I have been working in a Raspberry with an MCP2515 CAN bus device for read entire values of J1939 messages on broadcasting with python .
I would like to filter the J1939 messages, but i'm not undestand the meaning of the can-mask and how I discorver that. In the docs of python-can says :
All messages that match at least one filter are returned. If filters is None or a zero length sequence, all messages are matched.
[{"can_id": 0x11, "can_mask": 0x21, "extended": False}]
Even though I understood this 'None' part, i didn't understand how identify the mask for my ID message
Example:
I would like to just get by script just the messages with ID "0xCF00400" and "0x18fee927"
import can
# CAN Setting
can_interface = 'can0'
bus = can.interface.Bus(can_interface, bustype='socketcan_native')
while True:
message = bus.recv()
bus.set_filters([{"can_id":0xF004 , "can_mask": ?? , "extended": True},
{"can_id":0xfee9 , "can_mask": ?? , "extended": True}])
How i should fill each variable and how I determine the mask of an ID?
UPDATING 03/10/2021
I have tried the code below, but still returning all messages
import can
# CAN Setting
can_interface = 'can0'
can_filters = [{"can_id":0xCF00400, "can_mask": 0, "extended": True},
{"can_id":0x18fee927, "can_mask": 0, "extended": True}]
bus = can.interface.Bus(can_interface, bustype='socketcan_native',can_filters=can_filters)
while True:
message = bus.recv()
print(message)
Output:
Timestamp: 1615382511.238233 ID: 18fee500 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.238893 ID: 18fef100 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.247038 ID: 18fef100 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.247611 ID: 18fee500 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.248222 ID: 18fee900 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.248868 ID: 0cf00400 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.257056 ID: 0cf00400 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.257623 ID: 18fee900 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.258223 ID: 18fef100 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.258827 ID: 18fee500 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.267039 ID: 18fee500 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.267624 ID: 18fef100 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.268229 ID: 0cf00400 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.268835 ID: 18fee900 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.277035 ID: 18fee900 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.277620 ID: 0cf00400 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.278220 ID: 18fee500 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
Timestamp: 1615382511.278823 ID: 18fef100 X DLC: 8 ff ff ff ff ff ff ff ff Channel: can0
You can use "can_id":0xCF00400, "can_mask": 0xFFFFFFF
to efficiently filter (at the kernel level) for the cob ID you want, with no mask/filter required. The mask of 0xFFFFFFF
(all mask bits set to 1) requires an exact match on the can_id.
bus.set_filters([{"can_id":0xCF00400, "can_mask": 0xFFFFFFF, "extended": True},
{"can_id":0x18fee927, "can_mask": 0xFFFFFFF, "extended": True}])
For example, the documentation says:
A filter matches, when
<received_can_id> & can_mask == can_id & can_mask
. Ifextended
is set as well, it only matches messages where<received_is_extended> == extended
. Else it matches every messages based only on the arbitration ID and mask.
So as an example:
# The following just equals zero
0xCF00400 & 0 == 0 # True
# The following equals 0xCF00400 (217056256 in decimal) exactly
0xCF00400 & 0xFFFFFFF == 0xCF00400 # True
0xCF00400 & 0xFFFFFFF == 217056256 # True
# The following can_id would not get through the filter + mask:
0x18fee500 & 0xFFFFFFF == 0xCF00400 & 0xFFFFFFF # False
# The following obviously would get through the filter + mask:
0xCF00400 & 0xFFFFFFF == 0xCF00400 & 0xFFFFFFF # True
I'd put the bus.set_filters()
before the while True
loop, and before the bus.recv
. It's a setting, so you only need to set it once at the start.
Preferable, you would add it when you initialize the bus, like this:
can_filters = [{"can_id":0xCF00400, "can_mask": 0xFFFFFFF, "extended": True},
{"can_id":0x18fee927, "can_mask": 0xFFFFFFF, "extended": True}]
bus = can.Bus(
interface="socketcan",
channel="can0",
can_filters=can_filters
)
Also, I believe bustype='socketcan_native'
is deprecated in favour of interface="socketcan"
. I've been using the latter with success for quite some time, and no warning messages.