Hector Martin

I guess I might as well tell that story because it's so fun. So Glasgow was supposed to be resilient against Vio short circuits. revC0 wasn't. But in the most amazing way. When you shorted an active Vio, the whole thing would crash and power down, and stay there eating current.https://twitter.com/marcan42/status/1341653266816356352 …

Removing the short would instantly make it come back to life. But power cycling it (USB disconnect/connect) with the short intact didn't. However, if you shorted it with Vio off, then power cycled it, it was fine.

If you remove the short while it's dead and powered, it comes back. You can then restore the short and it's fine. Power cycle it, fine. But if you power it down, remove the short, put it back, then power it on again... still dead. It remembers whether it was dead at power-down.

I know what you're thinking, stray voltage on capacitors. Nope. I could power it down while shorted/dead, short/discharge everything, every pin on the board. Power it back on, still dead. 1-bit nonvolatile memory. ⸘‽

Eventually I figured out that our Vio LDOs had memory characteristics. Charge would remain on the pass transistor gate. Turns out, LDOs need power to *turn themselves off*. So if an LDO crowbars its own input rail, then it doesn't matter if you lower the enable signal.