Channel example - J1939

This example demonstrates how to configure a channel to accept a single J1939[1] PGN.

We want to only accept messages with PGN \(61444_{10} = \text{F004}_{16}\)[3] (EEC1).

J1939 message frames use extended 29-bit CAN-IDs[2]. The Parameter Group Number (PGN) is defined by 18 of the 29 bits. The remaining 11 bits define the priority and source address of the message. It is often useful to configure a filter to accept a specific PGN regardless of the source address and the priority. Ignoring the source and priority bits can be done using the filter message ID mask (msgIDMask).

Below, the left red bits represent the 3-bit priority, the green bits the 18-bit PGN and the right red bits the 8-bit source address of the 29-bit CAN-ID.

\[\color{red}{000}\color{green}{111111111111111111}\color{red}{00000000}_{2} = \text{3FFFF00}_{16}\]

Message ID bits in positions with zero bits in the filter message ID mask (msgIDMask) are ignored. By using \(\text{3FFFF00}_{16}\) as filter mask, the source and priority are ignored.

To only accept PGN \(61444_{10}\) we configure the filter as:

• The filter msgID is set to the PGN value with the rightmost 8 bits representing the (ignored) source address (HEX value \(\text{F00400}_{16}\))

• The filter msgIDMask is set to the only consider the 18 PGN bits as derived above (HEX value \(\text{3FFFF00}_{16}\))

Note

The filter mask \(\text{3FFFF00}_{16}\) can be used for most J1939 PGN messages.

Examples of IDs accepted / rejected by the filter

ID (HEX)	Filter	Note
0F00300	Reject	PGN F003
0F00400	Accept	PGN F004 with source address 0
0F00401	Accept	PGN F004 with source address 1
0F00464	Accept	PGN F004 with source address 100
4F00400	Accept	PGN F004 with priority 1
0F00500	Reject	PGN F005

The resulting channel configuration becomes:

Compact overview of channel configurations

CHN[4]	channelEnb	extendedID	DSP[5]	filteringEnb	msgID	msgIDMask
1	true	true	1	true	F00400	3FFFF00
2	false	false	1	false	00000000	1FFFFFFF
3	false	false	1	false	00000000	1FFFFFFF
4	false	false	1	false	00000000	1FFFFFFF
5	false	false	1	false	00000000	1FFFFFFF
6	false	false	1	false	00000000	1FFFFFFF
7	false	false	1	false	00000000	1FFFFFFF
8	false	false	1	false	00000000	1FFFFFFF

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[1]

J1939 is a protocol often used in heavy-duty automotive

[2]

A extended ID uses 29 bits. All 29 bits set becomes \(\texttt{11111111111111111111111111111}_{2} = \texttt{1FFFFFFF}_{16}\)

[3]

For more on the notation used see Number bases

[4]

Channel

[5]

downSamplePrescaler