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Sleepy sketches

Keeping the microcontroller asleep as much as possible is a key goal for a sensor system, so it makes sense to organise the entire software process around that.

The standard Arduino software model is, well, standard: programs ("sketches") are structured in terms of a setup() function that runs once when the system restarts and a loop() function that is run repeatedly. This suggests that the system spends its time running, which possibly isn't all that desirable: a sensor system typically tries to stay in a low-power mode as much as possible. The easiest way to do this is to provide a programming framework that handles the sleeping, and where the active bits of the program are scheduled automatically.

There are at least two ways to do this. The simplest is a library that lets loop() sleep, either directly or indirectly. This is good for simple programs and not so good for more complicated ones, as it means that loop() encapsulates all the program's logic in a single block. A more modern and compositional approach is to let program fragments request when they want to run somehow, and have a scheduler handle the sleeping, waking up, and execution of those fragments. That lets (for example) one fragment decide at run-time to schedule another

If we adopt this approach,we have to worry about the fact that one fragment might lock-out another. A desktop system might use threads; this is more problematic for a microcontroller, but an alternative is to force all fragments to only execute for a finite amount of time, so that the scheduler always gets control back. This might lead to a fragment not running when it asked (if other fragments were still running), but if we assume that the system spends most of its time asleep anyway, there will be plenty of catch-up time. Doing this results in an actor system where the fragments are actors that are scheduled from an actor queue.

Turning this into code, we get the SleepySketch library: a library for building Arduino sketches that spend most of their time sleeping.

SleepySketch design

There are a few wrinkles that need to be taken care of for running on a resource-constrained system. Firstly, the number of actors available is fixed at start-up (defaulting to 10), so that we can balance RAM usage.(With only 2k to play with, we need to be careful). Secondly, we use a class to manage the sleeping functionality in different ways: a BusySleeper that uses the normal delay() function (a busy loop) with no power-saving functions, a HeavySleeper that uses the watchdog timer to shut the system down as far as possible, and possibly some other intermediate strategies. Actors are provided by sub-classing the Actor class and providing a behaviour. We also allow pre- and post-behaviour actions to define families of actors, for example sensor observers. We separate the code for an actor from its scheduling.

The standard library uses singleton classes quite a lot, so for example the Serial object represents the USB connection from an Arduino to its host computer and is the target for all methods. We use the same approach and define a singleton, Sleepy

The program structure then loops something like this. If we assume that we've defined an actor class PingActor, then we can do the following:

void setup() {
   Serial.begin(9600);
   Sleepy.begin(new HeavySleeper());

   Sleepy.scheduleIn(new PingActor("Ping!"), 10000);
}

void loop() {
   Sleepy.loop();
}

The setup() code initialises the serial port and the sleepy sketch using a HeavySleeper, and then schedules an actor to run in 10000ms. The loop() code runs the actors while there are actors remaining to schedule. If the PingActor instance just prints its message, then there will be no further actors to execute and the program will end; alternatively the actor could schedule further actors to be run later, and the sketch will pick them up. The sketch will remain asleep for as long as possible (probably for over 9s between start-up and the first ping), allowing for some fairly significant power saving.

This is a first design, now just about working. It's still not as easy as it could be, however, and needs some testing to make sure that the power savings do actually materialise.

Understanding Arduino sleep modes: the watchdog timer

The Arduino has several sleep modes that can be used to reduce power consumption. The most useful for sensor networks is probably the one that uses the watchdog timer.

[mathjax]

Powering-down the Arduino makes a lot of sense for a sensor network: it saves battery power allowing the system to survive for longer. Deciding when to power the system down is another story, but in this post we'll concentrate on documenting the mechanics of the process. The details are necessarily messy and low-level. (I've been greatly helped in writing this post by the data sheet for the Atmel ATmega328P microcontroller that's used in the Arduino Uno, as well as by a series of blog posts by Donal Morrissey that also deal with other sleep modes for the Atmel.)

