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
  • CANSetTxIdleLevelConnect CAN Protocol with PIC

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,
Connect CAN Protocol with PIC schematich_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 &

 

 

For more detail: Connect CAN Protocol with PIC




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