AUTOSAR COM
Repeats and the minimum gap
Two extras for sending. One sends an important packet several times. The other stops a packet from being sent too often.
Repeats: say it more than once
A packet can get lost on a noisy wire. For important news, such as "brake pressed", COM can send the same packet several times in a row after one write.
set per packet
Number of repeats
How many extra sends follow the first one. With 2 repeats the packet goes out 3 times.
set per packet
Repeat spacing
The time between those sends.
If the application writes a new value while repeats are still running, COM drops the remaining repeats and starts again with the new value. Old news is not worth repeating.
Minimum gap: not too often
An application might write a triggered signal a hundred times a second. Sending a packet each time would flood the bus. The minimum gap is a pause COM keeps after sending a packet before it sends that same packet again.
A send that falls inside the pause is not lost. COM holds it back and sends when the pause is over, with the newest values.
Try it
Speed is a triggered signal in a packet with no timer. Click "Write speed" a few times quickly, with different settings. Repeats are one second apart here.
Things to notice. With repeats, a second write cuts the first series short. With a gap, quick writes produce fewer packets than writes, and the packet that finally goes carries the newest speed.
The triggered family
Lesson 2 said a signal is triggered or pending. Triggered comes in four flavours. They differ in two questions: does every write count, or only a changed value? And are repeats used?
| Setting of the signal | A write sends when… | Repeats |
|---|---|---|
TRIGGERED | always | yes |
TRIGGERED_WITHOUT_REPETITION | always | no, once |
TRIGGERED_ON_CHANGE | the value is different from before | yes |
TRIGGERED_ON_CHANGE_WITHOUT_REPETITION | the value is different from before | no, once |
PENDING | never | – |
As in lesson 2, all of this only matters when the packet is DIRECT or MIXED.
Check yourself
Optional. Answer from memory.
The AUTOSAR words
| Plain word here | AUTOSAR word |
|---|---|
| Repeats | Number of repetitions, also "N-times transmission" |
| Repeat spacing | Repetition period |
| Minimum gap | Minimum delay time, kept by the minimum delay timer (MDT) |
Source: AUTOSAR COM specification, R25-11, sections 7.2.3.1, 7.2.3.4 and 7.2.5.5 (SWS_Com_00330, 00767, 00734, 00768, 00279, 00305, 00698). Finer points: COM counts a repeat only once the lower layers confirm the packet was sent. A separate setting decides whether the minimum gap also applies to timer sends and repeats; in this toy it applies to every send.