Controller Area Network (CAN) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other without a host computer. Originally developed by Bosch for automotive applications, CAN-Bus has become a widely adopted communication protocol in automotive, industrial, and embedded systems due to its reliability, real-time performance, and fault tolerance.

CAN-Bus features:

• Multi-master, multi-drop serial communication protocol
• Message-based communication (not address-based)
• Built-in error detection and fault confinement
• Real-time capabilities with deterministic latency
• High noise immunity and fault tolerance
• Automatic retransmission of corrupted messages
• Priority-based message arbitration
• Operating speeds from 10 kbps to 1 Mbps
• Network length up to 1000m at lower speeds
• Support for up to 110 nodes (practical limit ~64 nodes)

CAN networks use a linear bus topology with termination resistors at both ends:

    ECU 1         ECU 2         ECU 3         ECU 4
     │             │             │             │
     │             │             │             │
 ────┴─────────────┴─────────────┴─────────────┴────
CAN_H ═══════════════════════════════════════════════
                                                    
CAN_L ═══════════════════════════════════════════════
 ────┬─────────────┬─────────────┬─────────────┬────
     │             │             │             │
   [120Ω]                                    [120Ω]
Termination                              Termination
 Resistor                                 Resistor

• 120Ω termination resistors required at both ends of the bus
• Total bus resistance: 60Ω (two 120Ω resistors in parallel)
• Stub lengths: Maximum 0.3m per node connection
• Unterminated network: Results in signal reflections and errors

SOF  Identifier  RTR IDE r0  DLC    Data Field     CRC    CRC  ACK ACK EOF  IFS
 │   ←─11 bits─→  │   │   │  ←4bits→ ←─0 to 64 bits─→ ←15bits→ DEL │  DEL ←7bits→
 │               │   │   │          │                      │   │   │
 1      ID        0   0   0       Data                   CRC  0   1   1111111
bit   (0-2047)   bit bit bit    (0-8 bytes)            Field bit bit  bits

Field Breakdown:
SOF    = Start of Frame (1 bit dominant)
ID     = Message Identifier (11 bits)
RTR    = Remote Transmission Request (0=data, 1=remote)
IDE    = Identifier Extension (0=standard, 1=extended)
r0     = Reserved bit (must be 0)
DLC    = Data Length Code (4 bits, 0-8 bytes)
Data   = Data payload (0-64 bits)
CRC    = Cyclic Redundancy Check (15 bits)
CRC_D  = CRC Delimiter (1 bit recessive)
ACK    = Acknowledge slot (1 bit)
ACK_D  = ACK Delimiter (1 bit recessive)
EOF    = End of Frame (7 bits recessive)
IFS    = Inter Frame Space (3 bits minimum)

SOF  Base ID  SRR IDE  Extended ID   RTR r1 r0 DLC   Data Field    CRC   CRC ACK ACK EOF
 │  ←11bits→  │   │   ←──18 bits───→  │   │  │ ←4b→ ←0 to 64 bits→ ←15b→ DEL │  DEL ←7b→
 1     ID     1   1       ID         0   0  0        Data         CRC   0   1   1  1111111

Extended Frame Additional Fields:
SRR    = Substitute Remote Request (1 bit recessive)
IDE    = Identifier Extension (1=extended frame)
Ext_ID = Extended Identifier (18 bits)
r1     = Reserved bit (must be 0)

• Purpose: Transmit data from sender to receivers
• RTR bit: 0 (dominant)
• Data field: 0-8 bytes of payload data
• Most common: Regular communication messages

• Purpose: Request data from specific node
• RTR bit: 1 (recessive)
• Data field: Empty (DLC indicates requested data length)
• Response: Node responds with corresponding data frame

• Purpose: Signal detection of bus errors
• Structure: Error flag (6 bits) + Error delimiter (8 bits)
• Transmission: Any node detecting error
• Effect: Destroys current message, forces retransmission

• Purpose: Provide extra delay between frames
• Structure: Similar to error frame
• Usage: Node needs more time to process messages

• Lower identifier = Higher priority
• Bitwise arbitration: Dominant bits (0) win over recessive bits (1)
• Non-destructive: Losing nodes automatically become receivers

