Once I got the taste of making all sorts of measurements with sensors, I happily started to collect samples. Then I quickly ran into the background sensor monitoring problem. The use case is very simple: the phone is in idle state, keyguard is locked but it collects and processes sensor data. For example I wanted to record acceleration sensor data while cycling. I started the sensor application, started the measurement and locked the keyguard. On the Nexus One it means that there will be no further sensor data delivered to the application until the screen is on.
The source of the problem is that the sensor is powered down when the screen is off. This is implemented below the Android framework (either in the kernel driver or in hardware, I would be curious if anyone knows the answer) because if you recompile the latest sources from android.git.kernel.org, sensor data will be delivered nicely to the application in the emulator even if the keyguard is locked (the stock 2.1 emulator's Goldfish sensor does not emit any event, that is why you have to recompile from source) . The fact remains: if you want acceleration sensor data on the Nexus One, the screen must be on. This pretty much kills all the user-friendly applications that want to analyze sensor data in the background while the phone is idle. In the worst case, the screen must be constantly on (e.g. for a pedometer that needs to measure constantly because you never know when the user makes a step) but the situation for a simple context reasoner service is not much better. Such a service (read the vision paper for background, you need free registration to access) may want to sample the sensors periodically (e.g. collecting 5 sec. of samples in every minute). In order to perform the sampling, the screen should be switched on for this duration which would result in a very annoying flashing screen and increased power consumption.
You can observe these effects in the improved Sensors program that is able to capture sensor data in the background.
Click here to download the example program.
Start and deploy this program and you will see the familiar sensor list. If you click on any list item, the old interactive display comes up. The improvement here is that the application measures the sensor sampling frequency. The rate can be set in the settings menu, from the main activity. If, however, you long-press on the list item, the sampling will be started in the background. In this case there is no interactive display of the samples, they are always saved into the /sdcard/capture.csv file. The background sampler does respect the sampling rate setting, however. The background sampler is implemented as a service.
So far there is nothing extraordinary. You may observe the ScreenOffBroadcastReceiver class in SamplingService that intercepts ACTION_SCREEN_OFF intent broadcasts and does something weird in response. In order to understand its operation, you must be aware that the power manager deactivates all the wake locks that are stronger than PARTIAL_WAKE_LOCK when the keyguard powers the device down. Obtaining for example a SCREEN_BRIGHT_WAKE_LOCK in the broadcast receiver would be of no use because at this point the screen is still on and the wake lock would be deactivated when the screen is turned off. Instead, the broadcast receiver starts a thread, waits for 2 sec (experimental value) and then it activates the wake lock with wakeup option. Try to push the power button when the background sampling is on, the screen will go off for 2 seconds but then it is turned on again and you will see the keyguard screen. If you check the log, you will see that the sensor sampling stops for that 2 seconds then it comes back on. Don't worry, when the background sampling is stopped (by long-pressing the sensor's name in the list again) the screen will turn off correctly.
Ugly? You bet. Not only the solution is a bad hack but it also prevents the device from switching off the screen and those AMOLED displays plus the high-powered processors eat battery power like pigs. The decision that the sensors are powered down when the screen is off prevents the implementation of some of the most exciting mobile applications and significantly decreases the value of the platform.
But enough of the grunting. This is the state of the art in background sensor sampling in Android (as of version 2.1 of the platform), I will continue with signal processing algorithms.