Category: C/C++

I am building an ESP32 based system that uses CAN bus communication to transfer data between processors. The ESP32 firmware is being developed using the Arduino programming environment. My plan is to purchase ESP32 modules and plug them on interface boards that are interconnected using the CAN bus. The maximum system is about 16 processors.

Controller Area Network (CAN) bus was officially released in 1986 at the Society of Automotive Engineers conference. It was designed as an in-vehicle communications backbone between engine control unit processor, sensors, actuators, indicators, displays, and other vehicle components. CAN bus defines the OSI network model physical and data link layers.

The physical network is a multi-drop bus that uses a single twisted pair wire with approximately one twist per inch and 120 ohm terminations at the ends. The Digikey blog CAN bus image below shows four CAN bus nodes (controller and transceiver), cabling, and terminations.

The CAN bus 2.0 has two frame versions, standard 2.0A and extended 2.0B. The difference between these frame types is the arbitration field with the length of the identifier (11-bits vs. 29-bits) and the control field. The picture from Typhoon HIL Documentation shows the two frame formats.

CAN bus operates on a decentralized networking principle where all nodes are equal. Any node can transmit a data frame when the bus is free. Each frame contains identification information and supports variable data lengths from 0 up to 8 bytes. CAN bus supports a multitude of bits rates from 25kb/s to 1000kb/s and up to 5.0Mb/s for CAN FD. Because it is a decentralized network, there exists bus arbitration and robust error detection used to create a reliable network.

I selected the ESP32 because it has a built in CAN bus 2.0A/B controller (called two-wire automotive interface (TWAI)) that supports bit rates from 12.5kb/s to 1000kb/s and there are available Arduino libraries. The ESP32 does not support CAN FD protocol. The lowest rates 12.5kb/s to 20kb/s are only available on the ESP32 revision 2 or later. To use the ESP32 you must add a CAN bus physical layer transceiver interface.

I setup a ESP32 CAN bus test with two devices. This simple setup should have been straight forward. There are examples with the Sandeep Mistry Arduino CAN bus library (version 0.3.1) I am using and multiple examples on the Internet. I ran into two problems using this library. The first was solved quickly while the second took some digging.

The first problem was with an include file. I’m using the latest Arduino IDE (2.3.2) with ESP32 support. I got a complier error stating that “esp_intr.h” could not be found. A quick search showed a change was needed in the ESP32SJA1000.cpp file changing the reference to #include “esp_intr_alloc.h.” This allowed the code to compile.

// Copyright (c) Sandeep Mistry. All rights reserved.
// Licensed under the MIT license. See LICENSE file in the project root for full license information.

#ifdef ARDUINO_ARCH_ESP32

//#include "esp_intr.h"
#include "soc/dport_reg.h"
#include "driver/gpio.h"
#include "esp_intr_alloc.h"
#include <rom/gpio.h>

#include "ESP32SJA1000.h"

In my test I had two ESP32 connected via CAN bus transceivers with the correct terminations. The two devices would not communicate even after testing several different bit rates. I used two different manufacturer’s modules, purchased at different times so I tried two devices from the same manufacturer’s and that worked! Then I tried the other manufacturer’s modules and that worked! But the two different manufacturers modules would not work together.

Finally I hooked up my oscilloscope and measured the bit times. I configured the system to run at 1Mb/s. On the newer ESP32 modules the bit rate was only 0.5Mb/s. The bit rate was correct on the older ESP32 modules.

Espressif Systems in newer ESP32 revisions support CAN bus bit rates less than 25kb/s. This update has caused an issue with the Sandeep Mistry Arduino CAN bus library (and I suspect other libraries as well). The library writes all bits to the REG_IER register that in newer ESP32 revisions has a bit that enables a divide by 2 CAN bit rate. The fix is to modify the REG_IER bit for ESP32 revisions 2 or greater. The modification is needed in the ESP32SJA1000.cpp begin() method after REG_IER write as shown below. After making these changes the CAN bus is now interoperable between different ESP revisions and manufacturer’s modules.

modifyRegister(REG_BTR1, 0x80, 0x80); // SAM = 1
writeRegister(REG_IER, 0xff); // enable all interrupts
  
if (ESP.getChipRevision() >= 2) {
   modifyRegister(REG_IER, 0x10, 0); // From rev2 used as "divide BRP by 2"
}

Besides learning about the CAN bus library capabilities I wanted to stress test the CAN bus to determine how well it will work in my application. The stress test uses four senders and one receiver (I only had 5 CAN bus transceivers for the test) to pass messages.

