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MSP430F55x power-up current

Other Parts Discussed in Thread: MSP430F5503

Dear MSP430 team,

my customer is using MSP430F5503 in his application.
When he is powering-up the device and keeps the device in Reset (force RST pin low) the MSP430 starts to raw several 100uA of current.

The ramping sequence of the power is quite slow. Ramp 0V -> 3V in  t > 100ms

If he ramps the power with RST pulled high, the device works fine and does not show this behaviour.Is there any sequencing requirement of power and RST signal?

 

 

  

  • The RST pin has an internal pull-up. So holding it low causes some current. I don't have the exact value at hand, but on 3V and 39k resistance, it would be 77µA more than when RST is tied high.
    However, it is requires to hold RST low during ramp-up, because when the brownout voltage is reached, the MSP will start while the supply voltage is still too low for proper operation (there is a gap between brownout trigger point and minimum required voltage, as the brownout detector cannot use a power-consuming precision reference source). The lower the supply ramps up, the higher the chance the MSP will crash at start.

    If the ramp-up time is not too long, a capacitor on RST pin will be sufficient – it keeps RST below the threshold until VCC stops rising. At the same time, the rising voltage on the capacitor will partly reduce the current through the pull-up.

    Critical is the time between reaching the brownout trigger point (which releases the internal brownout reset) to reaching the minimum voltage for operation. Both points vary across devices. The brownout adds a small delay before releasing the CPU, but if ramp-up is slower...

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