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First draft of a README file with good practices

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Riccardo 2017-08-04 18:42:08 -07:00
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README.md
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@ -3,9 +3,45 @@ opendbc
The project to democratize access to the decoder ring of your car.
Integrated with [cabana](https://community.comma.ai/cabana/)
Use [panda](https://github.com/commaai/panda) to connect your car to a computer
### DBC file basics
A DBC file encodes, in a humanly readable way, the information needed to understand a vehicle's CAN bus traffic. A vehicle might have multiple CAN buses and every CAN bus is represented by its own dbc file.
Wondering what's the DBC file format? [Here](http://www.socialledge.com/sjsu/index.php?title=DBC_Format) a good overview.
### How to start reverse engineering cars
[opendbc](https://github.com/commaai/opendbc) is integrated with [cabana](https://community.comma.ai/cabana/).
Use [panda](https://github.com/commaai/panda) to connect your car to a computer.
### Good practices for contributing to opendbc
- Comments: the best way to store comments is to add them directly to the DBC files. For example:
```
CM_ SG_ 490 LONG_ACCEL "wheel speed derivative, noisy and zero snapping";```
is a comment that refers to signal `LONG_ACCEL` in message `490`. Using comments is highly recommended, especially for doubts and uncertainties. [cabana](https://community.comma.ai/cabana/) can easily display/add/edit comments to signals and messages.
- Units: when applicable, it's recommended to convert signals into physical units, by using a proper signal factor. Using a SI unit is preferred, unless a non-SI unit rounds the signal factor much better.
For example:
```
SG_ VEHICLE_SPEED : 7|15@0+ (0.00278,0) [0|70] "m/s" PCM
```
is better than:
```
SG_ VEHICLE_SPEED : 7|15@0+ (0.00620,0) [0|70] "mph" PCM```
However, the cleanest option is really:
```
SG_ VEHICLE_SPEED : 7|15@0+ (0.01,0) [0|70] "kph" PCM
```
- Signal's size: always use the smallest amount of bits possible. For example, let's say I'm reverse engineering the gas pedal position and I've determined that it's in a 3 bytes message. For 0% pedal position I read a message value of `0x000000`, while for 100% of pedal position I read `0x640000`: clearly, the gas pedal position is within the first byte of the message and I might be tempted to define the signal `GAS_POS` as:
```
SG_ GAS_POS : 7|8@0+ (1,0) [0|100] "%" PCM
```
However, I can't be sure that the very first bit of the message is referred to the pedal position: I haven't seen it changing! Therefore, a safer way of defining the signal is:
```
SG_ GAS_POS : 6|7@0+ (1,0) [0|100] "%" PCM
```
which leaves the first bit unallocated. This prevents from very erroneous reading of the gas pedal position, in case the first bit is indeed used for something else.