Getting started with the core language

Connecting to the core device

As a very first step, we will turn on a LED on the core device. Create a file led.py containing the following:

from artiq.experiment import *


class LED(EnvExperiment):
    def build(self):
        self.setattr_device("core")
        self.setattr_device("led")

    @kernel
    def run(self):
        self.core.reset()
        self.led.on()

The central part of our code is our LED class, which derives from artiq.language.environment.EnvExperiment. Among other features, EnvExperiment calls our build() method and provides the setattr_device() method that interfaces to the device database to create the appropriate device drivers and make those drivers accessible as self.core and self.led. The kernel() decorator (@kernel) tells the system that the run() method must be compiled for and executed on the core device (instead of being interpreted and executed as regular Python code on the host). The decorator uses self.core internally, which is why we request the core device using setattr_device() like any other.

Copy the file device_db.py (containing the device database) from the examples/master folder of ARTIQ into the same directory as led.py (alternatively, you can use the --device-db option of artiq_run). You will probably want to set the IP address of the core device in device_db.py so that the computer can connect to it (it is the host parameter of the comm entry). See The device database for more information. The example device database is designed for the nist_clock hardware adapter on the KC705; see FPGA board ports for RTIO channel assignments if you need to adapt the device database to a different hardware platform.

Note

To obtain the examples, you can find where the ARTIQ package is installed on your machine with:

python3 -c "import artiq; print(artiq.__path__[0])"

Run your code using artiq_run, which is part of the ARTIQ front-end tools:

$ artiq_run led.py

The process should terminate quietly and the LED of the device should turn on. Congratulations! You have a basic ARTIQ system up and running.

Host/core device interaction (RPC)

A method or function running on the core device (which we call a “kernel”) may communicate with the host by calling non-kernel functions that may accept parameters and may return a value. The “remote procedure call” (RPC) mechanisms handle automatically the communication between the host and the device of which function to call, with which parameters, and what the returned value is.

Modify the code as follows:

def input_led_state() -> TBool:
    return input("Enter desired LED state: ") == "1"

class LED(EnvExperiment):
    def build(self):
        self.setattr_device("core")
        self.setattr_device("led")

    @kernel
    def run(self):
        self.core.reset()
        s = input_led_state()
        self.core.break_realtime()
        if s:
            self.led.on()
        else:
            self.led.off()

You can then turn the LED off and on by entering 0 or 1 at the prompt that appears:

$ artiq_run led.py
Enter desired LED state: 1
$ artiq_run led.py
Enter desired LED state: 0

What happens is the ARTIQ compiler notices that the input_led_state() function does not have a @kernel decorator (kernel()) and thus must be executed on the host. When the core device calls it, it sends a request to the host to execute it. The host displays the prompt, collects user input, and sends the result back to the core device, which sets the LED state accordingly.

RPC functions must always return a value of the same type. When they return a value that is not None, the compiler should be informed in advance of the type of the value, which is what the -> TBool annotation is for.

Without the break_realtime() call, the RTIO events emitted by self.led.on() or self.led.off() would be scheduled at a fixed and very short delay after entering run(). These events would fail because the RPC to input_led_state() can take an arbitrary amount of time and therefore the deadline for submission of RTIO events would have long passed when self.led.on() or self.led.off() are called. The break_realtime() call is necessary to waive the real-time requirements of the LED state change. It advances the timeline far enough to ensure that events can meet the submission deadline.

Real-time Input/Output (RTIO)

The point of running code on the core device is the ability to meet demanding real-time constraints. In particular, the core device can respond to an incoming stimulus or the result of a measurement with a low and predictable latency. We will see how to use inputs later; first, we must familiarize ourselves with how time is managed in kernels.

Create a new file rtio.py containing the following:

from artiq.experiment import *


class Tutorial(EnvExperiment):
    def build(self):
        self.setattr_device("core")
        self.setattr_device("ttl0")

    @kernel
    def run(self):
        self.core.reset()
        self.ttl0.output()
        for i in range(1000000):
            delay(2*us)
            self.ttl0.pulse(2*us)

In its build() method, the experiment obtains the core device and a TTL device called ttl0 as defined in the device database. In ARTIQ, TTL is used roughly synonymous with “a single generic digital signal” and does not refer to a specific signaling standard or voltage/current levels.

When run(), the experiment first ensures that ttl0 is in output mode and actively driving the device it is connected to. Bidirectional TTL channels (i.e. TTLInOut) are in input (high impedance) mode by default, output-only TTL channels (TTLOut) are always in output mode. There are no input-only TTL channels.

