PIC16C5X
7.7
Time-Out Sequence and Power Down
Status Bits (TO/PD)
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.
The TO and PD 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, or a MCLR or WDT wake-up reset.
To reset PIC16C5X devices when a brown-out occurs,
external brown-out protection circuits may be built, as
shown in Figure 7-14 and Figure 7-15.
TABLE 7-6:
TO/PD STATUS AFTER
RESET
FIGURE 7-14: BROWN-OUT PROTECTION
CIRCUIT 1
TO
PD
RESET was caused by
Power-up (POR)
1
u
1
u
(1)
VDD
MCLR reset (normal operation)
1
0
0
0
1
0
MCLR wake-up reset (from SLEEP)
WDT reset (normal operation)
VDD
33k
WDT wake-up reset (from SLEEP)
Q1
Legend: u= unchanged
10k
MCLR
Note 1: The TO and PD bits maintain their status (u) until
a reset occurs. A low-pulse on the MCLR input
does not change the TO and PD status bits.
40k
PIC16C5X
These STATUS bits are only affected by events listed
in Table 7-7.
This circuit will activate reset when VDD goes below Vz +
0.7V (where Vz = Zener voltage).
TABLE 7-7:
EVENTS AFFECTING TO/PD
STATUS BITS
Event
TO PD
Remarks
1
0
1
1
1
u
0
1
Power-up
FIGURE 7-15: BROWN-OUT PROTECTION
CIRCUIT 2
WDT Time-out
No effect on PD
SLEEPinstruction
CLRWDTinstruction
Legend: u= unchanged
VDD
VDD
Note: A WDT time-out will occur regardless of
the status of the TO bit. A SLEEPinstruc-
tion will be executed, regardless of the sta-
tus of the PD bit.
R1
Q1
MCLR
R2
40k
PIC16C5X
Table 7-3 lists the reset conditions for the special
function registers, while Table 7-4 lists the reset
conditions for all the registers.
This brown-out circuit is less expensive, although
less accurate. Transistor Q1 turns off when VDD
is below a certain level such that:
R1
= 0.7V
VDD •
R1 + R2
1998 Microchip Technology Inc.
Preliminary
DS30453B-page 41
MICROCHIP [ MICROCHIP ]
