Bus conflict damage is largely a myth, at least for modern ICs. Output drivers are a lot more robust than you think. More likely the extra current will cause a reset or glitch if the power supply circuitry/decoupling can't keep up with the current spike.
Really? So a CMOS driver that's driving a high logic level which is connected to a CMOS driver that's driving a low logic level won't get destroyed by the resultant short circuit?
As far as I understand (and I admit that I might be wrong here) a typical CMOS driver outputs a high logic level by connecting the output pin to Vcc (via a low-resistance FET) and it outputs a low logic level by connecting the output pin to GND (also via a low-resistance FET). And the circuit traces on the bus are also fairly low resistance. Wouldn't this short circuit result in dangerously high currents flowing?
Certainly high enough (>100mA) to violate the device's "absolute maximum ratings", the ones that you aren't supposed to exceed even momentarily?
I'd be very interested to hear more about the robustness of output drivers and the amount of abuse they can tolerate.
You're correct about how the outputs work, but the practical reality is that currents end up limited enough by the FET's on resistance that nothing gets damaged. Yes, it's outside the spec, but exceeding the AMRs doesn't mean your chip dies. I had a 5V microcontroller survive being put across 12V once. And I had a Threadripper motherboard die by shorting out its CPU Vcore FET, which would send the PSU's 12V rail into the CPU, and the CPU survived (PSU shut down before any damage was done).
If shorting a pin to the opposite rail destroyed your IC people would be destroying Arduinos left and right with trivial mistakes while experimenting, and they wouldn't be able to get away with having no I/O protection :-). You usually don't get >100mA out of a single IO line short - maybe 50mA. Having a bunch of paralleled bus contention can cause more damage (not to the drivers, but to power routing and other shared resources), but at that point you should be hitting PSU current protection limits (which are more important for overall design robustness).
Console modchips of olde (PS1/2/GameCube/etc) worked by overdriving bus lines with a stronger driver (often multiple lines ganged together). No consoles were hurt by this.
I did part of the design and board layout for Glasgow revC, an FPGA-based USB interface board, and I stress tested our IO level shifter chips for short circuit robustness. Leaving the outputs shorted hard to the opposite rail overnight did no apparent harm (bypassing the protection resistors), other than getting the chip nice and toasty for the duration of the test.
Edit: just remembered a personal anecdote. I only recall ever killing an output driver with a short, on a typical microcontroller, once in my life (could've been a fluke). And I've done many stupid experiments. Shorting outputs briefly for experimentation or unorthodox workarounds is solidly in the "no big deal" category in my mind, and it's practically never been a problem. E.g. "I don't know which side is TX and RX in this UART, so I'll just try both" "This device is bricked so let me short out the flash to force it to fall back to bootloader mode", etc.
> I stress tested our IO level shifter chips for short circuit robustness. Leaving the outputs shorted hard to the opposite rail overnight did no apparent harm (bypassing the protection resistors), other than getting the chip nice and toasty for the duration of the test.
That's impressive!
Thanks for the reply, sounds like I can be a bit less cautious without fear of blowing things up.
