TM4C1294NCPDT: How to discard CAN messages from a particular ID

Hi,

  Below is example to receive CAN messages with ID 0x1001 only and filter out all others. 

/* Initialize a message object to receive CAN messages with ID 0x1001.
* The expected ID must be set along with the mask to indicate that all
* bits in the ID must match. */
sCANMessage.ui32MsgID = 0x1001;
sCANMessage.ui32MsgIDMask = 0xfffff;
sCANMessage.ui32Flags = (MSG_OBJ_RX_INT_ENABLE | MSG_OBJ_USE_ID_FILTER |
MSG_OBJ_EXTENDED_ID);

If you want to exclude ID 0x2 then you would set the ID to 0x00000000 and the mask to 0x00000002. Bear in mind that filtering is applied by masking the specified bit(s) from comparison. It is not a equality comparison. Say for example that the CAN ID field is only three bits instead of 11 bits. If you specify ID equal to 0x0 and the mask equal to 0x2 then the ID 0x2 will be filtered out but also ID 0x3, 0x6 and 0x7. 

" If you specify ID equal to 0x0 and the mask equal to 0x2 then the ID 0x2 will be filtered out but also ID 0x3, 0x6 and 0x7. "

Can u please explain why 3,6,7 is getting filtered out and where it is mentioned in reference manual.

If the mask is equal to 0x2 as in 010b then bit 1 is used for acceptance filtering. If you specify an ID of 0x0 as in 000b for which bit 1 is equal to 0, any IDs that has bit 1 equal to 1 will be filtered out. Only IDs with bit 1 equal to 0 will be accepted. Look at a three-bit ID example, this means 010, 011, 110 and 111 will be filtered out because for these four IDs their bit 1 is equal to 1. This is what I mean not only an ID of 0x2 is filtered out but also 0x3, 0x6 and 0x7. If you want to filter out only ID=0x2 and pass everything else, why don't you consider disabling the acceptance filtering but in your ISR, check for the incoming ID equal to 0x2 and ignore the processing of it. 

I need to reduce overflow in my recieve Buffer so would have to discard it before recieving

I have explained how the hardware works. Below thread has the similar discussion. it is up to you how you want to implement it. I have no other suggestions.

https://electronics.stackexchange.com/questions/240278/controller-area-network-received-message-filtering-mechanism

Hi Charles,

Below was our understanding,

ARB ID- 000
(Bitwise OR)
Mask - 010
= 010

Node ID 1 - 001 - 000(AND) - Don't filter

Node ID 2 - 010 - 010(AND) - Filter

Node ID 3 - 011 - 010(AND) - Filter

Node ID 4 - 100 - 000(AND) - Don't filter

Node ID 5 - 101 - 000(AND) - Don't filter

Node ID 6 - 110 - 010(AND) - Filter

Node ID 7 - 111 - 010(AND) - Filter

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If output of AND operation is 0, don't filter the received CAN message else filter the receive

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But when we tried with below combination that understanding didnt work

ARB ID - 010
(Bitwise OR)
Mask - 111
= 111

Node ID 1 - 001 - 001(AND) - Filter

Node ID 2 - 010 - 010(AND) - Filter But it didnt Filter

Node ID 3 - 011 - 010(AND) - Filter

Node ID 4 - 100 - 000(AND) - Filter

Node ID 5 - 101 - 000(AND) - Filter

Node ID 6 - 110 - 010(AND) - Filter

Node ID 7 - 111 - 010(AND) - Filter

Please help me to understand.

For the mask bit that is equal to 1, it is like doing a XOR logic between the incoming ID on the bus with the specified ID stored in the sCANMessage.ui32MsgID for the corresponding bit. The result of the XOR must be equal to 1. For the mask bit(s) that is equal to 0, you do not need to compare. 

so if the result of the XOR is equal to 1 then the filter, otherwise do not filter?

You first do a XOR between between the incoming ID with what you specify in the sCANMessage.ui32MsgID. The result of the XOR is again XOR with the mask you store in sCANMessage.ui32MsgIDMask. The individual bits of that XOR result is then NOR to determine if a incoming ID should be filtered or not. 

Take you own example 1:

ARB ID- 000
Mask - 010

First take XOR between the incoming ID (there are 8 combinations using a 3bit ID example) and ARB ID (000 as in your example)

Incoming ID     Result1 (XOR result with 000)

000                  111

001                  110

010                  101

011                   100

100                   011

101                  010

110                  001

111                   000

Now take the XOR result of the above with the mask (010 as you specify in your example)

Result1            Result2 (XOR result with the mask 010)

111                     010                                    

110                    010

101                    000

100                    000

011                    010

010                    010

001                    000

000                    000

Result2             Result3 (OR of all three bits)

010                   1

010                   1

000                   0

000                   0

010                   1

010                    1

000                    0

000                    0

Incoming ID   Result3 

000                1               Passed through

001                1               Passed through

010                0               Filtered out

011                0               Filtered out

100                1                Passed through

101                1               Passed through

110                 0               Filtered out

111                  0              Filtered out

Why don't you write some code to see if this is what you are getting when you apply ARB=000 and Mask=010 and inject the 8 different combinations of message IDs on the bus. 

Hi,

  Are you clear with my explanation now?

Hi,

 Do you have any update as I have not heard back from you?