Working With Interrupts

Top  Previous  Next

Some platforms have a number of I/O lines that can work as interrupt inputs. To enable a particular interrupt line, select it using the io.intnum property and enable the line with the io.intenabled property:



'enable interrupt line #0





Once the line has been enabled, the change in this line's state will generate an on_io_int event. The linestate argument of this event is bit-encoded: each bit of the value represents one interrupt line. For the PL_INT_NUM_0, the corresponding bit is bit 0, for PL_INT_NUM_1 — bit 1, and so on. A particular bit of the linestate argument is set when the state change (from LOW to HIGH, or from HIGH to LOW) has been detected on the related interrupt line. Event handler for the on_io_int event can then determine what triggered the interrupt:



Sub On_io_int(linestate As Byte)

  'check if it is the interrupt line #0 that has caused the interrupt

  If linestate And &h01 <>0 Then

    'yes, interrupt has been triggered by the interrupt line #0


  End If

End Sub



Please, note that the word "interrupt" is used here is a somewhat loose sense. On traditional microcontrollers, interrupt line status change causes a near-instantaneous pause to the execution of the main code and a jump to an "interrupt routine". Hence, the term "interrupt" — the execution of the main code gets interrupted.

With the io object, the interrupt line state change does not disrupt the execution of the any event handler or reordering of pending events in the event queue. The on_io_int event is added to the end of the event queue and is not handled until all earlier events are processed. Therefore, nothing is actually interrupted. Note also that there is no guaranteed interrupt response speed here — the time between line state change detection and the execution of the on_io_int event handler depends on the number of prior events waiting in the queue and, hence, cannot be pre-determined with any certainty.

Further, there may be only one on_io_int event waiting in the event queue. Another such event will not be generated unless the previous one is processed (this prevents the event queue from getting overflown). Therefore, some short-lived state changes may remain undetected.

Bottom line: the "interrupts" of the io object should be viewed as a more convenient alternative to programmatic polling of the I/O lines.

Note that interrupt line names, such as "PL_INT_NUM_0" are defined by the pl_int_num enum, which is platform-specific. The declaration of this enum can be found in the your device's platform documentation (for example, EM1000's is here).