On 2021-02-02 20:19, Oliver Hartkopp wrote:
On 02.02.21 12:12, Vincent MAILHOL wrote:
The CAN_ERR_FLAG has been renamed in the documentation to indicate
"error messages" from the CAN controller, as an error frame is something
completely different.
Now as we are talking about having CAN_ERR_FLAG in the TX path besides
the vcan testing stuff, we should think about an API for the really
outgoing frames.
We could not only think about "create an error frame right now" but also
think about a more intelligent CAN node, which also offers to destroy
one or more specific CAN ID(s) at a specific bit position after
detecting that CAN ID.
My original idea was to leave it unspecified until a device was
actually capable of doing such a thing. But I am not against
defining the API now :) We might just have to wait a long time
for someone to actually implement it.
We could use the CAN_RTR_FLAG and the data[] section of the outgoing
error CAN frames for such an API.
We are not limited to the CAN_RTR_FLAG and the data[]
First, we have to list the use cases.
As I wrote before, there are only two forms for the error flag:
- The active error flag: 6 consecutive dominant bits
- The passive error flag: 6 consecutive recessive bits
IMO this passive error flag stuff is pointless.
The passive error does not have any effect on the bus. Nobody sees it. It's just a measure to continue counting error counters inside the CAN controller. IMO it's a read-only feature about the controller internal status.
I agree.
The device can either inject the flag either during:
- bus idle
Is an error flag defined at bus idle?
Error flags are intended to destroy *other CAN controllers* transmissions when detecting protocol violations. There can not be a protocol violation at idle time, right?
This is what the Kvaser CAN controller does. It will wait for the bus to become idle, before an active error flag is transmitted.
- while it is transmitting a frame
What's the use-case for destroying your own transmission?
- while it is receiving a frame
This makes sense, especially when you can destroy specific CAN ID frames at a specific bit position.
Or for any CAN ID at a specific bit position.
Or for any CAN ID at an undefined bit position.
I agree.
The error flag can occur at any time.
Sure? (see above)
Of course we might also provide some pump gun mode which just sends an error flag at some (any) time.
As above.
But for what reason?
Testing purpose, e.g if you develop software where you want to keep track of bus errors, this makes it possible to test such software in a controlled way.
We also use this ourselves when testing the transitions ERROR_ACTIVE <-> ERROR_WARNING <-> ERROR_PASSIVE, for Rx.
Regards,
jimmy
