title: CanBus Shield seeedStudio not capturing data
body: <p>I have purchased a Seeed Studio Can Bus Shield V1.2 and I’m having problems listening to a bus at 200kbps but I had success listing to my car's bus.</p>
  
  <p>I have an undocumented bus running at 200kps (it’ not a car bus) that I need to sniff, there are 3 devices on the bus. The impedance between canh and canl is 60ohm. I have cut the P1 jumper and checked the impedance is still 60 ohm when wiring on the screw terminal.
  I have plug the bus ground to the shield ground.</p>
  
  <p>I’m using the receive_check example provided with the lib. I have set the bus speed to 200kbps.</p>
  
  <p>This bus inits well but receives nothing. The Rx led blinks when receiving data, I have signal on pin 4 of  MCP2551, but nothing is displayed.</p>
  
  <p>To be sure to be doing things well, I have plugged the shield/arduino to my car. My car is running the bus at 500kps. I got it working the right way and the Arduino started displaying a message.</p>
  
  <p>I have a picoscope oscilloscope which can decode CAN BUS messages.</p>
  
  <p>My oscilloscope decodes the can bus messages with no difficulties. The bit time is about 5 micro sec,  the oscilloscope auto-configured itself to 200kbps. </p>
  
  <p>I have compared my car’s and my devices messages and they look really very similar. I use exactly the same settings on my oscillon (except for speed) to decode both messages. 
  The only difference I can see is that my device signal is not as clean as my car’s signal.</p>
  
  <p>This is my car’s signal (500kbps)</p>
  
  <p><a href="https://i.stack.imgur.com/OUR4y.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/OUR4y.png" alt="my can&#39;s canbus message"></a></p>
  
  <p>This is my device’s signal (200kbps)</p>
  
  <p><a href="https://i.stack.imgur.com/uW674.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/uW674.png" alt="my devie canbus message"></a></p>
  
  <p>Sometimes it’s a bit noisy, in that case, the oscillo cannot decode, but most of the time it is successful.</p>
  
  <p><a href="https://i.stack.imgur.com/3qB9b.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/3qB9b.png" alt="low quality signal"></a></p>
  
  <p>Could this be part of the problem?
  Should I anyway get a clear message from time to time? I’m not getting any.</p>
  
  <p>I have been wondering whether the config was right for the 200kbps speed so I installed the Chinese tool mentioned on the wiki page.</p>
  
  <p>For my speed 200kbps, the lib config is :</p>
  
  <pre><code>#define MCP_16MHz_200kBPS_CFG1 (0x01)
  #define MCP_16MHz_200kBPS_CFG2 (0xFA)
  #define MCP_16MHz_200kBPS_CFG3 (0x87)
  </code></pre>
  
  <p>I have tried other configurations for this speed but with no success.</p>
  
  <pre><code>#define MCP_16MHz_200kBPS_CFG1 (0x01)
  #define MCP_16MHz_200kBPS_CFG2 (0xF5)
  #define MCP_16MHz_200kBPS_CFG3 (0x85)
  </code></pre>
  
  <p>I have also turned the debug mode on, in the CanBus lib
  I get more messages on init but the arduino keeps silent when data is received.</p>
  
  <pre><code>CAN BUS Shield init ok!
  Enter setting mode success 
  set rate success!!
  Enter Normal Mode Success!!
  CAN BUS Shield init ok!
  </code></pre>
  
  <p>I'm using a Arduino Mega.</p>
  
  <p>What can I do? What is the problem? </p>
  
  <p>Thank you for your help</p>