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AHT10 - I2C problem

msimon

I just bought AHT10 humidity and temperature sensor. I have seen that it supports I2C and it is cheap so I didn't think thoroughly and bought two. I wanted to try them then I encounter a problem that only a single AHT10 can be connected to single I2C bus. I thought it is related to static address but it is worse (breakout board also showing two address but no details in the datasheet). The datasheet says that "Only a single AHT10 can be connected to the I2C bus and no other I2C devices can be connected." It sounds like traditional serial communication like RS232 not I2C. 

Is it normal to advertise it as an I2C device? and I also wonder what is the technical issue preventing to connect other devices with different address. Did you have any experience similar to this on any other sensor?

  • I had a quick look at the data sheet. They don't appear to offer an explanation for why it doesn't work with other devices.

    I've used several other I2C humidity sensors and lots of I2C devices of other kinds. This is the first one I've seen that doesn't really work in an I2C bus system at all. (Some others have issues.)

    The good news is that it is very easy to make I2C devices work with normal IO pins so it should might be possible for you to connect it up without killing your I2C bus if you need that for other things.

    If you describe your system a bit more (schematic, controller type, pictures) it might be possible to make some definite suggestions.

    MK

  • Does look like you’re pretty stuffed with that device using I2C directly.  You could just swallow the cost and get an AHT20 (or alternately, if you describe your requirements someone here may be able to better advise).  As Michael says, you can skip the I2C bus and use other pins.  This post links to a GitHub library that provides a solution: https://www.stm32duino.com/viewtopic.php?t=1338

  • It might be possible to use two or more AHT10 devices with a 'chip select' type setup.

    The AHT20 data sheet shows a power control application design like this for power saving applications which may be able to be applied to the AHT10: 

    image

    'GND_CTRL' effectively becomes a chip select line allowing you to activate the device you want to communicate with using the same I²C addresses.

  • Have you tried using the AHT10 with another device on the same I²C bus to see what happens ?

    It may be that it doesn't observe the stop condition so doesn't release itself from the host and continues responding to any data present on the bus. 

    If so, issuing a soft reset after the last measurement might be worth a try.

  • in reply to beacon_dave

    Both datasheets (AHT10 & AHT20) show that the sensor operates on microAmps, so powering by GPIO is possible too...

    image

  • in reply to BigG

    That design disconnects the I²C bus pull-up resistors from VDD presumably to prevent current drain through the pull-ups and driver IC to ground in power saving applications.

    If not concerned about power saving, then another approach may be to leave the sensor powered and look at connecting/disconnecting the SDA/SCL lines from the I²C bus with the 'chip select'.

    Keeping the sensor powered may improve measurement accuracy if it requires time to stabilise after power up. Although a 'chip deselect' approach may also work here or simply factoring in stabilisation times prior to a measurement.

  • in reply to michaelkellett

    Hi   , I am planning to use it for Save the Bee design challenge. I can use GPIO as you suggested, use I2C mux, or simply change the sensor but I wonder is it normal? I just bought it because it supports I2C so I thought I could use the same pins for various devices but it is not possible. This is a cheap product so I don't mind. I wonder it is my mistake or its product's fault. I also want to learn is there any weird things similar to this. 

  • in reply to msimon

    It's definitely not normal. I can't think of any other I2C chips that make such a requirement.

    I've used TI and Sensirion Humidity  sensors with no problems.

    MK

  • in reply to Andrew J

    Hi   , it is not an expensive product so money is not an issue. However AHT20 also not a good option as it has also have a static addressing. But it works with other I2C devices as mentioned datasheet. I am thinking bit banging on normal GPIO. I may buy other sensors but I want to see options. And also question it is normal or not. I think it is not normal and many people share the same opinion on the link you have provided. 

  • in reply to BigG

    Hi  , I will try it to see it works or not. Sounds logical but I am not sure if the product brings other problems.