Datasheet
1996 Microchip Technology Inc. DS30430B-page 47
PIC16F8X
8.7 Time-out Sequence and Power-down
Status Bits (TO/PD)
On power-up (Figure 8-10, Figure 8-11, Figure 8-12
and Figure 8-13) the time-out sequence is as follows:
First PWRT time-out is invoked after a POR has
expired. Then the OST is activated. The total time-out
will vary based on oscillator configuration and PWRTE
configuration bit status. For example, in RC mode with
the PWRT disabled, there will be no time-out at all.
TABLE 8-5: TIME-OUT IN VARIOUS
SITUATIONS
Since the time-outs occur from the POR reset pulse, if
MCLR
is kept low long enough, the time-outs will
expire. Then bringing MCLR
high, execution will begin
immediately (Figure 8-10). This is useful for testing
purposes or to synchronize more than one PIC16F8X
device when operating in parallel.
Table 8-6 shows the significance of the T
O and PD bits.
Table 8-3 lists the reset conditions for some special
registers, while Table 8-4 lists the reset conditions for
all the registers.
TABLE 8-6: STATUS BITS AND THEIR
SIGNIFICANCE
Oscillator
Configuration
Power-up Wake-up
from
SLEEP
PWRT
Enabled
PWRT
Disabled
XT, HS, LP 72 ms +
1024TOSC
1024TOSC 1024TOSC
RC 72 ms — —
TO PD Condition
11Power-on Reset
0xIllegal, T
O is set on POR
x0Illegal, PD is set on POR
01WDT Reset (during normal operation)
00WDT Wake-up
11MCLR
Reset during normal operation
10MCLR
Reset during SLEEP or interrupt
wake-up from SLEEP
8.8 Reset on Brown-Out
A brown-out is a condition where device power (VDD)
dips below its minimum value, but not to zero, and
then recovers. The device should be reset in the event
of a brown-out.
To reset a PIC16F8X device when a brown-out occurs,
external brown-out protection circuits may be built, as
shown in Figure 8-14 and Figure 8-15.
FIGURE 8-14: BROWN-OUT PROTECTION
CIRCUIT 1
FIGURE 8-15: BROWN-OUT PROTECTION
CIRCUIT 2
This circuit will activate reset when VDD goes below
(Vz + 0.7V) where Vz = Zener voltage.
VDD
33k
10k
40k
V
DD
MCLR
PIC16F8X
This brown-out circuit is less expensive, although less
accurate. Transistor Q1 turns off when VDD is below a
certain level such that:
VDD •
R1
R1 + R2
= 0.7V
R2
40k
VDD
MCLR
PIC16F8X
R1
Q1
V
DD