Worst case is prior to the chip being powered up, or while the rails are being sequentially raised. In that case, internal power can be a low as zero, and ESD diodes on each pin can conduct at 0.7 Volts. You damage individual pins or can avalanche the whole chip if you can drive enough current. --- Graham
== On Saturday, December 16, 2017 at 3:08:43 PM UTC-6, Stuart Longland wrote: > > On 17/12/17 00:34, Graham wrote: > > An FTDI 3.3 Volt USB to UART0 connection will not blowup or hurt > anything. > > You an hook it up and leave it hooked up, and not have to worry about > > damaging the Pocket Beagle, > > A 10K pullup to +3.3 V is also OK, but not actually required for the > > FTDI cable to work. > > > > Something like the TTL-232R-3V3-WE for wire pigtails. > > > > The damage warning is for things that will try to drive pins with > > currents high enough to damage semiconductor circuits and/or ESD > > protection circuits. > > Or, in the case of the boot pins, something that will over-ride a 100K > > resistor. > > Ahh okay… yeah I used UART here as it's an example of something that > would be generally wired up continuously and has the property of > presenting a voltage when idle using push-pull logic. I'd likely use > the MAX232 for a console port. > > By the sounds of things, the problem is more down to inrush current than > voltage alone, and clearly the AM3358 doesn't have its pins in > high-impedance mode at boot-up or else it'd be impossible to achieve > those current levels. 3v3 in series with a megohm or more is never > going to produce more than 3µ3A, and GPIOs in "input" mode often have a > resistance in that ballpark. A GPIO driven as a low output however, > could get nasty. > > The plan was to have a series resistor, so this should be sufficient to > prevent catastrophe. > -- > Stuart Longland (aka Redhatter, VK4MSL) > > I haven't lost my mind... > ...it's backed up on a tape somewhere. > -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to beagleboard+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/beagleboard/0d7c8b93-fe86-4ba1-993a-fc137424f498%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
