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

You have no guarantees about latency to userland without real time operating system. You're at the mercy of the kernel and it's slice time and preemption rules. Which could be higher than your maximum 100us.

In order for a workstation to respond to a hardware event you have to use interrupts and a device driver. Your options are limited to interfaces that offer an IRQ:

• Hardware serial/parallel port.
• PCI
• Some interface bridge on PCI.

Or. If you're into abusing IO, the soundcard.

USB is not one of them, it has a 1kHz polling rate. Maybe Thunderbolt does, but I'm not sure about that.

Answer 2

USB Raw HID with hacked 8KHz poll rate (125us poll interval) combined with Teensy 3.2 (or above). Mouse overclockers have achieved 8KHz poll rate with low USB jitter, and Teensy 3.2 (Arduino clone) is able to do 8KHz poll rate with a slightly modified USB FTDI driver on the PC side.

Barring this, and you need even better, you're now looking at PCI-Express parallel ports, to do lower-latency signalling via digital pins directly to pins on the parallel port. They must be true parallel ports, and not through a USB layer. DOS apps on gigahertz-level PCs were tested to get sub-1us ability (1.4Ghz Pentium IV) with parallel port pin signalling, but if you write a virtual device driver, you can probably get sub-100us within Windows.

Use raised priority & critical sections out of the wazoo, preferably a non-garbage-collected language, minimum background apps, and essentially consume 100% of a CPU core on your critical loop, and you can definitely reliably achieve <100us. Not 100% of the time, but certainly in the territory of five-nines (and probably even far better than that). If you can tolerate such aberrations.

Answer 3

For info, I just ran some tests on a Windows 10 PC fitted with two dedicated PCIe parallel port cards.

Sending TTL (square wave) pulses out using Python code (actually using Psychopy Builder and Psychopy coder) the 2 channel osciloscope showed very consistant offsets between the two pulses of 4us to 8us.

This was when the python code was run at 'above normal' priority. When run at normal priority it was mostly the same apart from a very occassional 30us gap, presumably when task switching took place)

Answer 4

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.

Answer 5

To answer the question, there are two low latency methods:

1. Serial or parallel port. It is possible to get latency down to the millisecond scale, although your performance may vary depending on manufacturer. One good brand is Brainboxes, although their cards can cost over $100!

2. Write your own driver. It should be possible to achieve latencies on the order of a few hundred micro seconds, although obviously the kernel can interrupt your process mid-way if it needs to serve something with a higher priority. This how a lot of scientific equipment actually works. (and a lot of the people telling you that a PC can't be made to work on short deadlines are wrong).

Answer 6

Ethernet

Look for a board that has a gigabit ethernet port directly connected to the microcontroller, and connect it to the PC directly with a crossover cable.