Time    Node A (ID=0x123)  Node B (ID=0x124)  Node C (ID=0x200)  Bus State
t0      Starts transmission Starts transmission Starts transmission  
t1      Sends 0 (dominant)  Sends 0 (dominant)  Sends 0 (dominant)   0
t2      Sends 0 (dominant)  Sends 0 (dominant)  Sends 0 (dominant)   0
t3      Sends 0 (dominant)  Sends 0 (dominant)  Sends 1 (recessive)  0
t3+     Continues          Continues          Stops (lost arbit.)    
t4      Sends 1 (recessive) Sends 0 (dominant) Silent               0
t4+     Stops (lost arbit.) Continues          Silent               
Result: Node B wins arbitration and transmits message

Explanation:
- All nodes start simultaneously
- Node C loses at bit 3 (sends 1, but bus shows 0)
- Node A loses at bit 4 (sends 1, but Node B sends 0)
- Node B has lowest ID, wins arbitration

• Cable specifications: Twisted pair, shielded
• Connector types: DB9, OBD-II, proprietary
• Termination: 120Ω at both ends
• Maximum nodes: 110 theoretical, 64 practical

• Frame formatting: Standard and extended frames
• Error detection: CRC, bit monitoring, frame check
• Error signaling: Error and overload frames
• Fault confinement: Error counters and states

• Detection: Node monitors its own transmission
• Condition: Transmitted bit differs from bus bit
• Exception: Arbitration and acknowledge phases
• Action: Transmit error frame

• Detection: More than 5 consecutive identical bits
• Bit stuffing rule: Insert complementary bit after 5 identical bits
• Purpose: Maintains synchronization
• Action: Receiver sends error frame

• Detection: Received CRC doesn't match calculated CRC
• Coverage: SOF to end of data field
• Polynomial: x^15 + x^14 + x^10 + x^8 + x^7 + x^4 + x^3 + 1
• Action: Receiver sends error frame

• Detection: Invalid frame format
• Examples: Wrong delimiter bits, invalid reserved bits
• Action: Error frame transmission

• Detection: No acknowledgment received
• Condition: No node acknowledges valid frame
• Cause: All receivers detected errors
• Action: Transmitter sends error frame

• Normal operation: Can transmit and receive normally
• Error counters: Both TEC and REC ≤ 127
• Error frames: Transmits active error frames (6 dominant bits)

• Restricted operation: Can receive normally, limited transmission
• Error counters: TEC or REC between 128-255
• Error frames: Transmits passive error frames (6 recessive bits)
• Transmission: Must wait 8 bit times before retransmission

• No communication: Cannot transmit or receive
• Error counter: TEC ≥ 256
• Recovery: Automatic after 128 occurrences of 11 recessive bits
• Manual intervention: May require application reset

Transmit Error Counter (TEC) Rules:
- Increment by 8 for each error during transmission
- Increment by 1 for each error during reception
- Decrement by 1 for each successful transmission
- Set to 0 when bus off recovery completes

Receive Error Counter (REC) Rules:  
- Increment by 1 for each error during reception
- Increment by 8 if node transmits error frame during reception
- Decrement by 1 for each successful reception (if >127)
- Stays at 0-127 range during error active state

CAN Bit Time Structure:
 
  Sync_Seg   Prop_Seg    Phase_Seg1    Phase_Seg2
     │           │            │            │
     │←─1TQ─→│←─1-8TQ─→│←─1-8TQ─→│←─2-8TQ─→│
     │       │         │         │         │
   Sync    Propagation  Phase     Phase
 Segment    Delay      Buffer1   Buffer2
     │                     │         │
     └─────Sample Point────┴─────────┘

Parameters:
- Sync_Seg: 1 Time Quantum (TQ) - synchronization
- Prop_Seg: 1-8 TQ - signal propagation delay
- Phase_Seg1: 1-8 TQ - phase buffer before sample point
- Phase_Seg2: 2-8 TQ - phase buffer after sample point
- Sample Point: Typically at 87.5% of bit time
- Total Bit Time: Sync_Seg + Prop_Seg + Phase_Seg1 + Phase_Seg2

System Clock: 16 MHz
Desired Baud Rate: 500 kbps
Bit Time: 2 μs (1/500k)

