UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
User manual Rev. 3 — 19 December 2013 23 of 841
NXP Semiconductors
UM10360
Chapter 3: LPC176x/5x System control
3.5 Brown-out detection
The LPC176x/5x includes a Brown-Out Detector (BOD) that provides 2-stage monitoring
of the voltage on the V
DD(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. This signal can be enabled for interrupt in the Interrupt
Enable Register in the NVIC in order to cause a CPU interrupt; if not, software can monitor
the signal by reading the Raw Interrupt Status Register.
The second stage of low-voltage detection asserts Reset to inactivate the LPC176x/5x
when the voltage on the V
DD(REG)(3V3)
pins falls below the BOD reset trip level (typically
1.85 V under nominal room temperature conditions). This Reset prevents alteration of the
flash as operation of the various elements of the chip would otherwise become unreliable
due to low voltage. The BOD circuit maintains this reset down below 1 V, at which point
the Power-On Reset circuitry maintains the overall Reset.
Both the BOD reset interrupt level and the BOD reset trip level thresholds include some
hysteresis. In normal operation, this hysteresis allows the BOD reset interrupt level
detection to reliably interrupt, or a regularly-executed event loop to sense the condition.
But when Brown-Out Detection is enabled to bring the LPC176x/5x out of Power-down
mode (which is itself not a guaranteed operation -- see Section 4.8.7 “
Power Mode
Control register (PCON - 0x400F C0C0)”), the supply voltage may recover from a
transient before the wake-up timer has completed its delay. In this case, the net result of
the transient BOD is that the part wakes up and continues operation after the instructions
that set Power-down mode, without any interrupt occurring and with the BOD bit in the
RSID being 0. Since all other wake-up conditions have latching flags (see Section 3.6.2
“External Interrupt flag register (EXTINT - 0x400F C140)” and Section 27.6.2), a wake-up
of this type, without any apparent cause, can be assumed to be a Brown-Out that has
gone away.
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