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HDC3020: Pulse on the SDA line

R.Fukunaga
Expert 2505 points
Part Number: HDC3020

Tool/software:

Hi all,

As shown in the figure below, pulses are generated on the SDA.

I think this device can be acceptable to tLPF=50ns.

However, actual measurement shows that the data rate is 400kbps, V0p=1.4V, and the width exceeds 400ns.
We are considering countermeasures.

1) Looking at the waveform, it occurs at the timing of SCL is Low.
When we use I2C, the data is not recognized when SCL=Low, so I understand that even if a pulse occurs, the data is not recognized. Is it correct?

2) Is there a way to reduce the pulse?
I tried the following methods, but there was no effect.
・I changed the damping resistance to SDA line from 22 to 50Ω, but there was no change.
・I changed the pull-up resistance from 10k to 5 kΩ, but the peak value of the pulse increased. I changed it to 20kΩ, and it became smaller.
If you have a recommended pull-up resistance value, I would appreciate it if you let me know.

Best Regards,

Ryusuke

  • 0
    TI__Expert 4900 points

    Hello Ryusuke,

    Could you please confirm the operating voltage you are running the HDC3020 at? It looks like you're running at 1.4V but I wanted to double check.

    Regards,


    Harry

  • 0 in reply to HarryG
    Expert 2505 points

    Hi Harry,

    I confirmed operating voltage as follows.I'll send you the waveform as soon as I get it.

    Vdd: DC3.3V, Vripple:±0.1

    SDA(Low): -0.1~0.5V

    SDA(High): 3.3V 

    SCL(Low): -0.1~0.5V

    SCL(High): 3.3V

    Best Regards,

    Ryusuke

  • +1
    Guru 99575 points

    These spikes are occurring at the handover (Transmitter->Receiver) for the ACK cycle. If the Receiver is slow to drive SDA (or the Transmitter quick to release it), SDA briefly goes Hi-Z in the gap and is pulled up. Stronger pullups will accentuate the effect.

    I2C allows pretty much any activity on SDA as long as SCL is low. This phenomenon is not uncommon, and is harmless (however disturbing it looks on the scope).

  • +1 in reply to Bruce McKenney
    Guru 99575 points

    You didn't mention what your Master device is, but some allow adjusting the time SDA is held after SCL goes low. The STM32C0 series, e.g. calls this Tsdadel (in the TIMINGR). This is the kind of thing I try to avoid messing with, as long as there's no trouble.

  • +1 in reply to R.Fukunaga
    TI__Expert 4900 points

    Thank you Ryusuke,

    These pulses can occur particularly after a device ACKs on the SDA line and when the controller and target device are exchanging control of the line. This can be due to the hold time for the controller being short, but this behavior should not have a negative impact on the performance of the HDC3020 and is fairly normal in I2C communication. Are you receiving valid temperature and humidity measurements?

    Regards,


    Harry

  • 0 in reply to HarryG
    Expert 2505 points

    Hi Harry and Bruce,

    The temperature and humidity measurement seems to be normal.

    Thank you for your response. It was helpful.

    Best Regards,

    Ryusuke

  • +1 in reply to R.Fukunaga
    TI__Expert 4900 points

    Happy to help!

    Harry