LPC2131/2132/2138 User Manual - mct.net

Philips Semiconductors Preliminary User Manual
ARM-based Microcontroller
BROWN-OUT DETECTION
LPC2131/2132/2138
The LPC2131/2132/2138 includes 2-stage monitoring of the voltage on the Vdd pins. If this voltage falls below 2.9V, the Brown-
Out Detector (BOD) asserts an interrupt signal to the Vectored Interrupt Controller. This signal can be enabled for interrupt in the
Interrupt Enable Register (VICIntEnable - 0xFFFFF010, Read/Write) on page 65; if not, software can monitor the signal by
reading the Raw Interrupt Status Register (VICRawIntr - 0xFFFFF008, Read Only) on page 64.
The second stage of low-voltage detection asserts Reset to inactivate the LPC2131/2132/2138 when the voltage on the V3 pins
falls below 2.6V. 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 1V, at which point the Power-On Reset
circuitry maintains the overall Reset.
Both the 2.9V and 2.6V threshholds include some hysteresis. In normal operation, this hysteresis allows the 2.9V detection to
reliably interrupt, or a regularly-executed event loop to sense the condition.
But when Brown-Out Detection is enabled to bring the LPC2131/2132/2138 out of Power-Down mode (which is itself not a
guaranteed operation -- see page 50), the supply voltage may recover from a transient before the Wakeup 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 RISR being 0. Since all other wakeup
conditions have latching flags (see the EXTINT register on page 33 and the RTC ILR on page 209), a wakeup of this type, without
any apparent cause, can be assumed to be a Brown-Out that has gone away.
System Control Block 52 November 22, 2004