Header files and general information

To use the watchdog timer, a sketch needs to include three header files:
#include <avr/power.h>
#include <avr/wdt.h>

These provide definitions for various functions and variables needed to control the watchdog timer and manage some of the other power functions.

Power modes

A power (or sleep) mode is a setting for the microcontroller that allows it to use less power in exchange of disabling some of its functions. Since a microcontroller is, to all intents and purposes, a small computer on a chip, it has a lot of sub-systems that may not be needed all the time. A power mode lets you shut these unneeded sub-systems down. The result saves power but reduces functionality.

Power modes are pretty coarse control mechanisms, and can shut down more than you intend. If your project is basically software-driven, with the Arduino making all the decisions, then a "deep" power-saving mode is ideal; on the other hand, if you rely on hardware-based signals at all, a "deep" sleep will probably ignore your hardware and the Arduino may never wake up.

The watchdog timer is used to manage the "power-down" mode, the deepest sleep mode with the biggest power savings.

Watchdog timer

The Arduino's watchdog timer is a countdown timer that's driven by its own oscillator on the microcontroller. It's designed to run even when all the other circuitry is powered down, meaning that the microcontroller is drawing as little power as possible without actually being turned off completely.

Why "watchdog" timer? The basic function of a watchdog timer is to "bite" after a certain period, where "biting" means raising an interrupt, re-setting the system, or both. A typical use of a watchdog is to make a system more robust to software failures. Since the watchdog is handled by the microcontroller's hardware, independent of any program being run, it will still bite even if the software gets stuck in an infinite loop (for example). Some designers set the watchdog ahead of complex operations, so that if the operation fails, the system will reset in a short amount of time and end up back in a known-good configuration. At the end of a successful operation, the program disables the watchdog (before it bites) and carries on. Of course this assumes that the operation completes before the watchdog bites, which means the programmer needs to have a good idea of how long it will take.

Setting the time-out period

It's as well to understand how watchdog timers on microcontrollers work. Typically they have a fairly coarse resolution, counting a fixed number of timer ticks before "biting" and performing some function. In the case of the Arduino, the watchdog timer is driven by the internal oscillator running at 128KHz and counts off some multiple of ticks before biting. This value -- the number of ticks counted -- is referred to as the "prescalar" for the timer.

The prescalar is controlled by the values of four bits in the watchdog timer's control register, WDTCSR. To set them up, you pick the value of prescalar you want and set the appropriate bits. If the bits contain a number ( i ), then the watchdog will bite after ( (2048 << i) / 128000 ) seconds. So ( i = 0) means the watchdog bites after 16ms; ( i = 1 ) produces  delay of 32ms; and so on up to ( i = 9 ) (the largest value allowed) means the watchdog bites after about 8s.

The word "about" is important here: the oscillator's exact frequency depends on the supply voltage to the chip and some other factors, meaning that you should be conservative about relying on the delay time.

Writing the appropriate value of ( i ) into the control register involves representing ( i ) as a four-digit binary number and then writing these bits into four bits of the register -- and unfortunately these bits aren't consecutive. if ( i = 7 ) for example, then this is 0b0111 in binary, so we write 1 into bits WDP0, WDP1 and WDP2, and 0 into bit WDP3, and 0 into all the other bits:

WDTCSR = (1 << WDP0) | (1 << WDP1) | (1 << WDP2);

The phrases of the form (1 << WDP0) simply takes a binary digit 1 and shifts it left into bit position WDP0. The | symbols logically OR these bits together to generate the final bit mask that is assigned to the control register.

Actually there's a little bit more to it than this, as we can't change the watchdog's configuration arbitrarily. Instead we have to notify the chip that it's configuration is about to be changed, by setting two other bits in the control register and then performing the updates we want:

WDTCSR |= (1 << WDCE) | (1 << WDE);

Setting WDCE enables changes in configuration to be made in the next few processor cycles, i.e. immediately. Setting WDE resets the timer.