Time Quantum (TQ) = System Clock / (BRP × (1 + Prop_Seg + Phase_Seg1 + Phase_Seg2))

Example Configuration:
BRP (Baud Rate Prescaler): 2
Sync_Seg: 1 TQ
Prop_Seg: 2 TQ  
Phase_Seg1: 3 TQ
Phase_Seg2: 2 TQ
Total TQ: 8

TQ = 16MHz / (2 × 8) = 1MHz → 1μs per TQ
Bit Time = 8 × 1μs = 8μs? No, this gives 125kbps

Correct Calculation:
BRP = 4, Total TQ = 8
TQ = 16MHz / 4 = 4MHz → 0.25μs per TQ  
Bit Time = 8 × 0.25μs = 2μs = 500kbps ✓

[Node1]─┬─────────[Node2]─┬─────────[Node3]─┬─────────[Node4]
        │                 │                 │               
      [120Ω]             │                 │             [120Ω]
   Termination           │                 │          Termination
                         │                 │
                    [Node5]           [Node6]
                    
Maximum stub length: 0.3m per connection
Total bus length: Depends on baud rate

                    [Node1]
                       │
                       │
[Node4]────────[CAN Hub]────────[Node2]  
                       │
                       │
                    [Node3]
                    
Advantages: Centralized control, easier diagnostics
Disadvantages: Single point of failure, more complex

MCP2551 Pinout (8-pin DIP/SOIC):
     ┌─────┐
TxD  │1   8│  VDD (+5V)
VSS  │2   7│  CANH
VRef │3   6│  CANL  
RxD  │4   5│  Rs
     └─────┘

Pin Functions:
1. TxD: Transmit Data Input (from microcontroller)
2. VSS: Ground
3. VRef: Reference voltage output (VDD/2)
4. RxD: Receive Data Output (to microcontroller)
5. Rs: Slope control (normal/standby mode)
6. CANL: CAN Low bus line
7. CANH: CAN High bus line  
8. VDD: Power supply (+5V)

• Application layer: Standardized device profiles
• Object Dictionary: Structured data access
• PDO: Process Data Objects for real-time data
• SDO: Service Data Objects for configuration
• Network Management: Node guarding, heartbeat
• Applications: Industrial automation, robotics

• Based on: CAN with DeviceNet application layer
• Power and data: Single cable solution
• Device profiles: Standardized device behavior
• Configuration: Network configuration tools
• Applications: Factory automation, discrete manufacturing

• Automotive standard: Heavy duty vehicles
• 29-bit identifiers: Extended CAN frames
• Parameter Groups: Standardized data definitions
• Transport protocol: Multi-packet messages
• Applications: Trucks, buses, agricultural equipment

• Diagnostic protocol: Vehicle diagnostics
• UDS support: Unified Diagnostic Services
• Flow control: Multi-frame message handling
• Standardized PIDs: Parameter identification
• Applications: Automotive diagnostics, emissions testing

• Increased payload: Up to 64 bytes per frame (vs 8 bytes)
• Variable data rate: Faster transmission in data phase
• Improved CRC: Enhanced error detection
• Backward compatibility: Coexists with classic CAN

Arbitration Phase (Classic CAN speed):
SOF│ID│RTR│IDE│FDF│r0│BRS│ESI│DLC│

Data Phase (Higher speed):
       Data Field (up to 64 bytes)

CRC Phase (Classic CAN speed):
CRC│DEL│ACK│DEL│EOF

New Bits:
FDF = FD Format (1 = CAN-FD frame)
BRS = Bit Rate Switch (1 = switch to fast data rate)  
ESI = Error State Indicator

• Twisted pair: Reduces electromagnetic interference
• Shielded: Additional EMI protection in noisy environments
• Characteristic impedance: 120Ω ± 5%
• Capacitance: <100 pF/m between conductors
• Wire gauge: 18-24 AWG typical

• Propagation delay: Must be <2 bit times total
• Signal quality: Maintain rise/fall times
• Reflections: Proper termination essential
• Stub lengths: Maximum 0.3m per connection

