Datasheet

PIC18F2221/2321/4221/4321 FAMILY
DS39689F-page 50 © 2009 Microchip Technology Inc.
5.4 Brown-out Reset (BOR)
PIC18F2221/2321/4221/4321 family 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>
Configuration bits. There are a total of four BOR
configurations which are summarized in Table 5-1.
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 V
DD below VBOR (parameter D005) for
greater than T
BOR (parameter 35) will reset the device.
A Reset may or may not occur if V
DD falls below VBOR
for less than TBOR. The chip will remain in Brown-out
Reset until V
DD rises above VBOR.
If the Power-up Timer is enabled, it will be invoked after
V
DD rises above VBOR; it then will keep the chip in
Reset for an additional time delay, T
PWRT
(parameter 33). 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 V
DD 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.
5.4.1 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
control bit, SBOREN (RCON<6>). 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 its
environment without having to reprogram the device to
change BOR configuration. It also allows the user to
tailor device power consumption in software by elimi-
nating the incremental current that the BOR consumes.
While the BOR current is typically very small, it may
have some impact in low-power applications.
5.4.2 DETECTING BOR
When Brown-out Reset is enabled, the BOR bit always
resets to ‘0’ on any Brown-out Reset or Power-on
Reset event. This makes it difficult to determine if a
Brown-out Reset 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
bit is reset to ‘1’ in software
immediately after any Power-on Reset event. If BOR
is
‘0’ while POR
is ‘1’, it can be reliably assumed that a
Brown-out Reset event has occurred.
5.4.3 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.
TABLE 5-1: BOR CONFIGURATIONS
Note: Even when BOR is under software control,
the Brown-out Reset voltage level is still
set by the BORV<1:0> Configuration bits.
It cannot be changed in software.
BOR Configuration Status of
SBOREN
(RCON<6>)
BOR Operation
BOREN1 BOREN0
00Unavailable BOR disabled; must be enabled by reprogramming the Configuration bits.
01Available BOR enabled in software; operation controlled by SBOREN.
10Unavailable BOR enabled in hardware in Run and Idle modes, disabled during
Sleep mode.
11Unavailable BOR enabled in hardware; must be disabled by reprogramming the
Configuration bits.