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I2C fix READ of zero bytes hardware hang #2301

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Jan 10, 2019
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I2C fix READ of zero bytes hardware hang #2301

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6 changes: 6 additions & 0 deletions cores/esp32/esp32-hal-i2c.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1609,6 +1609,9 @@ i2c_err_t i2cFlush(i2c_t * i2c)
}

i2c_err_t i2cWrite(i2c_t * i2c, uint16_t address, uint8_t* buff, uint16_t size, bool sendStop, uint16_t timeOutMillis){
if((i2c==NULL)||((size>0)&&(buff==NULL))) { // need to have location to store requested data
return I2C_ERROR_DEV;
}
i2c_err_t last_error = i2cAddQueueWrite(i2c, address, buff, size, sendStop, NULL);

if(last_error == I2C_ERROR_OK) { //queued
Expand All @@ -1628,6 +1631,9 @@ i2c_err_t i2cWrite(i2c_t * i2c, uint16_t address, uint8_t* buff, uint16_t size,
}

i2c_err_t i2cRead(i2c_t * i2c, uint16_t address, uint8_t* buff, uint16_t size, bool sendStop, uint16_t timeOutMillis, uint32_t *readCount){
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if size is 0, why return an error? if size is not zero, but buf is null, then I understand, but size 0 is fine to return OK

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@stickbreaker stickbreaker Jan 10, 2019 •
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Returning Ok means that the SlaveID is valid and the slave Answered the request. A zero length READ cannot be executed. So, the slave may not exist, or it may be busy.

A zero byte read creates an invalid I2C bus state. Depending on what the first bit the SLAVE responds with, the i2c peripheral may not be able to complete the transmission. A zero byte write is possible, but a zero byte read is not allowed. The zero byte write is used as a presence detect event.:

Wire.beginTransmission(ID);
uin8_t error = Wire.endTransmission();
if(error == 2) { // slave did not answer, is not present
  Serial.printf("NAK, no slave present at 0x%02X\n",ID);
} else if (error == 0) {// Slave ready for communications
  Serial.printf("ACK, slave present at 0x%02X\n",ID);
}

The following code is invalid:

uint8_t requestSize = 0;
uint8_t count = Wire.requestFrom(ID,requestSize);
if( count == requestSize) {// Slave is present and ready for communications
   Serial.printf("ACK, slave present at 0x%02X, received %dbytes\n",ID,count);
   Serial.printf(" requestFrom succeeded =%d(%s)\n",Wire.lastError(),Wire.getErrorText(Wire.lastError()));
   for (uint8_t i = 0 ; i< count; i++){
     Serial.print(Wire.read());
  }
  Serial.println():
}
else { // problem
   Serial.printf(" requestFrom Failed =%d(%s)\n",Wire.lastError(),Wire.getErrorText(Wire.lastError()));
}

change requestSize to a non zero value and the code works a expected,
The act of requesting a zero byte read must error out.

A Zero byte Write will return information, Whether a Slave device is active on the bus.

A Zero byte Read is meaningless. and will only succeed if the correct circumstances exist, else it will hang the i2c bus creating a error cascade that is confusing.

Chuck.

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ok...

if((size == 0)||(i2c == NULL)||(buff==NULL)){ // hardware will hang if no data requested on READ
return I2C_ERROR_DEV;
}
i2c_err_t last_error=i2cAddQueueRead(i2c, address, buff, size, sendStop, NULL);

if(last_error == I2C_ERROR_OK) { //queued
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