Datasheet

PIC16C505
DS40192D-page 36  1999-2012 Microchip Technology Inc.
7.7 Time-Out Sequence, Power Down,
and Wake-up from SLEEP Status Bits
(TO/PD/RBWUF)
The TO, PD, and RBWUF bits in the STATUS register
can be tested to determine if a RESET condition has
been caused by a power-up condition, a MCLR or
Watchdog Timer (WDT) reset.
7.8 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 PIC16C505 devices when a brown-out
occurs, external brown-out protection circuits may be
built, as shown in Figure 7-12 and Figure 7-13.
FIGURE 7-12: BROWN-OUT PROTECTION
CIRCUIT 1
TABLE 7-7: TO
/PD/RBWUF STATUS
AFTER RESET
RBWUF TO PD RESET caused by
000
WDT wake-up from
SLEEP
00u
WDT time-out (not from
SLEEP)
010
MCLR wake-up from
SLEEP
011
Power-up
0uu
MCLR not during SLEEP
110
Wake-up from SLEEP on
pin change
Legend: u = unchanged
Note 1: The TO
, PD, and RBWUF bits maintain their
status (u) until a reset occurs. A low-pulse on the
MCLR
input does not change the TO, PD, and
RBWUF status bits.
This circuit will activate reset when VDD goes below
Vz + 0.7V (where Vz = Zener voltage).
Note 1: Pin must be confirmed as MCLR
.
33k
10k
40k*
V
DD
MCLR
(1)
PIC16C505
VDD
Q1
FIGURE 7-13: BROWN-OUT PROTECTION
CIRCUIT 2
FIGURE 7-14: BROWN-OUT PROTECTION
CIRCUIT 3
This brown-out circuit is less expensive, although
less accurate. Transistor Q1 turns off when V
DD
is below a certain level such that:
Note 1: Pin must be confirmed as MCLR.
VDD •
R1
R1 + R2
= 0.7V
R2
40k*
V
DD
MCLR
(1)
PIC16C505
R1
Q1
VDD
This brown-out protection circuit employs Microchip
Technology’s MCP809 microcontroller supervisor.
There are 7 different trip point selections to
accommodate 5V to 3V systems.
MCLR
PIC12C5XX
VDD
VDD
VSS
RST
MCP809
VDD
bypass
capacitor