What is the lowest latency communication method between a computer and a microcontroller?

Question

I have a project in which I need to have the lowest latency possible (in the 1-100 microseconds range at best) for a communication between a computer (Windows + Linux + MacOSX)

Answer 1

In short, PCs aren't set up to handle that short of deadline. Even using a bare metal RTOS on an Intel Core series processor you end up with interrupt latency (how fast the processor can respond to interrupts) in the 2-3 µS range. (see http://www.intel.com/content/dam/www/public/us/en/documents/white-papers/industrial-solutions-real-time-performance-white-paper.pdf)

That's ignoring any sort of communication link like USB or ethernet (or other) that requires packetizing data, handshaking, buffering to avoid data loss, etc.

USB stacks are going to have latency, regardless of how fast the link is, because of buffering to avoid data loss. Same with ethernet. Really, any modern stack driver on a full blown OS isn't going to be capable of low latency because of what else is going on in the system and the need for robustness in the protocols.

If you have deadlines that are in the single digit of microseconds (or even in the millisecond range), you really need to do your real time processing on a microcontroller and have the slower control loop/visualization be handled by the host.