ElectRoidWarE: Connect CAN Protocol with PIC

The mikroC PRO for PIC provides a library (driver) for working with the CAN module.

The CAN is a very robust protocol that has error detection and signalization, self–checking and fault confinement. Faulty CAN data and remote frames are re-transmitted automatically, similar to the Ethernet.

Data transfer rates depends on the distance. For example, 1 Mbit/s can be achieved at network lengths below 40m while 250 Kbit/s can be achieved at network lengths below 250m. The greater distance the lower maximum bitrate that can be achieved. The lowest bitrate defined by the standard is 200Kbit/s. Cables used are shielded twisted pairs.

CAN supports two message formats:

Standard format, with 11 identifier bits, and
Extended format, with 29 identifier bits

Important :

Consult the CAN standard about CAN bus termination resistance.

Library Routines

CANSetOperationMode
CANGetOperationMode
CANInitialize
CANSetBaudRate
CANSetMask
CANSetFilter
CANRead
CANWrite
CANSetTxIdleLevel

CANSetOperationMode

Prototype	void CANSetOperationMode(unsigned short mode, unsigned short wait_flag);
Returns	Nothing.
Description	Sets CAN to requested mode, i.e. copies `mode` to CANSTAT. Parameter `mode` needs to be one of `CAN_OP_MODE` constants . Parameter `wait_flag` needs to be either 0 or 0xFF: If set to 0xFF, this is a blocking call – the function won’t “return” until the requested mode is set. If 0, this is a non-blocking call. It does not verify if CAN module is switched to requested mode or not. Caller must use `CANGetOperationMode` to verify correct operation mode before performing mode specific operation.
Requires	Microcontroller must be connected to CAN transceiver (MCP2551 or similar) which is connected to CAN bus.
Example	CANSetOperationMode(_CAN_MODE_CONFIG, 0xFF);

CANGetOperationMode

Prototype	unsigned short CANGetOperationMode();
Returns	Current opmode.
Description	Function returns current operational mode of CAN module.
Requires	Microcontroller must be connected to CAN transceiver (MCP2551 or similar) which is connected to CAN bus.
Example	if (CANGetOperationMode() == _CAN_MODE_NORMAL) { ... };

CANInitialize

Prototype	void CANInitialize(char SJW, char BRP, char PHSEG1, char PHSEG2, char PROPSEG, char CAN_CONFIG_FLAGS);
Returns	Nothing.
Description	Initializes CAN. All pending transmissions are aborted. Sets all mask registers to 0 to allow all messages. Filter registers are set according to flag value: if (CAN_CONFIG_FLAGS & _CAN_CONFIG_VALID_XTD_MSG != 0) // Set all filters to XTD_MSG else if (config & _CAN_CONFIG_VALID_STD_MSG != 0) // Set all filters to STD_MSG else // Set half of the filters to STD, and the rest to XTD_MSG. Parameters: `SJW` as defined in datasheet (1–4) `BRP` as defined in datasheet (1–64) `PHSEG1` as defined in datasheet (1–8) `PHSEG2` as defined in datasheet (1–8) `PROPSEG` as defined in datasheet (1–8) `CAN_CONFIG_FLAGS` is formed from predefined constants
Requires	CAN must be in Config mode; otherwise the function will be ignored. Microcontroller must be connected to CAN transceiver (MCP2551 or similar) which is connected to CAN bus.
Example	init = _CAN_CONFIG_SAMPLE_THRICE & _CAN_CONFIG_PHSEG2_PRG_ON & _CAN_CONFIG_STD_MSG & _CAN_CONFIG_DBL_BUFFER_ON & _CAN_CONFIG_VALID_XTD_MSG & _CAN_CONFIG_LINE_FILTER_OFF; ... CANInitialize(1, 1, 3, 3, 1, init); // initialize CAN

