has to have a pull up to supply in order to switch. These are low valued 50 ohm pullups that also serve to terminate their lines.

"Being an open drain interface allows multiple cpu's to easily share the same bus. Since the P6 family of cpus was designed for multiprocessing this makes total sense. It also has the advantage that the IO voltage can easily scale with the CPU core for lower power consumption. The switching level is set by a resistor divider that sets the threshold to exactly 1/2 of the IO voltage. They typically use 2 50 ohm resistors for this.

"These resistors in the case of SLOT 1 were located on the CPU board. In the case of the Pentium Pro they were located on the motherboard. Why Intel chose to put some of these resistor packs on the bottom side of the chip instead of the motherboard, I am not sure. I suspect it has something to do with the multiprocessing or a higher frequency frontside bus. The original SLOT I could deal with no more than 2 loads on a couple of signals limiting the multiprocessors to 2.

"Note: I don't see all of them there. There are only 12 x 4 or 48 pullups. The bus unit has a lot more pins than that being a 64 bit bus. However, there may be more of these packs on the other side of that little board or maybe they determined that only certain signals were critical."

David from Norway reckons the mysterious gizmos are probably capacitors.

"It is quite common now to use multi-pack capacitors, where (in this example) 4 individual capacitors are combined in one package. They will be used mainly for power supply smoothing to help reduce noise - the number of capacitors used reflects the number of power pins on the CPU. It is more useful to use lots of small capacitors rather than a few big ones of the same combined capacitance, because the smaller capacitors react better to higher frequencies.

"I realise that was a fairly boring and unimaginative answer, rather than something conspirative (sic) like extra circuitry to automatically email Intel with your personal details every time you try to overclock the chip."

John from the US suggests:

"I can't tell for sure from the picture, but those mysterious gizmos under the Pentium 4 look like low inductance decoupling capacitors (AVX Interdigitated Capacitors). They could possibly be terminating resistor arrays for the GTL+ circuits. However, the purple color indicates that the body is made from ceramic, which would most likely be used to make a capacitor. (The resistor arrays are generally black in color).

"I assume the Pentium 4 is flip chip mounted, so placing the capacitors directly under the die, on the pin side of the package, would give an extremely low inductance electrical path to the die for better performance."

Thanks to everyone who wrote in. ®