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CAN Bus Communications Decoded: CAN Message Frequency

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CAN Message Frequency


00:00 - Sending out data frames just the once isn't going to be particularly useful.
00:04 All of the transmitted parameters will be constantly changing, so we need to be continually sending out these frames with the new updated values.
00:12 This does give us a couple of things that we need to consider.
00:15 The first is that the message frequency, which is how often we send out a particular frame of data, is not the same as the board rate of the bus.
00:24 Keep in mind that sometimes the board rate is called the CAN frequency, which is where the confusion can come from.
00:30 The board rate is the number of individual bits of data we can transmit on the bus in 1 second, whereas the message frequency is how often we transmit a specific data frame.
00:41 While these 2 things are related, because if the board rate increases, our data frames are transmitted quicker, so we can transmit them more often if required, keep in mind that they are 2 distinct different parameters.
00:53 Message frequency is another consideration when we're creating data frames.
00:56 It's relatively common to group together the parameters we want to transmit by how quickly we expect them to physically change and send out these groups as individual data frames.
01:07 As an example, have a think about transmitting engine coolant temperature.
01:10 The coolant has a substantial thermal mass and because of this, we don't expect its temperature to actually change that quickly.
01:18 This means that transmitting engine coolant temp only a couple of times a second is usually adequate for every device on the bus to have a good idea of what the actual coolant temperature is at any point in time.
01:30 We're not likely to see a situation where our engine coolant temperature will spike multiple degrees in between sending out these messages.
01:37 In contrast to this, other parameters like lambda readings and manifold pressures can change very quickly and will need to be sent out in data frames with a much higher message frequency.
01:49 Typically at least 100 Hz for these parameters.
01:52 Something to consider when using CAN to transmit data for logging is that there's no point in logging data faster than it's being updated by a CAN data frame.
02:01 If engine coolant temperature is being transmitted at 2 Hz, but another device on the bus is logging it at 100 Hz, that data log will just show 50 samples, all of the same value.
02:14 This can take up valuable logging file space so it's common to match a logging frequency to a message frequency as closely as possible.

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