http://www.canbushack.com/blog/index.php?title=determining-network-baud-rate
Determining Network Baud Rate
So you found a CAN BUS to reverse engineer, but you don‘t know it‘s buad rate.
There are a couple to find it out.
One.
Go one-by-one through a possible list.
CAN BUS baud rates tend to be of only certain baud rates.
I am not positive, but this most likely has to do with the common crystal speeds used in microprocessors.
The slower the crystal, the slower the bus speed.
So baud rates tend to be one of the following:
33,333 bps, 50 Kbps, 83,333 bps, 100 Kbps, 125 Kbps, 250 Kbps, 500 Kbps (Most common), 800 Kpbs, and 1,000 Kbps.
The last two being the least common as they suffer from reduce network length issues.
You will not find a baud rate that exceeds 1,000 Kbps as this violates the CAN BUS specification.
If I do not know the baud rate of the network that I wish to connect to,
I will typically go through this list and hope one works.
In some cases you may effectively decapitate the network by having the wrong baud rate set in your tool.
So make sure you are not switching baud rates while the vehicle is in motion! This could cause undesired effects.
Two.
Use an osciliscope or logic analyzer.
The foolproof way of measuring any serial data network such as CAN BUS is to use tool that can measure the one bit time.
Baud rate is simply the inverse of one bit time.
So if you can measure this, you simply take 1/(one bit time) in seconds and you have your baud rate.
The best location to find a single bit on CAN BUS is to look at the last transition.
This is typically the ACK bit (acknowledge).
This is where receiving nodes send back a single bit to say that they properly received the frame.
This is always a bit by itself as the bit to the left is recessive as well as the bit to the right whereas it is dominate.
So it will stand out and you should have little trouble finding it.
So look to that last transition and mesure from on transition to the next and you should get something like 0.000002 seconds.
In this case you will simply take 1/0.000002 and you will get 500,000.
Thus the baud rate will be 500,000 Kbps. Done!
具有can总线模块及自动速度检测的微控制器
一种用于初始化微控制器中的控制器区域网络CAN模块的方法，在所述方法中可提供以下步骤：
测量CAN信号的多个下降沿或上升沿之间的周期时间；
对所述周期时间进行排序；
确定所述经排序周期时间的相邻周期时间之间的差值；
对所述差值进行排序；
从所述经排序差值选择第一差值并从所述第一差值确定第一频率；
使用所述选定频率初始化所述CAN模块；
接收CAN信号帧；
确定是否发生错误；
如果发生错误，则选择下一频率并重复初始化所述CAN模块直到已发现有效的CAN频率为止。
CANBus Determining Network Baud Rate 自动速度检测,布布扣,bubuko.com