The resolution of Ticks
As described on the RTOS Tick documentation page, RTOS time
is based upon a periodic tick interrupt. When an RTOS task wants to wait for a specific time to pass, for
example by calling vTaskDelay(), or for an event to occur, for example by
specifying a block time in a call to xQueueReceive(), the task specifies the
maximum time to block (that is, to wait, without using any CPU cycles)
as a tick count. The macro pdMS_TO_TICKS() converts milliseconds to ticks. The requested block time starts
at the time the API is called, which will be between two tick interrupts. The tick count is an integer that
can't count partial tick periods, so the time between the API call and the next tick interrupt is counted
as the first tick of the delay (block) period. This will result in slight differences in the observed block
time (in wall-clock time) between different API calls that request the same block time. The diagrams below
demonstrate this using a call to vTaskDelay( 2 ) as an example, assuming a tick period of 1ms.
The first diagram shows the situation when the call to vTaskDelay() occurs immediately after a tick
interrupt. The observed block time will be nearly 2 milliseconds (most of the time between Tick 1 and
Tick 2, plus all of the time between Tick 2 and Tick 3).
The second diagram shows the situation when the call to vTaskDelay() instead occurs immediately before a
tick interrupt. The observed block time will be just over 1 millisecond (a small part of the time between
Tick 1 and Tick 2, plus all the time between Tick 2 and Tick 3).
With these examples in mind, it can be seen that the actual time delayed when a delay of N ticks is
specified will always fall between (N-1 ticks * tick_period) and (N * tick_period). For this specific
example, that means a total delay between 1.0000000001ms and 1.99999999999ms.
A few important notes regarding delay time:
Tick resolution is always up to the tick count specified as a parameter, but no less than (N-1)
ticks. Essentially, the range is from N-1 ticks to N ticks (non-inclusive).
To guarantee a minimum delay of N time, you'll need to delay for
(N / (tick_period)) + 1 ticks.
The duration (in time) of a delay is based on the point between RTOS ticks when the delay is
called — the closer the call to the upcoming tick, the shorter the delay will be in time.
Next: RTOS Implementation - The GCC Signal Attribute
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