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Philips Semiconductors
User’s Manual - Preliminary -
P89LPC906/907/908
POWER MONITORING FUNCTIONS
2003 Dec 8 53
7. POWER MONITORING FUNCTIONS
The P89LPC906/907/908 incorporates power monitoring functions designed to prevent incorrect operation during initial power-
on and power loss or reduction during operation. This is accomplished with two hardware functions: Power-on Detect and
Brownout Detect.
BROWNOUT DETECTION
The Brownout Detect function determines if the power supply voltage drops below a certain level. The default operation for a
Brownout Detection is to cause a processor reset. However, it may alternatively be configured to generate an interrupt by setting
the BOI (PCON.4) bit and the EBO (IEN0.5) bit.
Enabling and disabling of Brownout Detection is done via the BOPD (PCON.5) bit, bit field PMOD1-0 (PCON.1-0) and user
configuration bit BOE (UCFG1.5). If BOE is in an unprogrammed state, brownout is disabled regardless of PMOD1-0 and BOPD.
If BOE is in a programmed state, PMOD1-0 and BOPD will be used to determine whether Brownout Detect will be disabled or
enabled. PMOD1-0 is used to select the power reduction mode. If PMOD1-0 = ’11’, the circuitry for the Brownout Detection is
disabled for lowest power consumption. BOPD defaults to ’0’, indicating brownout detection is enabled on power-on if BOE is
programmed.
If Brownout Detection is enabled, the operating voltage range for V
DD
is 2.7V-3.6V, and the brownout condition occurs when
V
DD
falls below the Brownout trip voltage, V
BO
(see D.C. Electrical Characteristics), and is negated when V
DD
rises above V
BO
.
If Brownout Detection is disabled, the operating voltage range for V
DD
is 2.4V-3.6V. If the P89LPC906/907/908 device is to
operate with a power supply that can be below 2.7V, BOE should be left in the unprogrammed state so that the device can
operate at 2.4V, otherwise continuous brownout reset may prevent the device from operating.
If Brownout Detect is enabled (BOE programmed, PMOD1-0 ≠ ’11’, BOPD = 0), BOF (RSTSRC.5) will be set when a brownout
is detected, regardless of whether a reset or an interrupt is enabled, . BOF will stay set until it is cleared in software by writing ’0’
to the bit. Note that if BOE is unprogrammed, BOF is meaningless. If BOE is programmed, and a initial power-on occurs, BOF
will be set in addition to the power-on flag (POF - RSTSRC.4).
For correct activation of Brownout Detect, certain V
DD
rise and fall times must be observed. Please see the datasheet for
specifications.