How to use the select statement with a timeout

• version

  1.1.1

• scope

  Example.

  This code is provided as example code for a user to base their code on.

• description

  How to use the select statement with a timeout

• boards

  Unless otherwise specified, this example runs on the SliceKIT Core Board, but can easily be run on any XMOS device by using a different XN file.

A select statement waits for one of a set of inputs to become ready, performs the selected input and then executes a corresponding body of code. There are however occasions when it is not reasonable to wait on one of the inputs to become ready. In this instance a timer can be used to exit the select statement if no input has become ready within a defined timeout period.

The current time is input from timer t. The value of time is then extended by the TIMEOUT_PERIOD to give a time in the future.

t :> time;
time += TIMEOUT_PERIOD;

If data is available from the port before the TIMEOUT_PERIOD expires then the port_input case is executed and the data can be printed out. However, if no data becomes available within the TIMEOUT_PERIOD then the timeout case is executed and the select statement exits with no input received from the port.

select {
  case port_input when pinsneq(port_input_data) :> port_input_data :
    printstr("Input received on port_input : ");
    printintln(port_input_data);
    break;
  case t when timerafter(time) :> void :
    printstrln("Select statement timeout waiting on input.");
    break;
}