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MCT8315A: No replies of MCT when 3.3V is delayed !!!

Eric LOUVEL
Prodigy 115 points
Part Number: MCT8315A
Other Parts Discussed in Thread: MCF8316A

Tool/software:

Hi,

I use a MTC8315AIV on a custom board, and I have an issue about I2C (no response of MCT).
I make a long time to isolate the problem, but now I have only one parameter (+3.3V voltage to delayed, see description below) to reproduce it.

The hardware context on my custom board : 
The input voltage (VM=+7V) is connected to VM pins (9, 10 and 11) of MCT8315.
An internal voltage +3.3V is generated by other component (DC/DC converter) and appears 800ms after VM. This 3.3V voltage is used (with pull-up resistors) on SDA, SCL, FG and FAULT pins of MCT8315.

The hardware context with EVM board to reproduce/investigate it : 
I disconnect the MCT of my custom board (remove serial resictor on I2C bus) and I connect an EVM MCT8316 (Brushless 25 Click board) :GND, SDA and SCL signals.

Case 1 : To reproduce the same issue I2C as my custom board), I connect the 3.3V from my board (800ms of delayed) to EVM board input 3.3V and launch a I2C scan --> the MCT chip doesn't repond

Case 2 : I take the 3.3V voltage from AVDD MCT (pin 27) and connect it to EVM board input 3.3V, and launch a I2C scan --> The MCT chip replies to the I2C scan

Do you have an explanation of this behaviour ? With the 3.3V delayed, is the MCT in sleep mode (I2C disabled) ? if yes, how to exit ?

note : I can't use the AVDD signal from MCT to pull up the I2C, FG and Fault signals because the PCB is already done.

Thank to advance

Eric

  • 0
    Prodigy 115 points

    Please note that I make the I2C scan some seconds after the startup (I wait that the 3.3V is stable). I want to clarify this point because my screenshots can be confusing.

  • 0 in reply to Eric LOUVEL
    TI__Mastermind 24320 points

    Hi Eric,

    When using an external power supply for the pull-ups on FG and nFAULT we recommend having this supply come up before VM. This is because if FG and nFAULT are not pulled up before VM comes up, the MCT8315A can get into a test mode. When in test mode the I2C communication bus on the MCT8315A is disabled which is likely why you are not able to communicate with the device.

    Regards,

    Joshua

  • 0 in reply to Joshua Horlander Cruz
    Prodigy 115 points

    Hi Joshua, thank you for your response.

    I don't see this information in datasheet. Can you give me the paragraph number ?

    I understand that the I2C is disabled when FG and nFault (pull-up) signals are delayed. Is it not the case for SDA and SCL signals ? 

    How to exit of "test mode" ?

    PS: I checked to wire the FG and nFault with pull-up from AVDD (from MCT) but the MCT doesn't replies. Why ?

    Best regards

    Eric

  • 0 in reply to Eric LOUVEL
    TI__Mastermind 24320 points
    Eric LOUVEL said:
    I don't see this information in datasheet. Can you give me the paragraph number ?

    The information relating to having to pull up nFAULT and the FG pins before VM is not provided in the datasheet. I will make a request that this be added to the datasheets.

    Eric LOUVEL said:
    I understand that the I2C is disabled when FG and nFault (pull-up) signals are delayed. Is it not the case for SDA and SCL signals ?

    SDA and SCL pins not being pulled up before VM should not disable I2C. I2C communication will be able to continue when the pull-ups on SDA and SCL are  pulled high.

    Eric LOUVEL said:
    How to exit of "test mode" ?

    Unfortunately the only way to exit the test mode is to power cycle the MCF8316A. I have another solution in mind which I will look into but for this method FG and nFAULT will not be able to be used, is this scenario ok if so I will continue looking into this solution?

    Regards,

    Joshua

  • 0 in reply to Joshua Horlander Cruz
    Prodigy 115 points

    Hi Joshua,

    Thanks for your help.

    Sorry, I need nFault pin (but FG pin is not mandatory in my application).

    I have an additional comment: the MCT doesn't replies to I2C communication but the nFault pin toggle between : high=800ms <-> low=5sec. I don't understand the reason. Do you know this behaviour ? How to know the mode in progress ?

  • 0 in reply to Eric LOUVEL
    TI__Mastermind 24320 points

    Hi Eric,

    Some fault seems to be getting reported, can you check VCP, VFB_BK, VM and AVDD to make sure these voltage are at their expected values?

    Also, try enabling DRVOFF in case the MCT8315A is trying to spin a motor to see if the nFAULT stops switching?

    Regards,

    Joshua