• Theoretical maximum: 110 nodes (address space)
• Practical maximum: 64 nodes (electrical loading)
• Bus loading: Each node adds capacitance
• Current consumption: Termination and transceiver limits

Proper CAN Network Grounding:

Power Supply Ground ──┬── Node 1 Ground
                      ├── Node 2 Ground  
                      ├── Node 3 Ground
                      └── Shield Ground (one point only)

Guidelines:
- Single point grounding preferred
- Avoid ground loops
- Shield connection at one end only
- Separate digital and analog grounds in nodes

• Twisted pair cables: Reduces radiated emissions
• Shielding: Prevents interference pickup
• Ferrite cores: Suppress common-mode noise
• Proper PCB layout: Ground planes, trace routing
• Isolation: Galvanic isolation in harsh environments

#include <SPI.h>
#include <mcp2515.h>

MCP2515 mcp2515(10); // CS pin 10

struct can_frame canMsg;

void setup() {
  Serial.begin(9600);
  mcp2515.reset();
  mcp2515.setBitrate(CAN_500KBPS, MCP_8MHZ);
  mcp2515.setNormalMode();
  Serial.println("CAN BUS Shield init ok!");
}

void loop() {
  // Send CAN message
  canMsg.can_id = 0x123;
  canMsg.can_dlc = 8;
  canMsg.data[0] = 0x01;
  canMsg.data[1] = 0x02;
  canMsg.data[2] = 0x03;
  canMsg.data[3] = 0x04;
  canMsg.data[4] = 0x05;
  canMsg.data[5] = 0x06;
  canMsg.data[6] = 0x07;
  canMsg.data[7] = 0x08;
  
  mcp2515.sendMessage(&canMsg);
  delay(1000);
  
  // Receive CAN message
  if (mcp2515.readMessage(&canMsg) == MCP2515::ERROR_OK) {
    Serial.print("ID: 0x");
    Serial.print(canMsg.can_id, HEX);
    Serial.print(" Data: ");
    for (int i = 0; i < canMsg.can_dlc; i++) {
      Serial.print("0x");
      Serial.print(canMsg.data[i], HEX);
      Serial.print(" ");
    }
    Serial.println();
  }
}

// Set filters to accept only specific message IDs
void setCANFilters() {
  // Filter 0: Accept ID 0x123
  mcp2515.setFilter(RXF0, false, 0x123);
  
  // Filter 1: Accept ID range 0x200-0x20F  
  mcp2515.setFilter(RXF1, false, 0x200);
  mcp2515.setFilterMask(MASK0, false, 0xFF0);
  
  // Enable interrupt on message reception
  mcp2515.setConfigMode();
  mcp2515.setCanInterrupt(MCP2515::CANINTF_RX0IF | MCP2515::CANINTF_RX1IF);
  mcp2515.setNormalMode();
}

CAN Signal Quality Measurements:

Differential Voltage:
- Recessive state: 0V ± 0.5V
- Dominant state: 2.0V ± 0.5V

Common Mode Voltage:
- Both states: 2.5V ± 1.0V

Rise/Fall Times:
- 10%-90%: <100ns typical
- Slew rate: >1.5 V/μs

Bus Loading:
- Total capacitance: <400 pF
- DC resistance: 60Ω ± 5%

• ASIL ratings: Safety Integrity Levels A-D
• Fault detection: Error monitoring and reporting
• Fail-safe behavior: Defined responses to faults
• Redundancy: Backup communication paths
• Validation: Extensive testing requirements

• SIL ratings: Safety Integrity Levels 1-4
• Black channel: CAN as transmission medium only
• Safety protocols: Application-layer safety mechanisms
• Diverse systems: Multiple independent channels

• Powertrain: Engine, transmission control
• Chassis: ABS, steering, suspension
• Body: Lighting, HVAC, door controls
• Infotainment: Audio, navigation, connectivity
• Diagnostics: OBD-II, manufacturer diagnostics

• Factory automation: PLC communication
• Process control: Sensor networks
• Building automation: HVAC, security systems
• Medical devices: Equipment networking
• Marine systems: Engine, navigation networks

• IoT gateways: Industrial IoT connectivity
• Robotics: Distributed control systems
• Energy systems: Solar, battery management
• Transportation: Rail, aviation systems