The receiver tracks each individual sender identified by the unique 11-bit ID and verifies all messages are received by checking a message counter. When an error occurs a serial message is printed and the new message counter is used.

// CAN Receiver Stress Test
// Standard message (11 bit ID)
// Variable time between messages and message length
// Constant ID and message counter to check for lost messages
// Multiple senders

#include <CAN.h>
#define MAX_SENDERS 16

struct messageCheck {
  int id11;
  uint8_t messageNum;
} receivedData[MAX_SENDERS];

uint8_t seqNum;
int id11;

void setup() {
  Serial.begin(115200);
  while (!Serial);

  Serial.println("CAN Stress Receiver");

  CAN.setPins(22,21);
  // start the CAN bus at 1000 kbps
  if (!CAN.begin(1000E3)) {
    Serial.println("Starting CAN failed!");
    while (1);
  }

  // voltage translator OE
  pinMode(12, OUTPUT);
  digitalWrite(12, HIGH); // has 10K pull down

  // initialize test structure
  for(int i = 0; i < MAX_SENDERS; i++) {
    receivedData[i].id11 = 0;
    receivedData[i].messageNum = 0;
  }

}

void loop() {
  // try to parse packet
  int packetSize = CAN.parsePacket();
  if (packetSize) {

    id11 = CAN.packetId();
    // first packet id in list
    int i = 0;
    while(receivedData[i].id11 != 0) {
      if(receivedData[i].id11 == id11) {
        break;
      }
      i++;
    }

    if(i < MAX_SENDERS) {
      if(receivedData[i].id11 == 0) {   // new to list
        receivedData[i].id11 = id11;
        receivedData[i].messageNum = CAN.read();
      }
      else {  // id found
        receivedData[i].messageNum++;
        seqNum = CAN.read();
        if(receivedData[i].messageNum != seqNum)  { // error
          receivedData[i].messageNum = seqNum;
          Serial.print("Error ID "); Serial.println(receivedData[i].id11);
        }
      }
    }
    else {  // some other error occured
      Serial.println("i >= MAX_SENDERS Error");
    }
  }
}

Each sender transmits data at random times between 1 and 50ms and each frame has random number of bytes between 1 (message counter) and 8. The message counter, which is always present, is used to track message delivery by the receiver.

// CAN Sender Stress Test
// Standard message (11 bit ID)
// Variable time between messages and message length
// Constant ID and message counter to check for lost messages

#include <CAN.h>
#include "esp_mac.h"

int id11 = 0;             // 11 bit ID, use MAC address
uint8_t messageNum = 0;   // first byte of message data, increments over time for testing
uint8_t bytesRandom;      // random number of additional bytes to send 0 to 7
long waitRandom;          // random wait time in MS before sending next message

void setup() {
  Serial.begin(115200);
  while (!Serial);

  Serial.println("CAN Sender Stress Test");

 CAN.setPins(22,21);
  // start the CAN bus at 500 kbps
  if (!CAN.begin(1000E3)) {
    Serial.println("Starting CAN failed!");
    while (1);
  }
  
  // voltage translator OE
  pinMode(12, OUTPUT);
  digitalWrite(12, HIGH); // has 10K pull down

  uint8_t mac[8];
  esp_efuse_mac_get_default(mac);
  for(int i = 0; i < 8; i++) {
    id11 += mac[i];
  }
  id11 &= 0x07ff;
  randomSeed(analogRead(39));
}

void loop() {

  // wait random ms
  waitRandom = random(1,50);
  delay(waitRandom);

  // send packet: id is 11 bits, packet can contain up to 8 bytes of data
  Serial.print("Sending packet ID ... "); Serial.print(id11, HEX);

  bytesRandom = (uint8_t)random(7);       // random number of additional bytes
  CAN.beginPacket(id11);                  // ID
  CAN.write(messageNum++);                // message count used to measure reliability during stress test
  for(int i = 0; i < bytesRandom; i++) {  // add random number of bytes
    CAN.write((uint8_t)random());
  }
  CAN.endPacket();
  Serial.println(" done");
}

Overall I’m pleased with CAN bus performance and reliability. I connected an oscilloscope and observe random message timing and message collision events. I can disconnect/reconnect/reset senders and the receiver properly detects a node message error. I have run the test for hours without any dropped messages at 1Mb/s bit rate on a very short bus (<2feet). Future testing includes adding more nodes and increasing the bus wire length.