PIC17C75X Datasheet, PDF (28/320 Pages) Microchip Technology – High-Performance 8-Bit CMOS EPROM Microcontrollers

PIC17C75X
5.1.5 BROWN-OUT RESET (BOR)
PIC17C75X devices have an 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).
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.
A configuration bit, BODEN, 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 D035, the brown-out situation
will reset the chip. A reset is not guaranteed to occur if
VDD falls below BVDD for less than parameter D035.
The chip will remain in Brown-out Reset until VDD rises
above BVDD. The Power-up Timer will now be invoked
and will keep the chip in reset an additional 96 ms. If
VDD drops below BVDD 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 BVDD, the Power-up Timer will execute a
96 ms time delay. 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
10k
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
96 ms
Internal
Reset
VDD
< 96 ms _{96 ms}
Internal
Reset
96 ms
BVDD Max.
BVDD Min.
BVDD Max.
BVDD Min.
BVDD Max.
BVDD Min.
DS30264A-page 28
Preliminary
© 1997 Microchip Technology Inc.