CANSetBaudRate

Prototype	void CANSetBaudRate(char SJW, char BRP, char PHSEG1, char PHSEG2, char PROPSEG, char CAN_CONFIG_FLAGS);
Returns	Nothing.
Description	Sets CAN baud rate. Due to complexity of CAN protocol, you cannot simply force a bps value. Instead, use this function when CAN is in Config mode. Refer to datasheet for details. Parameters: `SJW` as defined in datasheet (1–4) `BRP` as defined in datasheet (1–64) `PHSEG1` as defined in datasheet (1–8) `PHSEG2` as defined in datasheet (1–8) `PROPSEG` as defined in datasheet (1–8) `CAN_CONFIG_FLAGS` is formed from predefined constants
Requires	CAN must be in Config mode; otherwise the function will be ignored. Microcontroller must be connected to CAN transceiver (MCP2551 or similar) which is connected to CAN bus.
Example	init = _CAN_CONFIG_SAMPLE_THRICE & _CAN_CONFIG_PHSEG2_PRG_ON & _CAN_CONFIG_STD_MSG & _CAN_CONFIG_DBL_BUFFER_ON & _CAN_CONFIG_VALID_XTD_MSG & _CAN_CONFIG_LINE_FILTER_OFF; ... CANSetBaudRate(1, 1, 3, 3, 1, init);

CANSetMask

Prototype	void CANSetMask(char CAN_MASK, long value, char CAN_CONFIG_FLAGS);
Returns	Nothing.
Description	Function sets mask for advanced filtering of messages. Given `value` is bit adjusted to appropriate buffer mask registers. Parameters: `CAN_MASK` is one of predefined constant values `value` is the mask register value `CAN_CONFIG_FLAGS` selects type of message to filter, either `_CAN_CONFIG_XTD_MSG` or `_CAN_CONFIG_STD_MSG`
Requires	CAN must be in Config mode; otherwise the function will be ignored. Microcontroller must be connected to CAN transceiver (MCP2551 or similar) which is connected to CAN bus.
Example	// Set all mask bits to 1, i.e. all filtered bits are relevant: CANSetMask(_CAN_MASK_B1, -1, _CAN_CONFIG_XTD_MSG); // Note that -1 is just a cheaper way to write 0xFFFFFFFF. Complement will do the trick and fill it up with ones.

CANSetFilter

Prototype	void CANSetFilter(char CAN_FILTER, long value, char CAN_CONFIG_FLAGS);
Returns	Nothing.
Description	Function sets message filter. Given `value` is bit adjusted to appropriate buffer mask registers. Parameters: `CAN_FILTER` is one of predefined constant values `value` is the filter register value `CAN_CONFIG_FLAGS` selects type of message to filter, either `_CAN_CONFIG_XTD_MSG` or `_CAN_CONFIG_STD_MSG`
Requires	CAN must be in Config mode; otherwise the function will be ignored. Microcontroller must be connected to CAN transceiver (MCP2551 or similar) which is connected to CAN bus.
Example	// Set id of filter B1_F1 to 3: CANSetFilter(_CAN_FILTER_B1_F1, 3, _CAN_CONFIG_XTD_MSG);

CANRead

Prototype	char CANRead(long id, char* data, char* datalen, char* *CAN_RX_MSG_FLAGS);
Returns	Message from receive buffer or zero if no message found.
Description	Function reads message from receive buffer. If at least one full receive buffer is found, it is extracted and returned. If none found, function returns zero. Parameters: `id` is message identifier `data` is an array of bytes up to 8 bytes in length `datalen` is data length, from 1–8. `CAN_RX_MSG_FLAGS` is value formed from constants
Requires	CAN must be in mode in which receiving is possible. Microcontroller must be connected to CAN transceiver (MCP2551 or similar) which is connected to CAN bus.
Example	char rcv, rx, len, data[8]; long id; // ... rx = 0; // ... rcv = CANRead(id, data, len, rx);

CANWrite

Prototype	unsigned short CANWrite(long id, char data, char* datalen, char CAN_TX_MSG_FLAGS);
Returns	Returns zero if message cannot be queued (buffer full).
Description	If at least one empty transmit buffer is found, function sends message on queue for transmission. If buffer is full, function returns 0. Parameters: `id` is CAN message identifier. Only 11 or 29 bits may be used depending on message type (standard or extended) `data` is array of bytes up to 8 bytes in length `datalen` is data length from 1–8 `CAN_TX_MSG_FLAGS` is value formed from constants
Requires	CAN must be in Normal mode. Microcontroller must be connected to CAN transceiver (MCP2551 or similar) which is connected to CAN bus.
Example	char tx, data; long id; // ... tx = _CAN_TX_PRIORITY_0 & _CAN_TX_XTD_FRAME; // ... CANWrite(id, data, 2, tx);

CANSetTxIdleLevel

Prototype void CANSetTxIdleLevel(char driveHighState);

Returns Nothing.

