Demonstrations for CAN operation. 
Prerequirement: 
(1)demo board don't have CAN transceiver, need to use out board transceiver;
(2)120 ohm resistor shoule be sealed in one board(master board or slave board) if the distance is near; if the distance is far, both boards should be sealed 120 ohm resistor. 
(3)Use PC4 for CAN_TX, PC3 for CAN_RX.

WIRE Connect: Master Board         Slave Board 
                CAN_H    <------>   CAN_H
				CAN_L    <------>   CAN_L
				
1. CAN receive and transmit demo : demo master board communicates with slave board through CAN.
Pay attention : 
(1)Master board software and slave board software are controled by "#define MASTER_BOARD      1"; 
(2)Run master board first, then run slave board.
   Master board waits for message from slave, if receives success, send repsonse to slave. Then waits for message from slave again. 
Slave board sends message to master, then waits for response, if success, then send the next message.
