sorry for my super late reply.
Am 08.11.2015 um 01:15 schrieb Niklas Hauser:
Since the only thing that you seem to be waiting for which is not an
interrupt is a time event you could implement a "time server" with a
queue which you can push wakeup events to. Essentially the time server
just sets the timer to go off at the nearest time point and then pops it
from the queue. Then you can just wait for interrupt at idle and have
all the power savings but can also potentially run at a finer
granularity. I have had success with this even using multiple soft uarts
(which require quite high timing precision).
This project looks great.
We also have an extensive concept for cooperative, stackless multitasking,
based on Dunkels Protothreads , which we expanded on with Resumable
Functions  which are used in all device drivers to make communication
non-blocking and allow multiple device drivers on the same bus to safely access
This form of multithreading is very resource friendly since it does not require
context switches and only the main call stack (literally 2 bytes of RAM per
However, this is quite an advanced topic and there are disadvantages to this
approach compared to a full real-time operating system (power efficiency, hard
real-time), such as mbed os or riot-os, however, they do not necessarily work
well on AVRs.
Could you please expand on the disadvantages to power-efficiency or hard
real-time ? And how bad are those ?
Some background first:
The protothreads are constantly being polled for state changes in their `run()`
method regardless of that being useful or not.
This makes it very simple to implement timer logics, since they do not require
to register callbacks, you simply wait until the timer expired:
xpcc::PeriodicTimer timer(200); // expire every 200ms
PT_BEGIN(); // protothread begins
PT_WAIT_UNTIL(timer.execute()); // yield until true
Led::toggle(); // toggle an led every 200ms
In your main loop you will have to call the `MyAmazingThread::run()`
continously for this to work.
Notice that this equals to an _implicit_ round robin scheduling, ie. you don’t
register with a dynamic scheduler class, but simply add your update function to
the main loop.
Other than that the Protothread behave very similarly to other thread
implementions, with the exception that you cannot have local variables, since
they are based on a stackless coroutine implementation).
This is a very simple multitasking concept and it works very well for most
I especially prefer the thread abstraction over using callbacks, as this can
lead to some very unreadable code.
Since you have to poll the threads even when no state change occurred, the CPU
is alway 100% utilized.
So this naive polling approach is obviously not very power efficient.
For our use this is not really a concern, since our robots consume around
10-15W just doing nothing, so the extra 10mA from the CPU doesn’t really matter
However, theoretically you could implement a sleep function here, by running
though the main loop only when a state change in hardware occurred.
For example, the timers only need to be polled every 1ms, since that is the
resolution of the clock.
So if all threads are waiting for a timer to expire, the CPU could to go sleep
and wake up on the next ms tick.
I agree, "hard real-time operating systems" are a myth in my mind, If
you need hard real-time you need both a high priority interrupt and a
sophisticated method of atomics /critical sections in order to guarantee
that you never turn off interrupts for too long. Disabling interrupts by
escalating the interrupt priority rather than completely disabling them
seemed to work on a proof of concept design for me in the past but the
product was never developed for other reasons so I don't technically
have field experience.
Similar approaches could be taken for peripherals. You only have to poll the
thread waiting on I2C/UART/SPI if a state change actually happened, etc.
I would like to investigate the possibilities of this in the distant future, as
that sounds promising.
Another reason for not being power-efficient is that no DMA support is
Even though we have a DMA HAL for the STM32, the Controller and Stream
selection for every peripheral still needs to be implemented.
This will likely happen before considering a sleep concept.
However, please note, that for true power efficiency, you need a full blown
resource manager, that not only manages CPU sleep but also powers down unused
peripherals (clock gating) and even external peripherals (ie. sensors, external
Such advanced schemes can become very complicated very quickly (there is a ton
of research done on doing this in clever way) and will probably never be part
Regarding hard real-time:
In my experience, you don’t need a hard real-time scheduler, when you can do
your things directly in hardware (using a timer for a motor controller for
example) or by enabling a high-priority interrupt handler and executing from
If you do need actual hard real-time in your program, having a implicit
cooperatively scheduled main loop is not the way to go, since you cannot make
any guarantee on latency whatsoever.
So if you synchronize on a flag set during an interrupt, you will first have to
wait for the main loop to loop around to the thread receiving (“reading”) the
flag and acting on it.
However, the main loop gets called many thousands of times per second, so you
will most likely be totally fine.
So the protothreads are definitely soft real-time (very fast though), but you
can have the hard real-time if you want with interrupts.
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