Finally we enable the watchdog timer interrupts by setting bit WDIE. When the watchdog timer bites, the microcontroller executes an interrupt handler, re-starts the main program, and clears WDIE. Any further interrupts, if the time is re-enabled, will then cause a system reset.

WDTCSR |= (1 << WDIE);

So the complete code the setting up the watchdog timer to bite in 2s is:

set_sleep_mode(SLEEP_MODE_PWR_DOWN);              // select the watchdog timer mode
MCUSR &= ~(1 << WDRF);                            // reset status flag
WDTCSR |= (1 << WDCE) | (1 << WDE);               // enable configuration changes
WDTCSR = (1 << WDP0) | (1 << WDP1) | (1 << WDP2); // set the prescalar = 7
WDTCSR |= (1 << WDIE);                            // enable interrupt mode
sleep_enable();                                   // enable the sleep mode ready for use
sleep_mode();                                     // trigger the sleep

/* ...time passes ... */

sleep_disable();                                  // prevent further sleeps</pre>

 Interrupt handler

What happens when the watchdog bites? It causes an interrupt that has to be handled before the program can continue. The interrupt could be used for all sorts of things, but there's often no point in worrying about it: but it still has to be there, to prevent the microcontroller just resetting. The following code installs a dummy interrupt handler:

ISR( WDT_vect ) {
  /* dummy */
}

The WDT_vect identifies the watchdog timer's interrupt vector.

While this might seem like a waste of time, it's important to have an interrupt handler as the default behaviour of the watchdog timer is to reset the microcontroller, which we want to avoid. It's also worth noting that, once enabled, the watchdog timer will keep biting, so the interrupt handler will be called repeatedly. (Put a print statement in the hander to see.) This doesn't cause any problems.

Representing samples

Any sensor network has to represent sampled data somehow. What would be the most friendly format for so doing?

Re-usable software has to take an extensible view of how to represent data, since the exact data that will be represented may change over time. There are several approaches that are often taken, ranging from abstract classes and interfaces (for code-based solutions) to formats such as XML for data-based approaches.

Neither of these is ideal for a sensor network, for a number of reasons.

A typical sensor network architecture will use different languages one the sensors and the base station, with the former prioritising efficiency and compactness and the latter emphasising connectivity to the internet and interfacing with standard tools. Typically we find C or C++ on the sensors and Java, JavaScript, Processing, or some other language on the base station. (Sometimes C or C++ too, although that's increasingly rare for new applications.) It's therefore tricky to use a language-based approach to defining data, as two different versions of the same structure would have to be defined and -- more importantly -- kept synchronised across changes.

That suggests a data-based approach, but these tend to fall foul of the need for a compact and efficient encoding sensor-side. Storing, generating, and manipulating XML or RDF, for example, would typically be too complex and too memory-intensive for a sensor. These formats also aren't really suitable for in-memory processing -- unsurprisingly, as they were designed as transfer encodings, not primary data representations. Even though they might be attractive, not least for their friendliness to web interactions and the Semantic Web, they aren't really usable directly.

There are some compromise positions, however. JSON is a data notation derived initially from JavaScript (and usable directly within it) but which is sufficiently neutral to be used as an exchange format in several web-based systems. JSON essentially lets a user form objects with named fields, whose values can be strings, numbers, arrays, or other objects. (Note that this doesn't include code-valued fields, which is how JSON stays language-neutral: it can't encode computations, closures, or other programmatic features.)

JSON's simplicity and commonality have raised the possibility of using it as a universal transport encoding: simpler than XML, but capable of integration with RDF, ontologies, and the Semantic Web if desired. There are several initiatives in this direction: one I came across recently is JSON-LD (JSON for Linked Data) that seeks to integrate JSON records directly into the linked open data world.