Description

This function sets the state of CANTX pin when recessive.
Parameters:

driveHighState: State of the CANTX pin. Valid values :

Description	Predefined library const
CANTX pin will drive VDD when recessive. Use it when using a differential bus to avoid signal crosstalk in CANTX from other nearby pins.	`_CAN_DRIVE_HIGH_STATE_ENABLE`
CANTX pin will be tri-state when recessive.	`_CAN_DRIVE_HIGH_STATE_DISABLE`

Requires Microcontroller must be connected to CAN transceiver (MCP2551 or similar) which is connected to CAN bus.

Example

CANSetTxIdleLevel(_CAN_DRIVE_HIGH_STATE_ENABLE);

CAN Constants

There is a number of constants predefined in CAN library. To be able to use the library effectively, you need to be familiar with these. You might want to check the example at the end of the chapter.

CAN_OP_MODE

CAN_OP_MODE constants define CAN operation mode. Function CANSetOperationMode expects one of these as its argument:


const char

    _CAN_MODE_BITS   = 0xE0,   // Use this to access opmode  bits

    _CAN_MODE_NORMAL = 0x00,
    _CAN_MODE_SLEEP  = 0x20,

    _CAN_MODE_LISTEN = 0x60,
    _CAN_MODE_LOOP   = 0x40,

    _CAN_MODE_CONFIG = 0x80;

CAN_CONFIG_FLAGS

CAN_CONFIG_FLAGS constants define flags related to CAN module configuration. Functions CANInitialize and CANSetBaudRate expect one of these (or a bitwise combination) as their argument:


const char

    _CAN_CONFIG_DEFAULT         = 0xFF,   // 11111111

    _CAN_CONFIG_PHSEG2_PRG_BIT  = 0x01,

    _CAN_CONFIG_PHSEG2_PRG_ON   = 0xFF,   // XXXXXXX1

    _CAN_CONFIG_PHSEG2_PRG_OFF  = 0xFE,   // XXXXXXX0

    _CAN_CONFIG_LINE_FILTER_BIT = 0x02,

    _CAN_CONFIG_LINE_FILTER_ON  = 0xFF,   // XXXXXX1X

    _CAN_CONFIG_LINE_FILTER_OFF = 0xFD,   // XXXXXX0X

    _CAN_CONFIG_SAMPLE_BIT      = 0x04,

    _CAN_CONFIG_SAMPLE_ONCE     = 0xFF,   // XXXXX1XX

    _CAN_CONFIG_SAMPLE_THRICE   = 0xFB,   // XXXXX0XX

    _CAN_CONFIG_MSG_TYPE_BIT    = 0x08,

    _CAN_CONFIG_STD_MSG         = 0xFF,   // XXXX1XXX

    _CAN_CONFIG_XTD_MSG         = 0xF7,   // XXXX0XXX

    _CAN_CONFIG_DBL_BUFFER_BIT  = 0x10,

    _CAN_CONFIG_DBL_BUFFER_ON   = 0xFF,   // XXX1XXXX

    _CAN_CONFIG_DBL_BUFFER_OFF  = 0xEF,   // XXX0XXXX

    _CAN_CONFIG_MSG_BITS        = 0x60,

    _CAN_CONFIG_ALL_MSG         = 0xFF,   // X11XXXXX

    _CAN_CONFIG_VALID_XTD_MSG   = 0xDF,   // X10XXXXX

    _CAN_CONFIG_VALID_STD_MSG   = 0xBF,   // X01XXXXX

    _CAN_CONFIG_ALL_VALID_MSG   = 0x9F;   // X00XXXXX

You may use bitwise AND (&) to form config byte out of these values. For example:


init = _CAN_CONFIG_SAMPLE_THRICE &
       _CAN_CONFIG_PHSEG2_PRG_ON &

       _CAN_CONFIG_DBL_BUFFER_ON &
_CAN_CONFIG_STD_MSG       &
       _CAN_CONFIG_VALID_XTD_MSG &

CANInitialize(1, 1, 3, 3, 1, init);   // initialize CAN
_CAN_CONFIG_LINE_FILTER_OFF;

...

