Electronic – Why does PCIe hotplug capability require hardware support

hardwarepcie

As far as I understand, hot-plugging a PCIe connection requires both hardware and software support.

Software-wise, it makes sense, since the OS has to re-scan the bus, and by default it doesn't do this after the initial scan.

Hardware-wise, I do imagine it would require special circuitry in order to send an interrupt to the CPU, so that the OS is informed about a new device being connected to the bus.

However, what I do not understand, is why there is a risk of burning/damaging components if I just plug-in a PCIe device while the computer is running.

  • How does electrical current flow differently from when the PC is turned on after the insertion?

Best Answer

There are chips that require power up to occur in particular sequences most chips require power to be present before that data lines are driven. You must consider what happens when ground makes contact late, for example a high speed LVDS pair (common mode voltage ~1V and abs max maybe 2.5V) connects before the ground makes contact and after the 3.3V (or worse the 12V) rail has connected.... If not thought about the answer is often smoke.

Hot plug is a pain to get right even if you are designing for it from scratch, and often involves extra logic and power switching hardware which no commodity box designer wants to include.

Then there is the ringing and bounce as contacts make and break during insertion, this is a pain in all ways, and there is a reason things like the industrial versions of PCI have connectors with three or four different length pins to ensure correct sequencing.

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