An IPDU contains multiple signals and signal-groups. If the cycle time of the PDU is low, that is, it is transmitted at a faster rate, there is a possibility that not all the signals in the PDU are not updated at the same rate. In this case, Update bit feature provides a mechanism where the receiver can identify if a particular signal was updated by the sender or not.

The update bit is mainly used to identify whether the sender has updated the signal/signal group. An update bit can be configured for each signal/signal group. The position of the update bit configurable by the configuration parameter ComUpdateBitPosition, within the same signal configuration container.

Update bits are internal to the COM module and are not visible to the upper layer or Software components. All operations are handled internally by COM module. It interprets the update-bit as shown below

Update bit processing on the Sender Side

  1. If the user(RTE or SW component) updates the value of a signal/signal group by calling Com_SendSignal() or Com_SendSignalGroup(), COM module will internally set the corresponding update-bit.
  2. When to clear the update bit is configurable via the parameter ComTxIPduClearUpdateBit at the PDU level. As a result, all the update bits of signals belonging to a PDU are cleared together. It can be one of the following three.
    • Transmit : Cleared when PduR_ComTransmit() returns E_OK.
    • Confirmation : Cleared when IPDU transmission was successfully transmitted.
    • TriggerTransmit : Cleared when Com_TriggerTransmit() is requested.

Update bit processing on the Receiver Side

  1. If the update-bit of the received signal/signal group is not set, it means that the sender did not update the signal and the receiver ignores the signal and no further processing is performed.
  2. If the RTE/SWC reads a signal with an associated update bit not set, the initial value or the last received value is returned by COM module.

Usage restrictions of Update bit

  1. If the IPDU transmission mode is DIRECT or MIXED with ComTxModeNumberOfRepetitions greater or equal 1 update-bit can not be configured (because it kinda defies the usage of UB).
  2. Each signal/signal group can have only one associated Update bit.
  3. The update bit must be in the same IPDU as the associated signal.

That’s all about Update bits in AutoSAR COM. See you!

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