Abstract

This memo documents how sensor boards are organized in TinyOS, and the general principles followed by the components that provide access to its sensors.

1. Introduction

This document defines the default organization of a sensor board in TinyOS. There likely will be sensor boards that cannot conform to this specification, but following as closely to its spirit as possible will simplify generic applications that use a range of sensor boards.

This document assumes that sensors return uninterpreted 16-bit values, and, optionally uninterpreted, arbitrary-size calibration data. Conversion of sensor values to something with actual physical meaning is beyond the scope of this document.

2. Directory Organization

• A sensor board MUST have a unique name, composed of letters, numbers and underscores. Case is significant, but two sensor boards MUST differ in more than case. This is necessary to support platforms where filename case differences are not significant. We will use SBOARD to denote the sensor board name in the rest of this document.
• Each sensor board MUST have its own directory named SBOARD; default TinyOS sensor boards are placed in tinyos-2.x/tos/sensorboards, but sensor board directories can be placed anywhere as long as the nesC compiler receives a -I directive pointing to the sensor board's directory.
• Each sensor board directory MUST contain a .sensor file. This file is a perl script which contains any additional compiler settings needed for this sensor board (this file will be empty in many cases).
• If the sensor board wishes to define any C types or constants, it SHOULD place these in a file named SBOARD.h in the sensor board's directory.
• The sensor board directory SHOULD contain sensor board components for accessing each sensor on the sensor board. The conventions for these components are detailed in Section 3.
• A sensor board MAY include additional components providing alternative or higher-level interfaces to the sensors (e.g., for TinyDB). These components are beyond the scope of this document.
• Finally, the sensor board MAY contain any number of components, interfaces, C files, etc for internal use. To avoid name collisions, all externally visible names (interface types, components, C constants and types) used for internal purposes SHOULD be prefixed with SBOARD. All such components should end in P.

A simple example: the basic sensor board is named basicsb, it's directory is tinyos-2.x/tos/sensorboards/basicsb. It has no basicsb.h file and its .sensor file is empty. It has two components, PhotoC and TempC representing its light and temperature sensors.

3. Sensor Board Components

We have not yet selected any naming conventions for sensor board components. Please select reasonable namesldots

A sensor board component MUST provide:

• An Init interface.
• A StdControl or SplitControl interface for power management.
• A non-empty set of AcquireData interfaces for sampling.

A sensor board component MAY provide:

• Some CalibrationData interfaces for obtaining calibration data. A calibration interface for a sensor accessed via interface X should be called XCalibration.
• Some AcquireDataNow and AcquireDataBuffered interfaces, for high-speed or low-latency data acquisition.
• Any other appropriate interface.

The CalibrationData interface is shown below, while AcquireData, AcquireDataNow and AcquireDataBuffered are in TEP 101. The AcquireData interface returns uinterpreted 16-bit data. This might represent an A/D conversion result, a counter, etc. The optional calibration interface returns uninterpreted, arbitrary-size data.

A sensor board component SHOULD be as lightweight as possible - it should just provide basic access to the physical sensors and SHOULD NOT attempt to do calibration, signal processing, etc. If such functionality is desired, it SHOULD be provided in separate components.

Some common setups for sensor board components are:

• A single AcquireData interface. This is probably the most common case, where a single component corresponds to a single physical sensor, e.g., for light, temperature, pressure and there is no expectation of high sample rates.
• Multiple AcquireData interfaces. Some sensors might be strongly related, e.g., the axes of an accelerometer. A single component could then provide a sensor interface for each axis. For instance, a 2-axis accelerometer which can be sampled at high speed, and which has some calibration data might be declared with:

• A parameterised AcquireData interface. If a sensor board has multiple similar sensors, it may make sense to provide a single component to access all of these, using a parameterised AcquireData interface. For instance, a general purpose sensor board with multiple A/D channels might provide an AcquireData interface parameterised by the A/D channel id.
• In all of these examples, if high-speed sampling makes sensor for the sensor (e.g., a microphone), and the sensor is connected in a way that supports high-frequency and/or low-latency access (e.g., via an on-microcontroller A/D converter), the component should offer AcquireDataNow and AcquireDataBuffered interfaces.

Sensor board components MUST respect the following conventions on the use of the Init, StdControl, and SplitControl interfaces. These are given assuming StdControl is used, but the behaviour with SplitControl is identical except that start and stop are not considered complete until the startDone and stopDone events are signaled. The conventions are:

1. Init.init: must be called at mote boot time.

2. StdControl.start: ensure the sensor corresponding to this component is

   ready for use. For instance, this should power-up the sensor if necessary. The application can call getData once StdControl.start completes.

   If a sensor takes a while to power-up, the sensor board implementer can either use a SplitControl interface and signal startDone when the sensor is ready for use, or delay dataReady events until the sensor is ready. The former choice is preferable.

3. StdControl.stop: put the sensor in a low-power mode. StdControl.start must be called before any further readings are taken. The behaviour of calls to StdControl.stop during sampling (i.e., when an dataReady event is going to be signaled) is undefined.

.sensor File

This file is a perl script which gets executed as part of the ncc nesC compiler frontend. It can add or modify any compile-time options necessary for a particular sensor board. It MAY modify the following perl variables, and MUST NOT modify any others:

• @new_args: This is the array of arguments which will be passed to nescc. For instance, you might add an include directive to @new_args with push @new_args, -Isomedir
• @commonboards: This can be set to a list of sensor board names which should be added to the include path list. These sensor boards must be in tinyos-2.x/tos/sensorboards.

Example: mts3x0

The mica sensor board (mts300/mts310) has five sensors (and one actuator, the sounder) -- the accelerometer and magnetometer are only present on the mts310:

+===============+===========+===================+==================+

Each physical sensor is represented by a separate component. Specific sensors that have more than one axis of measurement (AccelC and MagC) provide more than one AcquireData interface on a single component. Some sensors, such as the magnetometer and microphone, have additional functionality provided through sensor-specific interfaces.

Although light and temperature are represented by separate components, in reality they share a single microcontroller pin. The two components PhotoC and TempC sit on top of the PhotoTempP component, which controls access to the shared pin, and orchestrates which sensor is currently connected to it. From a programmer's perspective, they appear as individual sensors, even though their underlying implementation is a bit more complex.

The board's mts3x0.h file contains private configuration data (pin usage, ADC ports, etc).

The mica sensor board has an empty .sensor file.

6. Author's Address