This raises the possibility of using JSON to define the format of sensor data samples, sample collections (datasets), and the like, and linking those descriptions directly to ontological descriptions of their contents and meaning. There are some problems with this, of course. Foremost, JSON isn't very compact, and so would require more storage and wireless bandwidth than a binary format. However, one approach might be to define samples etc in JSON format and then either use them directly (server-side) or compile them to something more static but more efficient for use sensor-side and for exchange. This would retain the openness but without losing performance.

Temperature sensors working

Temperature sensing using digital temperature sensors is easy to get working.

The temperature sensing part of the project requires three sensors for ambient, high-up and low-down measurement. The DS18B20 temperature sensor seems well-suited for the job.

DS18B20

Three DS18B20 temperature sensors sharing a OneWire bus, standard (rail) power mode

Hooking-up a OneWire bus for the three sensors lets them share a single microcontroller pin -- which isn't important for hardware reasons in this project, but also saves some microcontroller RAM, which might be. The circuit is very simple, with the three sensors sharing power and ground lines and with a common data line pulled-up to the power rail through a 4.7K resistor. The DQ line is attached to one of the Arduino's digital lines. The OneWire library is then used to instantiate a protocol handler for that line, and passed to the temperature control library to manage the interaction with the devices, including their conversion from raw to "real" temperature values.

The resulting code is almost comically simple:

#include <DallasTemperature.h>

OneWire onewire(8);                  // OneWire bus on pin 8
DallasTemperature sensors(&onewire);

void setup(void) {
  Serial.begin(9600);
  sensors.begin();
}

void loop(void) {
  sensors.requestTemperatures();
  for(int i = 0; i < 3; i++) {
    float c - sensors.getTempCByIndex(i);
    Serial.print("Sensor ");   Serial.print(i);   Serial.print(" = ");
    Serial.print(c);   Serial.println("C");
  }
  delay(5000);
}

That's it! The temperature library packages everything up nicely, including the conversion and the interaction with the OneWire protocol (which is quite fiddly).

Three DS18B30s on a prototyping shield

One potential problem for the future is that access to the sensors is by index, not by any particular identifier, and it;s not clear whether the ordering is always the same: does the sensor closest to the microcontroller always appear as index 0, for example? If not, then we'll have to identify which sensor is which somehow to sample the temperature from the correct place, or run each one on a different OneWire bus instance.

There's also an interesting point about parasite power mode, which is where the DS18B20 draws its power from the data bus rather than from a dedicated power rail. This might make power management easier, since the sensor would be unpowered when not being used, such as when the Arduino is asleep. This suggests it's probably worth looking into parasite power a bit more.

DS18B20 digital thermometer

The DS18B20 is a programmable digital thermometer that needs no calibration and uses only one wire of the microcontroller.

The DS18B20 is extremely popular as a temperature sensor, for obvious reasons: they're digital, and require no calibration, in contrast to using a thermistor or similar analogue device which would need to be characterised against reference temperatures. They're not as cheap as analogue components, but their simplicity of use and accuracy probably make up for that in scientific applications.

The devices are also notable as using the OneWire protocol developed by Dallas Semiconductor (now Maxim) and used in (amongst other devices) their iButton devices. Essentially OneWire is an embedded systems equivalent of USB that allows a set of devices to be chained together and addressed using only one pin on a microcontroller. This means that there's no real limit on the number of sensors that even a small chip can make use of. The datasheet is available if needed, but they're so easy to use and have such good software support (see below) that there's no real need to refer to it.

The sensors are packaged almost like transistors, with three wires for ground, power, and data. The easiest way to use them is to power them directly with 5V and ground, and use the third wire for communications. (There's also a "parasitic" mode that takes power from the data bus, which I haven't got to work yet.) The communications line is the "one wire" that runs the communications protocol.

Using OneWire devices, and DS18B20s in particular, is made very simple by the existence of two libraries, providing the protocol driver and temperature conversion and packaging respectively. Links and installation instructions can be found on the 3rd-part tools and libraries page.