PIC18F23K22 Datasheet, PDF (62/492 Pages) Microchip Technology – 28/40/44-Pin, Low-Power, High-Performance Microcontrollers with nanoWatt XLP Technology

PIC18(L)F2X/4XK22
4.4 Brown-out Reset (BOR)
PIC18(L)F2X/4XK22 devices implement a BOR circuit
that provides the user with a number of configuration and
power-saving options. The BOR is controlled by the
BORV<1:0> and BOREN<1:0> bits of the CONFIG2L
Configuration register. There are a total of four BOR
configurations which are summarized in Table 4-1.
The BOR threshold is set by the BORV<1:0> bits. If
BOR is enabled (any values of BOREN<1:0>, except
‘00’), any drop of VDD below VBOR for greater than
TBOR will reset the device. A Reset may or may not
occur if VDD falls below VBOR for less than TBOR. The
chip will remain in Brown-out Reset until VDD rises
above VBOR.
If the Power-up Timer is enabled, it will be invoked after
VDD rises above VBOR; it then will keep the chip in
Reset for an additional time delay, TPWRT. If VDD drops
below VBOR while the Power-up Timer is running, the
chip will go back into a Brown-out Reset and the
Power-up Timer will be initialized. Once VDD rises
above VBOR, the Power-up Timer will execute the
additional time delay.
BOR and the Power-on Timer (PWRT) are
independently configured. Enabling BOR Reset does
not automatically enable the PWRT.
The BOR circuit has an output that feeds into the POR
circuit and rearms the POR within the operating range
of the BOR. This early rearming of the POR ensures
that the device will remain in Reset in the event that VDD
falls below the operating range of the BOR circuitry.
4.4.1 DETECTING BOR
When BOR is enabled, the BOR bit always resets to ‘0’
on any BOR or POR event. This makes it difficult to
determine if a BOR event has occurred just by reading
the state of BOR alone. A more reliable method is to
simultaneously check the state of both POR and BOR.
This assumes that the POR and BOR bits are reset to
‘1’ by software immediately after any POR event. If
BOR is ‘0’ while POR is ‘1’, it can be reliably assumed
that a BOR event has occurred.
4.4.2 SOFTWARE ENABLED BOR
When BOREN<1:0> = 01, the BOR can be enabled or
disabled by the user in software. This is done with the
SBOREN control bit of the RCON register. Setting
SBOREN enables the BOR to function as previously
described. Clearing SBOREN disables the BOR
entirely. The SBOREN bit operates only in this mode;
otherwise it is read as ‘0’.
Placing the BOR under software control gives the user
the additional flexibility of tailoring the application to the
environment without having to reprogram the device to
change BOR configuration. It also allows the user to
tailor device power consumption in software by
eliminating the incremental current that the BOR
consumes. While the BOR current is typically very small,
it may have some impact in low-power applications.
Note:
Even when BOR is under software control,
the BOR Reset voltage level is still set by
the BORV<1:0> Configuration bits. It
cannot be changed by software.
4.4.3 DISABLING BOR IN SLEEP MODE
When BOREN<1:0> = 10, the BOR remains under
hardware control and operates as previously
described. Whenever the device enters Sleep mode,
however, the BOR is automatically disabled. When the
device returns to any other operating mode, BOR is
automatically re-enabled.
This mode allows for applications to recover from
brown-out situations, while actively executing code,
when the device requires BOR protection the most. At
the same time, it saves additional power in Sleep mode
by eliminating the small incremental BOR current.
4.4.4 MINIMUM BOR ENABLE TIME
Enabling the BOR also enables the Fixed Voltage
Reference (FVR) when no other peripheral requiring the
FVR is active. The BOR becomes active only after the
FVR stabilizes. Therefore, to ensure BOR protection,
the FVR settling time must be considered when
enabling the BOR in software or when the BOR is
automatically enabled after waking from Sleep. If the
BOR is disabled, in software or by reentering Sleep
before the FVR stabilizes, the BOR circuit will not sense
a BOR condition. The FVRST bit of the VREFCON0
register can be used to determine FVR stability.
DS41412B-page 62
Preliminary
 2010 Microchip Technology Inc.