VEML6070 causes I2C lockup after power cycle

[eecharlie]

• Arduino board: Uno
• Arduino IDE version: 1.8.5
• Steps to reproduce:

If you run the sketch below, the code will hang on the second loop iteration at uv.begin(VEML6070_HALF_T);
If you uncomment the uv.clearAck() then it will hang there.
If you comment out the lines that power-down the breakout & I2C bus, then the loop runs indefinitely as expected.
The putative problem is the Wire library getting into a wait loop for a bad bus condition to clear. Why the bad bus condition is created is unknown, since the bus is clear at the start of communication.

#include <Wire.h>
#include <Adafruit_VEML6070.h>

// Pin assignments
#define POWER_PIN (11)  // VIN pin on VEML breakout board
#define ACK_PIN   (13)     // Note blue LED will turn *off* if ACK is set

Adafruit_VEML6070 uv = Adafruit_VEML6070();

void setup() {
  Serial.begin(115200);
  Serial.println("VEML6070 Test to Reveal Bug");
  
  pinMode(POWER_PIN, OUTPUT);
  digitalWrite(POWER_PIN, LOW);
  
  digitalWrite(ACK_PIN, INPUT_PULLUP);  // ACK is open-drain

}

bool testAck() {
  bool ack = (digitalRead(ACK_PIN) == LOW);
  if (ack) { Serial.println("ACK is set (low)"); Serial.flush(); }
}

bool i2c_ready(){
  bool ready = (digitalRead(SDA) == HIGH) && (digitalRead(SCL) == HIGH);
  if (!ready) { Serial.println("I2C bus locked"); Serial.flush(); }
  return ready;
}

void loop() {
  // Power DOWN the VEML 6070, and drive the I2C bus
  // to ground to ensure the sensor is actually power-cycled
  digitalWrite(POWER_PIN, LOW);
  pinMode(SDA, OUTPUT);
  digitalWrite(SDA, LOW);
  pinMode(SCL, OUTPUT);
  digitalWrite(SCL, LOW);
  pinMode(SDA, INPUT);
  pinMode(SCL, INPUT);
  delay(100);
  
  
  // Power up and initialize the VEML 6070
  digitalWrite(POWER_PIN, HIGH);
  Wire.begin();
  delay(1000);  // For device to power up

  i2c_ready();
  testAck();
  //if (uv.clearAck()) { Serial.println("Ack was set"); Serial.flush(); }
  i2c_ready();

  Serial.print("init.. "); Serial.flush();
  uv.begin(VEML6070_HALF_T);
  Serial.println("done");

  // Attempt to read the UV value after allowing for measurement delay
  Serial.print("UV: ");
  uint16_t value = uv.readUV();
  Serial.println(value);
}

[eecharlie]

FYI, for catalog of historic problems & unmerged solutions in Arduino Wire library:
arduino/Arduino#7328

[eecharlie]

I have submitted a pull request that solves this problem
#6