The Sming framework has an event-driven architecture. In other words, we need to think of our application as performing some action in response to an event, input or stimulus.
Let’s look at the Basic Blink sample. We define a couple of global variables:
bool state = true;
The one to note is procTimer, which is a software timer (see Timers and Clocks).
The startup entry point for a Sming application is the
This sets up a repeating timer, so that every 1000ms the blink function gets called:
state = !state;
Note that what we don’t do is sit in a loop waiting for something to happen.
The Basic Interrupts sample can be summarised as follows:
We have a button connected to GPIO0 as an input
- Button pressed
- Hardware interrupt on GPIO0
Change output state of LED via GPIO2
There are two ways we can determine when the state of our GPIO0 pin changes:
We read the input very quickly and compare the current value with the previous value, which we keep a note of. Our event occurs when the value changes, or if the input goes HIGH, or LOW, etc.
If we need to catch fast pulses then polling may not work as the CPU might be doing something else and we’ll miss it. So instead, we’ll get the hardware to monitor the input for us and generate an event when it changes.
Both techniques have advantages and disadvantages, and interrupts are certainly not appropriate in all situations.
Bear in mind that every time an interrupt occurs the CPU has to stop executing your regular program, save any critical registers then jump to the interrupt service routine to run your code. All this takes time, so if the input changes very fast and very frequently then it can consume a lot of CPU and make the system very sluggish (or even crash it).
In Sming, we generally register a callback function to be invoked when an event occurs, such as if a timer expires.
This can be a regular ‘C’ callback function, which you should use for handling interrupts.
For regular application code, a
Delegate provides more flexbility and allows you to create
simpler, cleaner code. See Delegation
for a bit of background.
The Basic Delegates sample shows how to use various types of callback.
One common requirement is to use a class method as a callback, which is easily done using Delegates.
This flexibility comes at a cost, however:
Speed. They are slower than their regular C-function counterparts
Memory. Some calls may use the heap in the background.
These are the main reasons why you should not use Delegates in an interrupt context.
See Pull Request #1734 for some further details about the relative speeds.