Defining CAN Signals and Messages
To get a higher level of abstraction, the CAN signals and messages can be used to abstract the information mappings to the bus communication objects.
Example: The information Nodestatus and CPU-Load shall be sent in a CAN frame called Status with the following definition:
1. CAN-Identifier = 0x150
2. DLC = 2
3. Payload Byte 0 = Current Nodestatus
4. Payload Byte 1 = CPU-Load
The Nodestatus shall be settable via the CAN frame called Command with the following definition:
5. CAN-Identifier = 0x140
6. DLC = 1
7. Payload Byte 0 = New Nodestatus
If the information Nodestatus is changed, the following actions shall be done:
8. New Nodestatus = 1: Start the task Load-Task
9. New Nodestatus = 2: Stop the task Load-Task
Source code (Part 1):
#include can_sig.h
/* Signal Definition */
enum {
S_NODESTATUS = 0, S_CPULOAD, S_MAX
};
const CANSIG_PARA CanSig[CANSIG_N] = {
/*--- S_NODESTATUS */
{ CANSIG_UNCHANGED, /* Initial Status */
1, /* Width in Bytes */
0, /* Initial Value */
StatusChange }, /* Callback Func. */
/*------ S_CPULOAD */
{ CANSIG_UNCHANGED, /* Initial Status */
1, /* Width in Bytes */
0, /* Initial Value */
0 } /* No Callback */
};
Additional Information
1. The CAN signals can be defined without any CAN communication knowledge. No information about sending or receiving the information is needed during this state.
2. The enumeration is used to simplify the message definition and ensure to get a consistent message to signal mapping.
3. The CAN signal configuration is done as a global constant to safe RAM space and get a write protected configuration.
4. This variable must be declared globally, because the configuration data will be read while using the CAN signal layer.
5. The callback function definition may be omitted by configuring the CANSIG_CALLBACK_EN parameter to 0.
Source code (Part 2):
#include can_msg.h
/* Message Definition */
enum {
M_STATUS = 0, M_COMMAND, M_MAX
};
const CANMSG_PARA CanMsg[CANMSG_N] = {
/*------- M_STATUS */
{ 0x150L, /* CAN-Identifier */
CANMSG_TX, /* Message Type */
2, /* DLC of Message */
2, /* No. of Links */
{ { S_NODESTATUS, /* Signal ID */
0 }, /* Byte Position */
{ S_CPULOAD, /* Signal ID */
1 } } }, /* Byte Position */
/*------ M_COMMAND */
{ 0x140L, /* CAN-Identifier */
CANMSG_RX, /* Message Type */
1, /* DLC of Message */
1, /* No. of Links */
{ { S_NODESTATUS, /* Signal ID */
0 } } } /* Byte Position */
};
Additional Information
1. The CAN messages can be defined without any knowledge of the information generation and/or usage of the payload. Only the mapping of information parts to the payload must be known.
2. The enumeration is used to simplify the message handling.
3. The CAN message configuration is done as a global constant to safe RAM space and get a write protected configuration.
4. This variable must be declared globally, because the configuration data will be read while using the CAN message layer.