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high. Disabled does not mean that the port is deactivated, only that there is no pull-up or pulldown resistor wired.
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Interrupt Ports
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In the previous section, you learned how to read the state from an input port in a continuous loop. If you want your application to wait for a key press, then using polling for that is not good practice; it s pedantic and not very efficient. For triggering a rising or falling edge of a pulse (see Figure 5-4), this approach is also unsuitable. Continuously reading a GPIO state will keep the processor busy and give it no idle time, so it can never go to low-power mode. Interrupt ports are the keys to solving this problem; the special class called InterruptPort extends the InputPort class. Interrupts are hardware events. If the microcontroller has nothing further to do than wait for a certain GPIO event, the processor can confidently go into powersaving mode. As soon as a signal change occurs at the input pin, the microcontroller wakes up and a certain method, the interrupt service routine (ISR), will be executed. You can configure an interrupt port to wait either for the rising edge of a pulse, falling edge of a pulse, both edges of a pulse, or for the high or low signal level in general (Figure 5-4 shows these).
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High level Rising edge Low level
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Figure 5-4. Anatomy of a pulse The constructor looks like the InputPort one, but with an additional parameter interrupt of the type Port.InterrupMode. public InterruptPort(Cpu.Pin portId, bool glitchFilter, Port.ResistorMode resistor, Port.InterruptMode interrupt); InterruptMode indicates on what you want to trigger. Figure 5-5 shows how the interrupt modes apply to a pulse. public enum InterruptMode { InterruptNone, InterruptEdgeLow, InterruptEdgeHigh, InterruptEdgeBoth, InterruptEdgeLevelHigh, InterruptEdgeLevelLow }
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Falling edge
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The port is deactivated falling edge (Change from high to low) rising edge (Change from low to high) both edges (Any state change) high low
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CHAPTER 5 AC CE SSING HA RDWA RE
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High level Rising edge = High edge Low level
Figure 5-5. Interrupt modes
Falling edge = Low edge
Triggering an Edge
The code in Listing 5-5 shows how you can trigger to both edges. The interrupt handler routine is assigned over the delegate (reference to a method) OnInterrupt. The custom interrupt handler port_OnInterrupt is then executed as soon as the state of the GPIO port changes. Listing 5-5. Triggering Both Edges using using using using System; System.Threading; Microsoft.SPOT; Microsoft.SPOT.Hardware;
namespace GpioInterruptPortEdgeSample { public class Program { public static void Main() { InterruptPort port = new InterruptPort(Cpu.Pin.GPIO_Pin3, false, //no glitch filter Port.ResistorMode.PullDown, Port.InterruptMode.InterruptEdgeBoth); port.OnInterrupt += new GPIOInterruptEventHandler(port_OnInterrupt); Thread.Sleep(Timeout.Infinite); } private static void port_OnInterrupt(Cpu.Pin port, bool state, TimeSpan time) { Debug.Print("Pin=" + port + " State=" + state + " Time=" + time); } } }
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Triggering to a Level
If you want to wait for a certain level, either high or low, you need to pass either InterruptEdgeLevelHigh or InterruptEdgeLevelLow as the interrupt mode. The interrupt handler will only be called once, as soon as the port achieves the desired state. The interrupt handler will be called again only if you clear the interrupt by calling ClearInterrupt. Listing 5-6 demonstrates how to trigger to a level. Listing 5-6. Triggering to a Level using using using using System; System.Threading; Microsoft.SPOT; Microsoft.SPOT.Hardware;
namespace GpioInterruptPortEdgeSample { public class Program { private static InterruptPort interruptPort; public static void Main() { interruptPort = new InterruptPort(Cpu.Pin.GPIO_Pin2, false, //no glitch filter Port.ResistorMode.PullUp, Port.InterruptMode.InterruptEdgeLevelLow); interruptPort.OnInterrupt += new GPIOInterruptEventHandler(port_OnInterrupt); Thread.Sleep(Timeout.Infinite); } private static void port_OnInterrupt(Cpu.Pin port, bool state, TimeSpan time) { Debug.Print("Pin=" + port + " State=" + state + " Time=" + time); interruptPort.ClearInterrupt(); } } } In Listing 5-6, the variable interruptPort cannot be declared locally in the Main method like it is in Listing 5-5, because the variable needs to be addressed in the interruptPort_ OnInterrupt interrupt method. Therefore, you need to declare the variable as a static class member.
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