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# CAN Bus Communications Decoded: CAN Message frequency

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## CAN Message frequency

### 02.25

 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.