The experiment then drives one million 2 µs long pulses separated by 2 µs each. Connect an oscilloscope or logic analyzer to TTL0 and run artiq_run.py rtio.py. Notice that the generated signal’s period is precisely 4 µs, and that it has a duty cycle of precisely 50%. This is not what you would expect if the delay and the pulse were implemented with register-based general purpose input output (GPIO) that is CPU-controlled. The signal’s period would depend on CPU speed, and overhead from the loop, memory management, function calls, etc, all of which are hard to predict and variable. Any asymmetry in the overhead would manifest itself in a distorted and variable duty cycle.

Instead, inside the core device, output timing is generated by the gateware and the CPU only programs switching commands with certain timestamps that the CPU computes.

This guarantees precise timing as long as the CPU can keep generating timestamps that are increasing fast enough. In case it fails to do that (and attempts to program an event with a timestamp smaller than the current RTIO clock timestamp), a RTIOUnderflow exception is raised. The kernel causing it may catch it (using a regular try... except... construct), or it will be propagated to the host.

Try reducing the period of the generated waveform until the CPU cannot keep up with the generation of switching events and the underflow exception is raised. Then try catching it:

from artiq.experiment import *


def print_underflow():
    print("RTIO underflow occured")

class Tutorial(EnvExperiment):
    def build(self):
        self.setattr_device("core")
        self.setattr_device("ttl0")

    @kernel
    def run(self):
        self.core.reset()
        try:
            for i in range(1000000):
                self.ttl0.pulse(...)
                delay(...)
        except RTIOUnderflow:
            print_underflow()

Parallel and sequential blocks

It is often necessary that several pulses overlap one another. This can be expressed through the use of with parallel constructs, in which the events generated by the individual statements are executed at the same time. The duration of the parallel block is the duration of its longest statement.

Try the following code and observe the generated pulses on a 2-channel oscilloscope or logic analyzer:

for i in range(1000000):
    with parallel:
        self.ttl0.pulse(2*us)
        self.ttl1.pulse(4*us)
    delay(4*us)

ARTIQ can implement with parallel blocks without having to resort to any of the typical parallel processing approaches. It simply remembers the position on the timeline when entering the parallel block and then seeks back to that position after submitting the events generated by each statement. In other words, the statements in the parallel block are actually executed sequentially, only the RTIO events generated by them are scheduled to be executed in parallel. Note that if a statement takes a lot of CPU time to execute (this different from the events scheduled by a statement taking a long time), it may cause a subsequent statement to miss the deadline for timely submission of its events. This then causes a RTIOUnderflow exception to be raised.

Within a parallel block, some statements can be made sequential again using a with sequential construct. Observe the pulses generated by this code:

for i in range(1000000):
    with parallel:
        with sequential:
            self.ttl0.pulse(2*us)
            delay(1*us)
            self.ttl0.pulse(1*us)
        self.ttl1.pulse(4*us)
    delay(4*us)

RTIO analyzer

The core device records the real-time I/O waveforms into a circular buffer. It is possible to dump any Python object so that it appears alongside the waveforms using the rtio_log function, which accepts a channel name (i.e. a log target) as the first argument:

from artiq.experiment import *


class Tutorial(EnvExperiment):
    def build(self):
        self.setattr_device("core")
        self.setattr_device("ttl0")

    @kernel
    def run(self):
        self.core.reset()
        for i in range(100):
            self.ttl0.pulse(...)
            rtio_log("ttl0", "i", i)
            delay(...)

Afterwards, the recorded data can be extracted and written to a VCD file using artiq_coreanalyzer -w rtio.vcd (see: Core device RTIO analyzer tool). VCD files can be viewed using third-party tools such as GtkWave.

Direct Memory Access (DMA)

DMA allows you to store fixed sequences of pulses in system memory, and have the DMA core in the FPGA play them back at high speed. Pulse sequences that are too fast for the CPU (i.e. would cause RTIO underflows) can still be generated using DMA. The only modification of the sequence that the DMA core supports is shifting it in time (so it can be played back at any position of the timeline), everything else is fixed at the time of recording the sequence.

Try this:

from artiq.experiment import *


class DMAPulses(EnvExperiment):
    def build(self):
        self.setattr_device("core")
        self.setattr_device("core_dma")
        self.setattr_device("ttl0")

    @kernel
    def record(self):
        with self.core_dma.record("pulses"):
            # all RTIO operations now go to the "pulses"
            # DMA buffer, instead of being executed immediately.
            for i in range(50):
                self.ttl0.pulse(100*ns)
                delay(100*ns)

    @kernel
    def run(self):
        self.core.reset()
        self.record()
        # prefetch the address of the DMA buffer
        # for faster playback trigger
        pulses_handle = self.core_dma.get_handle("pulses")
        self.core.break_realtime()
        while True:
            # execute RTIO operations in the DMA buffer
            # each playback advances the timeline by 50*(100+100) ns
            self.core_dma.playback_handle(pulses_handle)