No I2C ACK from Honeywell HIH-6120 humidity sensor

Question

There is an Arduino library for the HIH-6130 which has exact same I2C protocol as the sensor I am using which is the HIH-6120. I am using the same two lines of code to tell the sensor to take a reading:

Wire.beginTransmission(0x27);
ret = Wire.endTransmission();

ret is 2, which means master received NACK on transmit of address. Looking at this on my analogue scope it appears the sensor makes an attempt to let SDA rise to 1 but it is late and too weak: (about 10us per div, 2v per div, sync on falling SDA) A delayed by B gives (1us per div) Pullup resistors are 2k. Sensor address is 0x27 according to HP. Min clock frequency is 100 KHz which is what Arduino is giving.

What on earth is wrong?

Answer 1

It looks like it's ACK'ing fine to me but maybe or the code is interpreting it wrong?

I see the 0x27 (data latched on the rising clock edge), and then the read/write bit is 0 meaning you are doing a write, then the slave drives SDA low to indicate an ACK. You can see right when it happens there's a tiny step down to exactly 0 Volts.

Then on the falling edge of the clock the slave is releasing the SDA line (as it should) so the SDA rises up, then the master is taking over the bus and driving SDA line low. That's why you see that narrow blip.

So it looks fine. Somehow the HW is interpreting it wrong or your understanding of the API might be wrong?

Answer 2

See my comment above - the transaction itself looks OK.

However, all your code is doing is sending the address then immediately sending a STOP. You're not actually reading anything. This first read is a just a dummy wake-up command (thanks Janka).

According to the datasheet, you then need to send the address in READ mode then read either two bytes if you want just humidity, or four bytes if you want humidity plus temperature. Then you send a stop.

I think the code needs to look more like this:

Wire.beginTransmission(0x27); // write to wake-up
Wire.endTransmission();       // ignore this
delay(100);

Wire.begin();                 // optional for read
Wire.requestFrom(0x27, 4);    // request 4 bytes from slave device 
while (Wire.available())      // slave may send less than requested
{                            
    char c = Wire.read();     // receive a byte as character
    Serial.print(c);          // print the character
}
ret = Wire.endTransmission();

If this code messes up (i.e. if it still sees NAK where there is none), there's something fishy with the library perhaps.