AUTOSAR COM
Switching packets on and off
A control unit should not always talk. While the bus sleeps or during special modes, sets of packets are switched off, and on again later.
Packet groups
Packets are sorted into named sets, for example "everything I send on bus 1" or "everything I receive on bus 1". Such a set is a packet group. A group holds only sending packets or only receiving packets, never both. Each group can be started and stopped as a whole.
After switch-on
All groups are stopped. Nothing is sent until somebody starts a group.
Who starts and stops?
Not COM, and not the application. A separate manager in the control unit that knows the state of the bus: awake, going to sleep, diagnostics running.
A packet may belong to several groups. It is on as long as at least one of its groups is started. A packet that belongs to no group is simply always on.
What "stopped" means
| A stopped sending packet | A stopped receiving packet | |
|---|---|---|
| On the bus | Is not sent. Timer and waiting sends are cancelled. | Arriving copies are ignored. |
| Timeouts | Not watched | Not watched |
| Application writes / reads | The value is still stored, but COM answers "not available". A triggered write is not remembered for later. | A read gives the last known value, with the answer "not available". |
Try it
This packet has a timer (every 2 seconds) and a triggered speed signal. Its group starts out stopped, as after switch-on.
Things to notice. Write the speed while the group is stopped, then start it: no packet goes out at that moment, the old trigger is forgotten. But the first timer packet carries the value you wrote.
Starting fresh, or carrying on
When a group is started, the manager chooses one of two ways. "Fresh" puts all signals in the group's packets back to their start values. "Carry on" keeps whatever is stored. In both cases the timers, timeouts and update-bits start from the beginning.
There is one more, smaller switch for receiving groups: the timeout watching alone can be turned off and on, while the packets keep being received.
Check yourself
Optional. Answer from memory.
The AUTOSAR words
| Plain word here | AUTOSAR word |
|---|---|
| Packet group | I-PDU group |
| Start / stop a group | Com_IpduGroupStart / Com_IpduGroupStop |
| The manager | For example the Basic Software Mode Manager (BswM) |
| Turn timeout watching on / off | Com_EnableReceptionDM / Com_DisableReceptionDM |
Source: AUTOSAR COM specification, R25-11, section 7.2.5 (SWS_Com_00444, 00771, 00840, 00787, 00222, 00334, 00777, 00684, Tables 7.3 and 7.4). Finer point: how long after a start the first timer packet goes out is a setting of its own (a time offset), used to keep many packets from all going out at the same instant.