CAN_TX_MSG_FLAGS

CAN_TX_MSG_FLAGS are flags related to transmission of a CAN message:


const char

    _CAN_TX_PRIORITY_BITS = 0x03,

    _CAN_TX_PRIORITY_0    = 0xFC,   // XXXXXX00

    _CAN_TX_PRIORITY_1    = 0xFD,   // XXXXXX01

    _CAN_TX_PRIORITY_2    = 0xFE,   // XXXXXX10

    _CAN_TX_PRIORITY_3    = 0xFF,   // XXXXXX11

    _CAN_TX_FRAME_BIT    = 0x08,

    _CAN_TX_STD_FRAME    = 0xFF,   // XXXXX1XX

    _CAN_TX_XTD_FRAME    = 0xF7,   // XXXXX0XX

    _CAN_TX_RTR_BIT      = 0x40,

    _CAN_TX_NO_RTR_FRAME = 0xFF,   // X1XXXXXX

    _CAN_TX_RTR_FRAME    = 0xBF;    // X0XXXXXX

You may use bitwise AND (&) to adjust the appropriate flags. For example:


// form value to be used with CANSendMessage:

send_config = _CAN_TX_PRIORITY_0 &
_CAN_TX_XTD_FRAME  &

              _CAN_TX_NO_RTR_FRAME;
...

CANSendMessage(id, data, 1, send_config);

CAN_RX_MSG_FLAGS

CAN_RX_MSG_FLAGS are flags related to reception of CAN message. If a particular bit is set; corresponding meaning is TRUE or else it will be FALSE.


const char

    _CAN_RX_FILTER_BITS  = 0x07,  // Use this to access filter bits

    _CAN_RX_FILTER_1     = 0x00,
_CAN_RX_FILTER_2     = 0x01,

    _CAN_RX_FILTER_4     = 0x03,
_CAN_RX_FILTER_3     = 0x02,
    _CAN_RX_FILTER_5     = 0x04,

    _CAN_RX_OVERFLOW     = 0x08,  // Set if Overflowed else cleared
_CAN_RX_FILTER_6     = 0x05,


_CAN_RX_INVALID_MSG  = 0x10,  // Set if invalid else cleared

    _CAN_RX_XTD_FRAME    = 0x20,  // Set if XTD message else cleared

    _CAN_RX_RTR_FRAME    = 0x40,  // Set if RTR message else cleared

    _CAN_RX_DBL_BUFFERED = 0x80;  // Set if this message was hardware double-buffered

You may use bitwise AND (&) to adjust the appropriate flags. For example:


if (MsgFlag & _CAN_RX_OVERFLOW != 0) {
...

  // Receiver overflow has occurred.

  // We have lost our previous message.

}

CAN_MASK

CAN_MASK constants define mask codes. Function CANSetMask expects one of these as its argument:


#const char

    _CAN_MASK_B1 = 0,

    _CAN_MASK_B2 = 1;

CAN_FILTER

CAN_FILTER constants define filter codes. Function CANSetFilter expects one of these as its argument:


const char

    _CAN_FILTER_B1_F1 = 0,
_CAN_FILTER_B1_F2 = 1,

    _CAN_FILTER_B2_F2 = 3,
_CAN_FILTER_B2_F1 = 2,
    _CAN_FILTER_B2_F3 = 4,

    _CAN_FILTER_B2_F4 = 5;

Library Example

This is a simple demonstration of CAN Library routines usage. First node initiates the communication with the second node by sending some data to its address. The second node responds by sending back the data incremented by 1. First node then does the same and sends incremented data back to second node, etc.

Code for the first CAN node:


unsigned char Can_Init_Flags, Can_Send_Flags, Can_Rcv_Flags; // can flags

unsigned char Rx_Data_Len;                                   // received data length in bytes

char RxTx_Data[8];                                           // can rx/tx data buffer

char Msg_Rcvd;                                               // reception flag

const long ID_1st = 12111, ID_2nd = 3;                       // node IDs

long Rx_ID;



void main() {

  PORTC = 0;                                                // clear PORTC


TRISC = 0;                                                // set PORTC as output

  Can_Init_Flags = 0;                                       //


Can_Send_Flags = 0;                                       // clear flags

  Can_Rcv_Flags  = 0;                                       //

  Can_Send_Flags = _CAN_TX_PRIORITY_0 &                     // form value to be used

                   _CAN_TX_XTD_FRAME &                      //     with CANWrite

                   _CAN_TX_NO_RTR_FRAME;

  Can_Init_Flags = _CAN_CONFIG_SAMPLE_THRICE &              // form value to be used

