© Koninklijke Philips Electronics N.V. 2005. All rights reserved.
User manual
Rev. 03 — 7 June 2005
39 of 139
Philips Semiconductors
UM10119
P89LPC938 User manual
Enabling and disabling of Brownout Detection is done via the BOPD (PCON.5) bit, bit field
PMOD1/PMOD0 (PCON[1:0]) and user configuration bit BOE (UCFG1.5). If BOE is in an
unprogrammed state, brownout is disabled regardless of PMOD1/PMOD0 and BOPD. If
BOE is in a programmed state, PMOD1/PMOD0 and BOPD will be used to determine
whether Brownout Detect will be disabled or enabled. PMOD1/PMOD0 is used to select
the power reduction mode. If PMOD1/PMOD0 = ‘11’, the circuitry for the Brownout
Detection is disabled for lowest power consumption. BOPD defaults to logic 0, indicating
brownout detection is enabled on power-on if BOE is programmed.
If Brownout Detection is enabled, the brownout condition occurs when V
DD
falls below the
Brownout trip voltage, VBO (see
P89LPC938 data sheet, Static characteristics
), and is
negated when V
DD
rises above VBO. If the P89LPC938 device is to operate with a power
supply that can be below 2.7 V, BOE should be left in the unprogrammed state so that the
device can operate at 2.4 V, otherwise continuous brownout reset may prevent the device
from operating.
If Brownout Detect is enabled (BOE programmed, PMOD1/PMOD0
≠
‘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 a
logic 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 data sheet for specifications.