RTOS Task Notifications
Used As Light Weight Mailbox

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RTOS task notifications can be used to send data to a task, but in a much more restricted way than can be achieved with an RTOS queue because:

  1. Only 32-bit values can be sent
  2. The value is saved as the receiving task’s notification value, and there can only be one notification value at any one time
Hence the phrase ‘lightweight mailbox’ is used in preference to ‘lightweight queue’. The task’s notification value is the mailbox value.

Data is sent to a task using the xTaskNotify() and xTaskNotifyFromISR() API functions with their eAction parameter set to either eSetValueWithOverwrite or eSetValueWithoutOverwrite. If eAction is set to eSetValueWithOverwrite then the receiving task’s notification value is updated even if the receiving task already had a notification pending. If eAction is set to eSetValueWithoutOverwrite then the receiving task’s notification value is only updated if the receiving task did not already have a notification pending – as to update the notification value would overwrite the previous value before the receiving task had processed it.

A task can read its own notification value using xTaskNotifyWait().

See the documentation for the relevant API functions for examples.

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