PIC17C7XX Datasheet, PDF (29/328 Pages) Microchip Technology – High-Performance 8-Bit CMOS EPROM Microcontrollers with 10-bit A/D

PIC17C7XX
5.1.5 BROWN-OUT RESET (BOR)
PIC17C7XX devices have on-chip Brown-out Reset cir-
cuitry. 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 program
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).
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.
The BODEN configuration bit can disable (if clear/pro-
grammed) or enable (if set) the Brown-out Reset cir-
cuitry. If VDD falls below BVDD (Typically 4.0V,
parameter #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 parameter #35. The chip
will remain in Brown-out Reset until VDD rises above
BVDD. 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 VDD rises above
BVDD, 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:
VDD
33k
EXTERNAL BROWN-OUT
PROTECTION CIRCUIT 1
VDD
10kΩ
40 kΩ
MCLR
PIC17CXXX
This circuit will activate reset when VDD goes below
(Vz + 0.7V) where Vz = Zener voltage.
FIGURE 5-9:
VDD
R1
EXTERNAL BROWN-OUT
PROTECTION CIRCUIT 2
VDD
Q1
MCLR
R2
40 kΩ
PIC17CXXX
This brown-out circuit is less expensive, albeit less
accurate. Transistor Q1 turns off when VDD is below a
certain level such that:
VDD •
R1 _{= 0.7V}
R1 + R2
FIGURE 5-10: BROWN-OUT SITUATIONS
VDD
Internal
Reset
VDD
Greater of 96 ms
and 1024 Tosc
Internal
Reset
VDD
< 96 ms
Greater of 96 ms
and 1024 Tosc
Internal
Reset
Greater of 96 ms
and 1024 Tosc
BVDD Max.
BVDD Min.
BVDD Max.
BVDD Min.
BVDD Max.
BVDD Min.
© 1998 Microchip Technology Inc.
DS30289A-page 29