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PDR
PDR
Associate II
December 1, 2017
Question

Need to add an extra clock cycle to an I2C clock line

  • December 1, 2017
  • 1 reply
  • 1580 views
Posted on December 01, 2017 at 04:55

I am writing a driver for an old Lego NXT ultrasonic sensor. It uses I2C, which is fine in itself, but the sensor has a well-known bug such that it does not always follow standard I2C protocol. Specifically, it requires an extra clock cycle at a certain point in the communication.This is widely known, and if you examine the various solutions people have adopted, eg on Arduino, you will see what I mean. I am using a third-party board for this purpose that provides the necessary circuitry supporting the sensor, and I can communicate with the sensor successfully, except for the specific case in which this extra pulse is required.This third-party board is designed to be an extension for an Arduino, but I am using it with an STM32F4 Discovery board. It also has an H-bridge for the NXT motors, etc, and all works well.

The third-party board has a digital I/O line physically tied directly to the SCL line so that the Arduino software can add the clock cycle when required. Without the STM32 I2C driver running I can drive that digital line to add what would be the extra clock cycle, and have verified it with a logic analyzer. However, when the I2C device is applied, I cannot affect the SCL via this backdoor I/O line. I tried disabling the I2C unit, driving the digital line for the pulse, and then re-enabling the unit, but without any observable effect.

Hence my first question: should this be possible with an I2C unit on the STM32F4 Disco board?

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    1 reply

    Tesla DeLorean
    Tesla DeLorean
    Guru
    December 1, 2017
    Posted on December 01, 2017 at 05:09

    Would expect one could switch the GPIO in/out of AF mode, can say I've tried.

    I think I'd just entirely bit-bang the I2C interface, the STM32 I2C peripheral is as complex as it is annoying.

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    PDR
    PDRAuthor
    Associate II
    December 1, 2017
    Posted on December 01, 2017 at 16:52

    Reconfiguring the pin mode is certainly worth a try. I was hoping to avoid bit-banging it, but agree it may be the simplest approach in the long run.

    Thanks Clive!

    waclawek.jan
    waclawek.jan
    Super User
    December 2, 2017
    Posted on December 02, 2017 at 16:38

    However, when the I2C device is applied, I cannot affect the SCL via this backdoor I/O line. I

    That indicates something seriously broken, like the I2C pin not being set to open drain in GPIO_OTYPER.

    Reconfiguring the pin is just one write to GPIO_AFRL/AFRH; however, that internally would set the I2C module's SCL to 'unconnected' (which in my experience is 0, although I'd lȭve to hear ST confirm this together with a great dozen of other similar details, one very nice day)and I wouldn't be surprised if this would upset the internals of I2C machine - it has its quirks and after

    https://community.st.com/0D50X00009XkaUJSAZ

    of using it I still did not grab it completely. Roughly the same may apply to pulling the clock externally by another pin.

    So, in this particular case, I am with Clive: better bitbang.

    JW

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