There's knowing the time, which you can do with something like NTP. That the RPI can manage just fine.
And there's acting with precise timing, eg, if you need to control a mechanism and reliably react on a deadline of a few ms. A RPI doesn't perform well there, which is why 3D printers use microcontrollers instead.
Access to most of the hardware and real-time deterministic behavior. It’s a really great project and lets you twiddle those gpio pins at ridiculous speeds with perfect timing (less than a millisecond).
A PI comes with a whole bunch of great hardware baked in, so if you have one laying around, and want to do some microcontroller stuff, I think it’s a great choice.
It's still an A-profile MPU and not an R- or M-profile MCU, and while it will be fast it will have less deterministic behaviour than we might like. If you disable the caches and MMU you'll get better consistency. But wouldn't we expect ~microsecond accuracy from a properly-configured MCU?; ~millisecond accuracy is not a particularly high bar.
You can read pins (well one) with sub-microsecond latency using the Fast Interrupt Request, but I have not tried this myself. I think a PI would be more than capable of matching most microcontrollers just due to its very fast clock speed. Add multiple cores with the PI4 and you get a crazy amount of compute between each pulse as well.
There are a bunch of clocks that run plenty fast to enable high resolution timing as well.
The high clock speed and multiple cores are great. It's definitely a beefy system. But this is completely orthogonal to timing accuracy and consistency. Speed does not make it more consistent. Tiny low power MCUs have much more accurate and consistent timing.
Low latency can be a good thing, but it's also not related to consistency, particularly when you start looking at what the worst-case scenario can be.
A few ms? In my experience that seems well within the capabilities of Linux. I guess last time I measured wasn't on a Raspberry Pi. I'm kinda tempted to take a shot at profiling this and writing up a blog post since it seems like a useful topic, although it will probably be a few months until I can get around to it.
I think milliseconds overestimates by a few orders of magnitude, but non-real-time OSs really suffer for the intermediate IO stuff you expect in an embedded project (e.g. SPI, I2C, etc)
A long time ago, I was playing with Project Nerves on an Orange Pi running some flavor of debian. I was doing some I2C transaction (at 400 kHz, each bit is single-digit microseconds), and I ultimately had to have a re-attempt loop because the transaction would fail so often. I found a failure cutoff of 5 attempts was sufficient to keep going. I don't recall the failure rate, but basically, whenever a transaction failed, I'd have to reattempt 2-3 times before it eventually succeeded.
Meanwhile, on a bog-standard Arduino with an ATMega328P, I send the I2C traffic once, and unless the circuit is physically damaged, the transaction will succeed.
No, the consistency of the timing is terrible on Linux.
Seriously, stick a scope or logic analyser on e.g. an I2C line and look at the timing consistency. Even on specialised kernels for realtime use, you can have variable timing delays between each transaction on the bus. And this is all in-kernel stuff that's inconsistent--it looks like it's getting pre-empted during a single I2C_RDWR transaction between receipt of one response and sending of the next message. The actual transmission timing under control of the hardware peripheral is really tight, but the inter-transmission delays are all over the place. Compare it with an MCU where the timing is consistent and accurate, and it's night and day.
> control a mechanism and reliably react on a deadline of a few ms
I actually did measure this with an oscilloscope on embedded Linux (not a raspberry pi). A PPS signal was fed into Linux, and in response to the interrupt Linux sent a tune command to a radio. Tuning the radio itself had some unknown latency.
End-to-end, including the unknown latency of tuning the radio, I never observed a latency that would even round to 1 ms. That's unpatched and untuned Linux, no PREEMPT_RT. I didn't dig any further because it met our definition of "reliable" and was well, well within our timing budget.
I'll be the first to admit it wasn't some kind of rigorous test, just a casual characterization. I would not suggest anyone use Linux for a pacemaker, airplane flight controller, etc.
This is making me itch to buy an oscilloscope and run some more thorough tests. I'd like to see how PREEMPT_RT, loading, etc changes things.
My profiling was on an NXP i.MX8 MPU, which is a A-profile quad core SOC very similar to an RPi. I think it was with a PREEMPT_RT kernel, but I can't guarantee that, but I was fairly shocked at the lack of consistency in I2C timing when doing fairly trivial tasks (e.g. a readout of an EEPROM in a single I2C_RDWR request). You wouldn't see this when doing the equivalent on an M-profile MCU with a bare metal application or an RTOS.
What is acceptable does of course depend upon the requirements of your application, and for many applications Linux is perfectly acceptable. However, for stricter requirements Linux can be a completely inappropriate choice, as can A-profile cores. They are not designed or intended for this type of use.
Profiling this stuff is a really interesting challenge, particularly statistical analysis of all of the collected data to compare different systems or scenarios. I've seen some really interesting behaviours on Linux when it comes to the worst-case timings, and they can occasionally be shockingly bad.
I was referring to that yes, even if Linux performs well in the ideal case, it's not necessarily reliable, and the possible problems are hard to compensate for.
Eg, your process can randomly get stuck because something in the background is checking for updates and IO is being much slower than usual, or the system ran out of RAM and everything got bogged down by swap.
On a microcontroller you just don't have anything else running, so those risks don't exist. Eg, a 3D printer controls a MOSFET to enable/disable the heaters. The system can overheat and actually catch on fire if something makes the software get bogged down badly enough. On a Linux system there's a whole bunch of stuff that can go wrong, most of which is completely outside the software you actually wanted to run.
Being within the capabilities of something and guaranteeing that it will never exceed that are two different things. At least in the past real time guarantees for Linux came as part of an optional patch set for the kernel since guaranteeing that an algorithm would complete within a set time frame or that things like priority inversion issues would be handled correctly came with a performance cost.
I think we are talking about things like interrupt latency, not NTP synchronization.
MCU interrupt latency can be extremely deterministic. I ran some measurements for work and found Linux to be adequate for many uses, but it is a valid concern. There are some Linux kernel patches like PREEMPT_RT that attempt to bound Linux latencies, but generally MCUs are a lot better suited if latency is critical. In part because they just have less software running on them to interfere with timing.
That's not the kind of timing the original point was talking about AFAICT. Real time response is the issue with regular Linux, not vaguely accurate wall time.