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ESD Induced BOR on MSP430F5510

Edward Scheuermann
Prodigy 25 points
Other Parts Discussed in Thread: MSP430F5510

I have a project that I am working on using an MSP430F5510 that interfaces with a small brushed DC motor driver and 2.4GHz wireless transceiver.  All components (including the battery) are housed within a small cylindrical plastic enclosure with plastic end caps, i.e. no external connections.  The only metallic parts visible from outside the enclosure are some small fasteners used for mounting internal components.

The problem is that after applying ESD pulses to the assembled device (air discharge at 15kV according to IEC 61000-4-2 at various points on the enclosure), the MSP430 occasionally "resets".  Fortunately, the device quickly recovers but inspection of the SYSRSTIV register immediately following each reset returns a value of 02h indicating BOR.  This behavior is also consistent with what I've observed in practice using the wireless connection to query the cause of the previous device reset.

I understand that ESD related problems can induce a variety of system level upset conditions including device reset.  However, the cause of each reset is very consistent and always points to the brownout circuity of the MSP430.  With this in mind, I'm a bit lost of how next to proceed with diagnosing/correcting this issue.  I've tried searching the web for resources on system level ESD robustness (including SLAA530) and found several design practices that may help alleviate the problem.  However, I'm hesitant to redesign my PCB without fully understanding how or why a BOR is consistently caused by ESD.

Is a BOR reset even consistent with what one might expect during ESD testing or is it likely to indicate a problem elsewhere in the system (poor power supply, insufficient decoupling, etc.)?  Is there something that I might have overlooked?

My apologies if additional information is needed.  I'm happy to provide other materials if it may help.

Thanks in advance!

  • 0
    Guru 58965 points
    I think ESD could have caused the DVcc to dip below 1.3V locally and temporarily at the multiple DVcc/DVss pair of pins of MSP430. If that is the case, adequate multiple bypass capacitors between each pair of the DVcc/DVss pins near the pins may help.

    Also, is there a Thermal-Pad or something like that under the package? Was that soldered to the PCB? How was it grounded?
  • 0 in reply to old_cow_yellow
    Prodigy 25 points
    There is no thermal pad under the MSP430 (48 pin LQFP package). A common ground pour was used in the PCB layout to ground all components. Additionally, the DC motor can is grounded.

    For decoupling at the MSP430 DVcc/DVss pairs, two ceramic caps are used (10uF and 0.1uF). Each pair of decoupling caps is placed relatively close to the MSP430 but should probably be reoriented and placed even closer according to the recommendations in SLAA530. Unfortunately, this is something I can't test/verify without changing the PCB layout.

    Could it be that more/different decoupling capacitors are needed or is the PCB layout really that sensitive with respect to decoupling capacitor placement?
  • 0 in reply to Edward Scheuermann
    Guru 23000 points
    To protect against an ESD avoid ESD currents trough the PCB even through a (ground) copper pour.
    Ground each external device separately, antenna (base), motor (casing), mounting screws etc, using a good quality Litzen wire in a star connection.

    Capacitors gives less protection but if used, use 1nF or lower value. Ferrite beads will give a good protection against a low current ESD. To reduce an ESD current add small inductors or resistors in-between external connections (motor-drive).

    Depending on the plastic used for the enclosure, it can charge up highly. Use an inside screening or conductive spray.

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