1.8.9. IO Consistency

At the IO-update (reading inputs and writing outputs) all IO-channels must be transmitted task-consistent. That means only if a task has finished its IO-update completely, the IO-channels are allowed to transmit to the periphery!

An IO-Update must reach the following 3 different constraints:

1. Consistency

2. Jitter

3. Latency

Task consistency must always be maximum! That means, Ios must be updated by a task contiguous. This means not, that a task cannot be interrupted during an IO update by another task.

Jitter means the time shift between the calculated start time of a task and its real start time. This jitter should be minimum to avoid a bad real time influence.

Latency is the time shift between the Ios are updated until they are updated at the hardware. This should be minimum too, to have a maximum real time behavior.

To reach these 3 goals, the following mechanism are used in the runtime system:

• Bits are written in an atomic way, so Inputs and outputs must not be

  pre-processed

• Bus cycle must not be executed during Ios update of another task to

  realize consistency! To avoid jitter, the bus cycle is moved or delayed when it can be done. Only a latency will occur in this situation.

• A collision during a bus cycle can only be resolved by a delay of the

  interrupt task. This leads to a small jitter, but no latency. This is the only situation that is actually not handled in the runtime!