Embedded Systems & Automotive Intermediate 20 min

CAN-Bus 2.0B Telemetry Node with Industrial Sensor Fusion

Construct an automotive-grade CAN-Bus 2.0B communication node transmitting real-time atmospheric telemetry across differential twisted pairs.

2. Prerequisites & Background Theory:

Serial communication fundamentals, differential physical signaling, bit-timing synchronization, and CRC checksums.

STEP 1

Goal & Engineering Objectives

Build a robust physical-layer CAN-Bus network transmitting sensor telemetry frames at 500 kbps across noisy industrial environments without packet drops or frame collisions.

STEP 2

Governing Mathematical Formulation & Physics

                  Controller Area Network (CAN ISO 11898) Differential Signaling:

• Recessive Bit ('1'): CAN_H = 2.5V, CAN_L = 2.5V $\implies V_{\text{diff}} = 0.0\text{V}$
• Dominant Bit ('0'): CAN_H = 3.5V, CAN_L = 1.5V $\implies V_{\text{diff}} = 2.0\text{V}$

Non-Destructive Bitwise Arbitration:
If two nodes transmit simultaneously, the node transmitting a dominant '0' overwrites a recessive '1'. The node with the lower numerical message ID continues transmitting without interruption or data corruption.

Line Characteristic Impedance Matching:
$$Z_0 = 120\,\Omega \implies \text{Two } 120\,\Omega \text{ resistors at trunk ends } (R_{eq} = 60\,\Omega)$$                

STEP 3

System Architecture & Block Flow Diagram

[BME280 Sensor] --(I2C 0x76)--> [ESP32 TWAI Controller]
                                     |
                               (TXD / RXD TTL)
                                     v
                            [SN65HVD230 Transceiver]
                                     |
                           (CAN_H / CAN_L Differential)
                                     v
                      [120Ω Shielded Twisted Pair Bus]

STEP 4

Required Hardware Bill of Materials (BOM)

Hardware Component	Qty	Engineering Specification
ESP32-WROOM-32 Development Board	2	Integrated Two-Wire Automotive Interface (TWAI)
SN65HVD230 CAN Transceiver (3.3V)	2	3.3V logic, ISO 11898 compatible, up to 1 Mbps
BME280 Temperature, Humidity & Pressure Sensor	1	I2C interface, 3.3V operating voltage
120Ω 1/4W Metal Film Resistors	2	End-of-line bus termination resistors
High-Quality Twisted-Pair Cable (e.g., Cat5e/Cat6)	2m	120Ω characteristic differential impedance

Look up component pinouts in Components Catalog →

STEP 5

Wiring, Pinout & Schematics Mapping

                  • Node 1 (Transmitter):
  - ESP32 GPIO 5 (TX) -> SN65HVD230 TXD
  - ESP32 GPIO 4 (RX) -> SN65HVD230 RXD
  - ESP32 GPIO 21 (SDA) -> BME280 SDA
  - ESP32 GPIO 22 (SCL) -> BME280 SCL
• Differential Bus Interconnect:
  - SN65HVD230 CAN_H (Node 1) -> CAN_H (Node 2)
  - SN65HVD230 CAN_L (Node 1) -> CAN_L (Node 2)
  - Place 120Ω resistor across CAN_H and CAN_L at Node 1 and Node 2.                

STEP 6

Firmware Implementation & Algorithms

// ESP32 Native TWAI (CAN-Bus) Transmitter Implementation
#include "driver/twai.h"

void init_twai() {
    twai_general_config_t g_config = TWAI_GENERAL_CONFIG_DEFAULT(GPIO_NUM_5, GPIO_NUM_4, TWAI_MODE_NORMAL);
    twai_timing_config_t  t_config = TWAI_TIMING_CONFIG_500KBITS();
    twai_filter_config_t  f_config = TWAI_FILTER_CONFIG_ACCEPT_ALL();
    
    ESP_ERROR_CHECK(twai_driver_install(&g_config, &t_config, &f_config));
    ESP_ERROR_CHECK(twai_start());
}

void transmit_sensor_telemetry(int16_t temp_x100, uint32_t press_pa) {
    twai_message_t msg;
    msg.identifier = 0x360; // Cloud 360 Telemetry Frame ID
    msg.extd = 0;           // Standard 11-bit ID
    msg.data_length_code = 6;
    
    // Byte 0-1: Temperature (Signed 16-bit integer, degrees C * 100)
    msg.data[0] = (uint8_t)(temp_x100 >> 8);
    msg.data[1] = (uint8_t)(temp_x100 & 0xFF);
    
    // Byte 2-5: Atmospheric Pressure (Unsigned 32-bit integer in Pa)
    msg.data[2] = (uint8_t)(press_pa >> 24);
    msg.data[3] = (uint8_t)(press_pa >> 16);
    msg.data[4] = (uint8_t)(press_pa >> 8);
    msg.data[5] = (uint8_t)(press_pa & 0xFF);
    
    twai_transmit(&msg, pdMS_TO_TICKS(50));
}

Simulate and Edit in Coding Lab

STEP 7

Bench Testing & Waveform Verification

                  1. Disconnect bus power and measure resistance with a DMM between CAN_H and CAN_L: It must read approximately 60Ω (two 120Ω resistors in parallel).
2. Connect a 2-channel oscilloscope with channel math (CH1 - CH2): Observe sharp differential pulses between 1.5V and 3.5V with zero ringing.
3. Verify TWAI transmit error counter (TEC) remains at 0 over continuous 100,000-frame test runs.                

STEP 8

Troubleshooting & Common Failure Modes

                  • Problem: Bus is stuck in 'Bus-Off' state (TEC > 255).
  Solution: Verify that at least one other active receiving node is connected and acknowledging frames (Dominant ACK bit in slot 14).
• Problem: Ground loops causing transmission errors across separated benches.
  Solution: Connect a third common logic ground reference wire alongside the twisted pair or use galvanic digital isolators (ISO1050).                

STEP 9

Extensions & Undergraduate Research Challenges

                  1. Implement the CANopen higher-layer communication protocol with Service Data Objects (SDO) and Process Data Objects (PDO).
2. Incorporate hardware cryptographic HMAC message authentication into frame payloads to prevent automotive bus spoofing.                