CAN Bus Communications Decoded: Step Five: Determine Variable Scaling
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Step Five: Determine Variable Scaling
03.26
| 00:00 | The last step of our process is called determining parameter scaling. |
| 00:03 | In this instance it's more determining the exact response on the data stream there that corresponds to button presses on our keypad. |
| 00:13 | And in particular the one that we're looking for is our start/stop button here. |
| 00:17 | So by pressing that, I can see our first data byte of our frame there. |
| 00:25 | As I press it, it changes to 02 and as I release it it changes back to 00. |
| 00:32 | And every time I press that, our last data byte there, so that's data byte 4 is increasing and it's continually increasing every time I press any of the buttons on this keypad. |
| 00:48 | What that leads me to believe is that that last data byte is actually going to be a timestamp, it's going to be the number of, the amount of elapsed time since there was an input to the keypad. |
| 00:59 | That's not really information that I'm interested in at the moment so we're going to focus just on our first data byte there, data byte 0 which is the one that's changing when we're pressing a button. |
| 01:10 | So in particular we're looking at this data byte here. |
| 01:17 | So if I press our start/stop button, I can see that that is changing from 00 to 02. |
| 01:24 | If I open up our calculator app and I head into hexadecimal mode here, we're in the programmer mode of the calculator and I'm in hex. |
| 01:35 | If I put in 02 which is our response when that button is pressed, I can see in binary that's a value of 0010. |
| 01:47 | Now that 1 in binary is in the second bit position or bit position 1 because the numbering starts from 0. |
| 01:57 | What that's leading me to believe is that that bit position 1 is related to the state of our button here. |
| 02:06 | So when that bit position 1 is 1, the start button is pressed and when it's 0, the button isn't pressed. |
| 02:14 | Leading on from that, I think we could probably pretty safely determine that the bit positions in that first data byte are actually what are going to represent the button presses on our keypad here. |
| 02:26 | So I'll just close down our calculator app, once again looking at our first data byte there, I suspect if I press our system on/off button which is quite often a lightning bolt which we've got here, I think we're going to see that data change from 00 to 01 'cause that would be the bit in position 0 changing from a 0 to a 1. |
| 02:48 | Which is exactly what we see. |
| 02:50 | So I think it's pretty safe to say that our bit positions are going to be 0, 1, 2, 3 and then the log button is going to be bit position 5, 6, 7 and 8. |
| 03:02 | So that's going to be the relationship between our inputs to our keypad here and the data on the CAN bus completely determined. |
| 03:08 | We're going to be able to program our ECU to look for that particular data frame and respond to a key press which is going to be bit position 1 in the first data byte of that frame. |
