twr-k20d72m and can

[christos]

Hello all,

   I am using a twr-k20d72m board and started to try the flexCAN example with MQX 3.8.1 and CW 10.2.

   CW builds the example successfully but I can not see any activity on PTA12 pin 42 on the PK20DX256VLL7 using an oscilloscope. The terminal output is:

*********FLEXCAN TEST PROGRAM.*********                                       
  Message format: Standard (11 bit id)                                        
  Message buffer 0 used for Tx and Rx.                                        
  Interrupt Mode: Enabled               
  Operation Mode: TX and RX --> Normal                                        
***************************************                                        
                                                                               
selected frequency (Kbps) is: 125                                              
Data length: 1                                                                
FLEXCAN reset. result: 0x0                                                    
FLEXCAN initilization. result: 0x0                                            
FLEXCAN mode selected. result: 0x0                                             
FLEXCAN global mask. result: 0x0                                               
FLEXCAN Error ISR install, result: 0x0                                         
FLEXCAN Bus off ISR install, result: 0x0                                       
FLEXCAN error interrupt enable. result: 0x0                                    
FLEXCAN rx remote mailbox initialization. result: 0x0                          
FLEXCAN rx mailbox initialization. result: 0x0                                
FLEXCAN mailbox activation. result: 0x0                                        
FLEXCAN RX ISR install. result: 0x0                                            
FLEXCAN RX remote ISR install. result: 0x0                                     
FLEXCAN tx remote mailbox initialization. result: 0x0                          
FLEXCAN tx mailbox initialization. result: 0x0                                 
FLEXCAN tx mailbox activation. result: 0x0                                     
FLEXCAN TX ISR install. result: 0x0                                            
FLEXCAN TX remote ISR install. result: 0x0                                     
FLEXCAN started. result: 0x0                                                   
Data transmit: 0         Data transmit: 1                                                                
FLEXCAN tx update message. result: 0x0                                          
Data transmit: 2                                                                
FLEXCAN tx update message. result: 0x0                                          
Data transmit: 3                                                                
FLEXCAN tx update message. result: 0x0                                          
Data transmit: 4                                                                
FLEXCAN tx update message. result: 0x0    

When I configure the CAN in loopback mode it seems to work but still I can not see any activity on the pins with the oscillosope. For the loopback I use the code from another forum post. Re: Flexcan example on tower K60

*********FLEXCAN TEST PROGRAM.*********                                         
   Message format: Standard (11 bit id)                                         
   Message buffer 0 used for Tx and Rxion. result: 0x0                          
FLEXCAN mailbox activation. result: 0x0                                         
FLEXCAN RX ISR install. result: 0x0                                             
FLEXCAN RX remote ISR install. result: 0x0                                      
FLEXCAN tx remote mailbox initialization. result: 0x0                           
FLEXCAN tx mailbox initialization. result: 0x0                                  
FLEXCAN tx mailbox activation. result: 0x0                                      
FLEXCAN TX ISR install. result: 0x0                                             
FLEXCAN TX remote ISR install. result: 0x0                                      
FLEXCAN started. result: 0x0                                                    
Data transmit: 0                                                                
Received data: 0x0                                                              
ID is: 0x123                                                                    
DLC is: 0x1                                                                     
                                                                                
Data transmit: 1                                                                
FLEXCAN tx update message. result: 0x0                                          
Received data: 0x1                                                              
ID is: 0x123                                                                    
DLC is: 0x1 

Any help much appreciated.

Best regards,

Christos

[dereksnell · Accepted solution]

Hi Christos

The CAN protocol requires the CAN transmitter to verify itself whenever it transmits. While transmitting on the Tx pin, it also listens on the Rx pin to ensure it receives the same data being transmitted. If it doesn't, then it stops transmitting.

Therefore, if you are not using a CAN PHY that loops the Tx back to Rx, then you need to short the Tx pin on your Tower system to the Rx pin. Otherwise, you won't see any data transmitting on Tx.

And if that isn't enough to help you, ensure the pin mux for your CAN pins are set for the CAN peripheral. The pin mux is set in the PORTx_PCR registers.

[christos]

Hi Derek,

  Thank you very much for your reply. That makes sense now, as you say it needs the RX because it needs to determine the bus arbitration status. I believe this must be the problem indeed.

   I have a TWR-IND-IO board which has CAN PHY. I will give that a try ASAP and mark your answer as correct upon success.

Cheers,

Christos

[christos]

Hi Derek,

   Thank you very much for your help. I have actually have solved the problem and it was indeed what you advised  the CAN PHY was NOT connected.

  Just in case someone else has the same issue these are the modification I had to do to make the TWR-K20D72M work with the TWR-IND-IO.

• Populate 0Ohm resistors R25 and R26 on the TWR-K20D72M
• Populate 0Ohm resistors R10 and R11 on the TWR-IND-IO
• Depopulate (desolder) sholder bridges SH1 and SH2 (not sure if  this mode is necessary but I would rather have them disconnected because the K20 is routing VREFA1 and VREFA2 on the solder bridges).

Once again thank you very much.

Best regards,

Christos