Datasheet

 1998-2013 Microchip Technology Inc. DS30289C-page 31
PIC17C7XX
5.1.5 BROWN-OUT RESET (BOR)
PIC17C7XX devices have on-chip Brown-out Reset
circuitry. This circuitry places the device into a RESET
when the device voltage falls below a trip point (BVDD).
This ensures that the device does not continue pro-
gram execution outside the valid operation range of the
device. Brown-out Resets are typically used in AC line
applications, or large battery applications, where large
loads may be switched in (such as automotive).
The BODEN configuration bit can disable (if clear/
programmed), or enable (if set) the Brown-out Reset
circuitry. If V
DD falls below BVDD (typically 4.0 V,
paramter #D005 in electrical specification section), for
greater than parameter #35, the Brown-out situation
will reset the chip. A RESET is not guaranteed to occur
if VDD falls below BVDD for less than paramter #35. The
chip will remain in Brown-out Reset until V
DD rises
above BV
DD. The Power-up Timer and Oscillator Start-
up Timer will then be invoked. This will keep the chip in
RESET the greater of 96 ms and 1024 TOSC. If VDD
drops below BVDD while the Power-up Timer/Oscillator
Start-up Timer is running, the chip will go back into a
Brown-out Reset. The Power-up Timer/Oscillator Start-
up Timer will be initialized. Once V
DD rises above
BV
DD, the Power-up Timer/Oscillator Start-up Timer
will start their time delays. Figure 5-10 shows typical
Brown-out situations.
In some applications, the Brown-out Reset trip point of
the device may not be at the desired level. Figure 5-8
and Figure 5-9 are two examples of external circuitry
that may be implemented. Each needs to be evaluated
to determine if they match the requirements of the
application.
FIGURE 5-8: EXTERNAL BROWN-OUT
PROTECTION CIRCUIT 1
FIGURE 5-9: EXTERNAL BROWN-OUT
PROTECTION CIRCUIT 2
FIGURE 5-10: BROWN-OUT SITUATIONS
Note: Before using the on-chip Brown-out for a
voltage supervisory function, please
review the electrical specifications to
ensure that they meet your requirements.
VDD
33k
10k
40 k
V
DD
MCLR
PIC17CXXX
This circuit will activate RESET when VDD goes below
(Vz + 0.7V) where Vz = Zener voltage.
This brown-out circuit is less expensive, albeit less
accurate. Transistor Q1 turns off when V
DD is below a
certain level such that:
VDD •
R1
R1 + R2
= 0.7V
R2
40 k
V
DD
MCLR
PIC17CXXX
R1
Q1
V
DD
Greater of 96 ms
BV
DD Max.
BV
DD Min.
V
DD
Internal
RESET
BVDD Max.
BV
DD Min.
V
DD
Internal
RESET
< 96 ms
BVDD Max.
BV
DD Min.
V
DD
Internal
RESET
and 1024 TOSC
Greater of 96 ms
and 1024 T
OSC
Greater of 96 ms
and 1024 T
OSC