i2c communications on ESP32

By Official I2C protocol as UM10204 I2C-Bus Specification and User Manual, A valid I2C transaction must be a START, at least one Data Byte, and a STOP. The Arduino environment is base on the AVR processor series, These Atmel CPU's implemention the I2C protocol allows unlimited pauses between protocol elements. The Espressif ESP32 implement enforces a Strict TimeOut between elements. This has resulted in perceived I2C bus stability issues (834,811,741,682,659 and many more. @lonerzzz created a partial solution #751.

The I2C protocol arbitrates exclusive ownership of the bus to a single Master Device from a START signal to the next STOP. A ReSTART operation is just a START signal inserted into this START -> STOP flow. Schematically like this:

START (Read or Write) -> ReSTART (Read or Write) -> STOP.

The existing Arduino code base is reliant on the AVR's ability to infintely pause a i2c transaction. The standard coding practice of:

// set internal address pointer in I2C EEPROM from which to read
Wire.beginTransmission(ID);
Wire.write(highByte(addr));
Wire.write(lowByte(addr));
uint8_t err =Wire.endTransmission(false); // don't send a STOP, just Pause I2C operations
if(err==0){ // successfully set internal address pointer
  err=Wire.requestFrom(addr,len);
  if(err==0){ // read failed
    Serial.print("Bad Stuff!! Read Failed\n");
    }
  else {// successful read
    while(Wire.avaiable()){
      Serial.print((char)Wire.read());
      }
    Serial.println();
    }
  }

May not function correctly with the ESP32, actually usually will not function correctly. The current arduino-esp32 platform is built upon the espressif-idf which is built on FreeRTOS, a multi-process/processor operating system. The Arduino sketch is just one task executing on One of the Two processor cores. The TimeOut is triggered when the FreeRTOS Task Scheduler does a task switch between Wire.endTransmission(false); and the i2c activites inside Wire.requestfrom(). The maximum TimeOut interval is around 12ms.

This rewrite of Wire() is designed to avoid this TimeOut from ever being possible. To avoid the TimeOut this library uses a queuing system that does not allow an i2c transaction to start unless a STOP has been issued. But, alas, this creates some incompatibilities with the pre-exisiting Arduino code base. The changes to the standard Arduino Wire() coding are minimal, but, they are necessary:

// new, maybe Excessive Error Return codes for @stickbreaker:arduino-esp32
typedef enum {
    I2C_ERROR_OK=0,
    I2C_ERROR_DEV,
    I2C_ERROR_ACK,
    I2C_ERROR_TIMEOUT,
    I2C_ERROR_BUS,
    I2C_ERROR_BUSY,
    I2C_ERROR_MEMORY,
    I2C_ERROR_CONTINUE,
    I2C_ERROR_NO_BEGIN
} i2c_err_t;

// set internal address pointer in I2C EEPROM from which to read
Wire.beginTransmission(ID);
Wire.write(highByte(addr));
Wire.write(lowByte(addr));
uint8_t err =Wire.endTransmission(false); 
// don't send a STOP, just Pause I2C operations

//err will be I2C_ERROR_CONTINUE (7), an indication that the preceding
//transaction has been Queued, NOT EXECUTED

if(err == 7){ // Prior Operation has been queued
// it will be executed when the next STOP is encountered.
// if sendStop had equaled true, then a successful endTransmission() would have
// returned 0. So, if this if() was if(err==0), the Queue operation would be
// considered an error.  This is the primary Difference.

  err=Wire.requestFrom(addr,len);
  if(Wire.lastError()!=0){ // complete/partial read failure
    Serial.printf("Bad Stuff!!\nRead of (%d) bytes read %d bytes\nFailed"
      " lastError=%d, text=%s\n", len, err, Wire.lastError(), 
      Wire.getErrorText(Wire.lastError()));
    }
  // some of the read may have executed
  while(Wire.avaiable()){
    Serial.print((char)Wire.read());
    }
  Serial.println();
  }

Additionally this implementation of Wire() includes methods to handle local buffer data blocks. These local buffer can be up to 64k-1 bytes in length.

New Methods:

    i2c_err_t writeTransaction(uint8_t address, uint8_t* buff, size_t size, bool sendStop);// big block handling
    size_t requestFrom(uint8_t address, uint8_t* buf, size_t size, bool sendStop);
    size_t transact(size_t readLen); // replacement for endTransmission(false),requestFrom(ID,readLen,true);
    size_t transact(uint8_t* readBuff, size_t readLen);// bigger Block read
    i2c_err_t lastError(); // Expose complete error
    void dumpInts(); // diagnostic dump for the last 64 different i2c Interrupts
    size_t getClock(); // current i2c Clock rate
    void setTimeOut(uint16_t timeOutMillis); // allows users to configure Gross Timeout
    uint16_t getTimeOut();

transact() coding is:

// set internal address pointer in I2C EEPROM from which to read
Wire.beginTransmission(ID);
Wire.write(highByte(addr));
Wire.write(lowByte(addr));

uint8_t err=Wire.transact(len); // transact() does both Wire.endTransmission(false); and Wire.requestFrom(ID,len,true);
if(Wire.lastError != 0){ // complete/partial read failure
  Serial.printf("Bad Stuff!! Read Failed lastError=%d\n",Wire.lastError());
  }
  // some of the read may have executed
while(Wire.avaiable()){
  Serial.print((char)Wire.read());
  }
Serial.println();

This APLHA release should be compiled with ESP32 Dev Module as its target, and Set the "core debug level" to 'error'

There is MINIMAL to NO ERROR detection, BUS, BUSY. because I have not encounter any of them!

Chuck.