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I'm working with LPC1769, but this applies to any MCU.

The datasheet states that the power supply range is from 2.4V to 3.6V. The included brownout detection circuit triggers a warning interrupt below 2.2V and asserts the Reset below 1.85V.

What happens when the supply voltage is between 1.85V and 2.4V ? Why the BO circuit was not designed to reset the MCU below 2.4V ?

I know that the operation is guaranteed down to 2.4V and that the MCU might work very well even at 1.85V, but this is a grey/risky area.

Thank you.

enter image description here

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  • \$\begingroup\$ Flexibility, probably? You can write an interrupt handler to do the reset as you suggest... but you can also choose to utilize the risky/grey area as well. \$\endgroup\$
    – anrieff
    Commented Jan 28, 2020 at 0:07
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    \$\begingroup\$ It looks strange to me too: there's no point having BOD circuitry if it's triggering below reliable operational values, surely you design the BOD circuitry to do something sensible. Atmega datasheet says "VBOT may be below nominal minimum operating voltage for some devices. For devices where this is the case, the device is tested down to VCC = VBOT during the production test. This guarantees that a Brown-Out Reset will occur before VCC drops to a voltage where correct operation of the microcontroller is no longer guaranteed". Perhaps NXP has a similar statement? \$\endgroup\$
    – jonathanjo
    Commented Jan 28, 2020 at 0:20
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    \$\begingroup\$ It may make sense though if you think about what the BOD is supposed to do, give you enough time to get into a "safe" state. They may have determined that the external caps and internal voltage system gives you "x" amount of time from getting the ISR to the time the processor becomes unreliable, at least enough time to run the interrupt. \$\endgroup\$
    – Ron Beyer
    Commented Jan 28, 2020 at 0:38
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    \$\begingroup\$ I would go with a external BOR IC with value set to about 2.6 V. No risks. \$\endgroup\$
    – User323693
    Commented Jan 28, 2020 at 1:10
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    \$\begingroup\$ In the generic MCU case, there's usually a couple of voltages to pick from, and you can configure if you like it to trip at a certain level, or just give an interrupt and then trip at a lower level etc. This particular MCU just doesn't have very sophisticated brown-out peripheral. Keep in mind that on-chip brown-out detection is still something fairly new - it's only been industry standard for 10 years or so. Not like watchdogs and similar that have been around since the dawn of time. \$\endgroup\$
    – Lundin
    Commented Jan 28, 2020 at 9:29

2 Answers 2

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The datasheet states that the power supply range is from 2.4V to 3.6V. The included brownout detection circuit triggers a warning interrupt below 2.2V and asserts the Reset below 1.85V.

At first glance this appears to be silly. What is the point of having a brownout voltage set below the minimum operating voltage of the chip?

However the datasheet also says "below 1.85 V... operation of the various elements of the chip would otherwise become unreliable due to low voltage.", which implies that it is reliable (though perhaps not meeting all specs) above 1.85 V.

What happens when the supply voltage is between 1.85V and 2.4V ? Why the BO circuit was not designed to reset the MCU below 2.4V ?

We don't know exactly what might happen, but clearly it is expected to work well enough to respond to interrupts, and presumably take appropriate action in preparation for possible loss of supply voltage.

The BOD could have been designed to reset the MCU below 2.4V, but that would make it impossible to operate at 2.4V without risking brownout reset. That voltage would then have to be an absolute minimum rating for operation with BOD enabled, not a normal operating voltage. Which would be a problem, because the feature set includes "Single 3.3 V power supply (2.4 V to 3.6 V)". With no safety margin, how could they guarantee operation at 2.4V?

I'm working with LPC1769, but this applies to any MCU.

Other MCUs may be specified differently. For example the ATmega328p is specified for operation from 1.8 V to 5.5 V, and has nominal BOD voltages of 1.8 V, 2.7 V and 4.3 V. To reliably operate this chip at 1.8V you would to disable the BOD.

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From the user manual:

enter image description here

Please note the typical definitions (emphasis mine): What is missing is the characterization of the BOD module.

The LPC176x/5x includes a Brown-Out Detector (BOD) that provides 2-stage monitoring of the voltage on the VDD(REG)(3V3) pins. If this voltage falls below the BOD interrupt trip level (typically 2.2 V under nominal room temperature conditions), the BOD asserts an interrupt signal to the NVIC.

The second stage of low-voltage detection asserts Reset to inactivate the LPC176x/5x when the voltage on the VDD(REG)(3V3) pins falls below the BOD reset trip level (typically 1.85 V under nominal room temperature conditions).

The BOR feature is defined but the voltage range and the parameters are defined vaguely. There is no entry in the datasheet about the range in the specification table.

  1. As long as the supply range is 2.4 V to 3.3 V, all the operations are guaranteed.
  2. below 2.4 V (which is the range of typical 2.2 V BOR interrupt trip level), user can expect (not 100% guaranteed) an event (interrupt or bit set event)
  3. around 1.85 V, the controller will be reset

What happens when the supply voltage is between 1.85V and 2.4V ?

The Brown detection is meant to trigger below 2.4 V, considering the wide operating temperature range and the internal hardware Brown out detection tolerance, there is a grey area which the supplier did not mention in the datasheet.


There are MCUs where there is sophisticated Brown out detection/Reset implementation. Example below:
enter image description here


If there is a need to operate way up to 2.4 V and a glitch on the supply line is expected to disrupt the normal operation of the device see below

If the project is not limited by price, a dedicated voltage monitoring IC such as this will make sure that, any point in time, if the voltage dips below 2.4 V, the controller will be brought to the reset state.

  • The benefit of this during EMI tests is trivial.
  • The power on Reset delay upto 400 ms cannot be provided by internal BOR (there is no mention
  • the delay always makes sure that the controller will have a stabilized safe operating voltage range before the controller starts booting.
  • Below snippets from the example voltage monitoring IC.

enter image description here enter image description here

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