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SM320F28335-EP: MCU state when not programmed

Andrea Bonomi
Prodigy 20 points

Part Number: SM320F28335-EP

Goodmorning,

 

I was wondering in which state the microcontroller is when not programmed. Is it in IDLE or in another state? How can I estimate its current consumption?

I am asking because with oscilloscope I am measuring the +3.3V generated by a voltage regulator that feeds the VDD, VDDA pins and it presents a ripple higher than expected. Is it possible that the microcontroller not programmed disturbs the supply?

When I disconnect the regulator from the microcontroller, the ripple of the device improves.

 

Best regards,

 

A.B.

  • 0
    TI__Mastermind 33493 points

    Hi A.B.

    Thanks for reaching out here. Can you provide more information on the test setup? What is the voltage regulator being measured? Can you provide scopeshots of the voltage rail with and without the device connected?

    Are you connecting to the device in anyway during the testing?

    What is the bootmode configuration for the device?

    Glad to help get to the bottom of this.

    Best Regards,

    Zackary Fleenor

  • 0 in reply to Fleenor
    Prodigy 20 points

    Goodmorning Fleenor,

    I reply dot by dot your questions:

    1. I separated the switching voltage regulator in order to characterize it. I connect +5V on its input and measure a correct and stable output of +3.3V. The IC works in forced PWM at 2.5 MHz. I verified with the FFT measurement that it works at this switching frequency. Therefore, I deduce the L and Cout values have been estimated correctly. The measurements are performed with a Lecroy oscilloscope. I place the probe directly on the Cout terminal and the ground tip is connected to the nearest ground.

    2.  Figure 1 represents the power ON of the voltage regulator disconnected from the microcontroller. Figure 2 represents the power ON of the voltage regulator connected to the microcontroller. In this last picture, it can be viewed an initial correct steady state of the +3.3V rail, but after 2.78 ms, it starts oscillating.

    Figure 1                 

    Figure 2                               

    3. No.

    4. The BOOT selection consists of 4.7k resistors, each one connected among +3.3V and one of the following GPIOs: GPIO84, GPIO85, GPIO86, GPIO87 (pin 169, 172, 173, 174). Therefore, the boot mode configuration should be the MODE "F" : Jump to Flash.

    Thanks for your help,

    A.B.

  • 0 in reply to Andrea Bonomi
    TI__Mastermind 33493 points

    Hey Andrea,

    Thank you for providing this additional information.

    Currently, by using the jump to flash boot mode, the device will attempt to jump to flash memory space at completion of ROM boot, but will result in executing ITRAP(Illegal Instruction) since the flash is unprogrammed at 0xFFFF. The device will then attempt to jump to the ITRAP ISR, which will also be undefined. This will put the device in an undefined state and at some point, the device watchdog with trip and reset the device. This loop will continue to perform over and over.

    Our first suggestion would be to place the device into SCI boot mode (this will continuously poll the SCI bus for data - not executing any code) while performing your testing. Or provide a simple program in the flash to prevent the device entering the reset loop described previously.

    In general, we would expect some ripple/supply-disturbance when the device is connected, powered-on, and unprogrammed. Even though there isn't a program loaded, the device still executes ROM boot and then attempts to execute the secondary boot mode configuration. In order for this to happen, the Power Management Module (PMM) and associated voltage regulator and monitor circuits will be active and contribute to some level of transients/noise. Make sure all necessary decoupling capacitors are populated as prescribed by the device datasheet and perhaps over-compensate/over-engineer at this level to account for previously mentioned/expected noise/transients.

    Best Regards,

    Zackary Fleenor