                   _CAN_CONFIG_PHSEG2_PRG_ON &              // with CANInit

                   _CAN_CONFIG_XTD_MSG &

                   _CAN_CONFIG_DBL_BUFFER_ON &
_CAN_CONFIG_VALID_XTD_MSG;

  CANInitialize(1,3,3,3,1,Can_Init_Flags);                  // Initialize CAN module


CANSetOperationMode(_CAN_MODE_CONFIG,0xFF);               // set CONFIGURATION mode

  CANSetMask(_CAN_MASK_B1,-1,_CAN_CONFIG_XTD_MSG);          // set all mask1 bits to ones

  CANSetMask(_CAN_MASK_B2,-1,_CAN_CONFIG_XTD_MSG);          // set all mask2 bits to ones

  CANSetFilter(_CAN_FILTER_B2_F4,ID_2nd,_CAN_CONFIG_XTD_MSG);// set id of filter B2_F4 to 2nd node ID

  CANSetOperationMode(_CAN_MODE_NORMAL,0xFF);               // set NORMAL mode

  RxTx_Data[0] = 9;                                         // set initial data to be sent

  CANWrite(ID_1st, RxTx_Data, 1, Can_Send_Flags);           // send initial message

  while(1) {                                                               // endless loop

    Msg_Rcvd = CANRead(&Rx_ID , RxTx_Data , &Rx_Data_Len, &Can_Rcv_Flags); // receive message

    if ((Rx_ID == ID_2nd) && Msg_Rcvd) {                                   // if message received check id

      PORTC = RxTx_Data[0];                                                // id correct, output data at PORTC

      RxTx_Data[0]++ ;                                                     // increment received data

      Delay_ms(10);

      CANWrite(ID_1st, RxTx_Data, 1, Can_Send_Flags);                      // send incremented data back

    }
}

}

Code for the second CAN node:


unsigned char Can_Init_Flags, Can_Send_Flags, Can_Rcv_Flags; // can flags

unsigned char Rx_Data_Len;                                   // received data length in bytes

char RxTx_Data[8];                                           // can rx/tx data buffer

char Msg_Rcvd;                                               // reception flag

const long ID_1st = 12111, ID_2nd = 3;                       // node IDs

long Rx_ID;



void main() {

  PORTC = 0;                                                // clear PORTC


TRISC = 0;                                                // set PORTC as output

  Can_Init_Flags = 0;                                       //


Can_Send_Flags = 0;                                       // clear flags

  Can_Rcv_Flags  = 0;                                       //

  Can_Send_Flags = _CAN_TX_PRIORITY_0 &                     // form value to be used

                   _CAN_TX_XTD_FRAME &                      //     with CANWrite

                   _CAN_TX_NO_RTR_FRAME;

  Can_Init_Flags = _CAN_CONFIG_SAMPLE_THRICE &              // form value to be used

                   _CAN_CONFIG_PHSEG2_PRG_ON &              // with CANInit

                   _CAN_CONFIG_XTD_MSG &

                   _CAN_CONFIG_DBL_BUFFER_ON &
_CAN_CONFIG_VALID_XTD_MSG &

                   _CAN_CONFIG_LINE_FILTER_OFF;

  CANInitialize(1,3,3,3,1,Can_Init_Flags);                  // initialize external CAN module

  CANSetOperationMode(_CAN_MODE_CONFIG,0xFF);               // set CONFIGURATION mode

  CANSetMask(_CAN_MASK_B1,-1,_CAN_CONFIG_XTD_MSG);          // set all mask1 bits to ones

  CANSetMask(_CAN_MASK_B2,-1,_CAN_CONFIG_XTD_MSG);          // set all mask2 bits to ones

  CANSetFilter(_CAN_FILTER_B2_F3,ID_1st,_CAN_CONFIG_XTD_MSG);// set id of filter B2_F3 to 1st node ID

  CANSetOperationMode(_CAN_MODE_NORMAL,0xFF);               // set NORMAL mode

  while (1) {                                                              // endless loop

    Msg_Rcvd = CANRead(&Rx_ID , RxTx_Data , &Rx_Data_Len, &Can_Rcv_Flags); // receive message

    if ((Rx_ID == ID_1st) && Msg_Rcvd) {                                   // if message received check id

      PORTC = RxTx_Data[0];                                                // id correct, output data at PORTC

      RxTx_Data[0]++ ;                                                     // increment received data

      CANWrite(ID_2nd, RxTx_Data, 1, Can_Send_Flags);                      // send incremented data back

    }
}

}