Philips Semiconductors | User’s Manual - Preliminary - |
| | |
POWER MONITORING FUNCTIONS | P89LPC906/907/908 |
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 VDD is 2.7V-3.6V, and the brownout condition occurs when VDD falls below the Brownout trip voltage, VBO (see D.C. Electrical Characteristics), and is negated when VDD rises above VBO. If Brownout Detection is disabled, the operating voltage range for VDD 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 VDD rise and fall times must be observed. Please see the datasheet for specifications.
