AUTOSAR COM
Start values and update-bits
Two small helpers. A start value answers "what do I read before anything arrived?". An update-bit answers "is this value really new?".
Start values
After switch-on, an application may ask for a signal before any packet has arrived. COM must still answer. So every signal gets a start value chosen in advance. COM hands it out until a real value replaces it.
The same is true on the sending side: if a packet goes out before the application has written every signal in it, the missing ones travel with their start values.
Try it
The temperature's start value is 20.
Just switched on. Nothing has arrived yet.
Update-bits
A packet always carries all of its signals. Suppose a packet holds speed and temperature and goes out because the speed changed. The temperature travels along, unchanged. The receiver cannot tell whether that temperature is fresh or the same old one again.
An update-bit solves this. It is one extra bit in the packet that belongs to one signal and means "this signal was written since the last send".
sender
Set and clear
COM sets the bit when the application writes the signal, and clears it again after the packet was sent.
receiver
Check
Bit set: handle the signal as usual. Bit not set: ignore this signal completely, as if it had not been in the packet.
The application never sees the bit. COM handles it on both sides.
Try it
Write one or both signals, then let the packet go. Watch which signals the receiver accepts.
Sender: packet being prepared
Receiver: what happened to each packet
- No packet sent yet.
Things to notice. Send twice in a row without writing: the second packet has no bit set, and the receiver ignores everything in it.
Check yourself
Optional. Answer from memory.
The AUTOSAR words
| Plain word here | AUTOSAR word |
|---|---|
| Start value | Init value |
| Update-bit | Update-bit (also "update indication bit") |
Source: AUTOSAR COM specification, R25-11, sections 7.2.1.4 and 7.9 (SWS_Com_00098, 00228, 00061, 00062, 00324, 00802). Finer points: a setting chooses when the sender clears the bits: right after handing the packet down, after the bus confirmed it, or when a lower layer fetched it. A signal group (lesson 10) gets one update-bit for the whole group. All update-bits start cleared.