Datasheet

LPC2388_0 © NXP B.V. 2007. All rights reserved.
Preliminary data sheet Rev. 00.01 — 23 October 2007 34 of 57
NXP Semiconductors
LPC2388
Fast communication chip
The first option assumes that power consumption is not a concern and the design ties the
V
DD(3V3)
and V
DD(DCDC)(3V3)
pins together. This approach requires only one 3.3 V power
supply for both pads, the CPU, and peripherals. While this solution is simple, it does not
support powering down the I/O pad ring “on the fly” while keeping the CPU and
peripherals alive.
The second option uses two power supplies; a 3.3 V supply for the I/O pads (V
DD(3V3)
) and
a dedicated 3.3
V supply for the CPU (V
DD(DCDC)(3V3)
). Having the on-chip DC-to-DC
converter powered independently from the I/O pad ring enables shutting down of the I/O
pad power supply “on the fly”, while the CPU and peripherals stay active.
The VBAT pin supplies power only to the RTC and the Battery RAM. These two functions
require a minimum of power to operate, which can be supplied by an external battery.
When the CPU and the rest of chip functions are stopped and power removed, the RTC
can supply an alarm output that may be used by external hardware to restore chip power
and resume operation.
7.26 System control
7.26.1 Reset
Reset has four sources on the LPC2388: the RESET pin, the Watchdog reset, power-on
reset, and the BrownOut Detection (BOD) circuit. The
RESET pin is a Schmitt trigger input
pin. Assertion of chip Reset by any source, once the operating voltage attains a usable
level, starts the Wake-up timer (see description in
Section 7.25.3 “Wake-up timer”),
causing reset to remain asserted until the external Reset is de-asserted, the oscillator is
running, a fixed number of clocks have passed, and the flash controller has completed its
initialization.
When the internal Reset is removed, the processor begins executing at address 0, which
is initially the Reset vector mapped from the Boot Block. At that point, all of the processor
and peripheral registers have been initialized to predetermined values.
7.26.2 Brownout detection
The LPC2388 includes 2-stage monitoring of the voltage on the V
DD(3V3)
pins. If this
voltage falls below 2.95
V, the BOD asserts an interrupt signal to the Vectored Interrupt
Controller. This signal can be enabled for interrupt in the Interrupt Enable Register in the
VIC in order to cause a CPU interrupt; if not, software can monitor the signal by reading a
dedicated status register.
The second stage of low-voltage detection asserts Reset to inactivate the LPC2388 when
the voltage on the V
DD(3V3)
pins falls below 2.65 V. 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 2.95 V and 2.65 V thresholds include some hysteresis. In normal operation, this
hysteresis allows the 2.95
V detection to reliably interrupt, or a regularly executed event
loop to sense the condition.