Datasheet

 1997-2013 Microchip Technology Inc. Preliminary DS30453E-page 23
PIC16C5X
5.2 Device Reset Timer (DRT)
The Device Reset Timer (DRT) provides an 18 ms
nominal time-out on RESET regardless of Oscillator
mode used. The DRT operates on an internal RC oscil-
lator. The processor is kept in RESET as long as the
DRT is active. The DRT delay allows V
DD to rise above
V
DD min., and for the oscillator to stabilize.
Oscillator circuits based on crystals or ceramic resona-
tors require a certain time after power-up to establish a
stable oscillation. The on-chip DRT keeps the device in
a RESET condition for approximately 18 ms after the
voltage on the MCLR
/VPP pin has reached a logic high
(V
IH) level. Thus, external RC networks connected to
the MCLR
input are not required in most cases, allow-
ing for savings in cost-sensitive and/or space restricted
applications.
The Device Reset time delay will vary from chip to chip
due to V
DD, temperature, and process variation. See
AC parameters for details.
The DRT will also be triggered upon a Watchdog Timer
time-out. This is particularly important for applications
using the WDT to wake the PIC16C5X from SLEEP
mode automatically.
5.3 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 PIC16C5X devices when a brown-out
occurs, external brown-out protection circuits may be
built, as shown in Figure 5-6, Figure 5-7 and Figure 5-
8.
FIGURE 5-6: EXTERNAL BROWN-OUT
PROTECTION CIRCUIT 1
FIGURE 5-7: EXTERNAL BROWN-OUT
PROTECTION CIRCUIT 2
FIGURE 5-8: EXTERNAL BROWN-OUT
PROTECTION CIRCUIT 3
This circuit will activate RESET when VDD goes below Vz
+ 0.7V (where Vz = Zener voltage).
33K
10K
40K
V
DD
MCLR
PIC16C5X
VDD
Q1
This brown-out circuit is less expensive, although
less accurate. Transistor Q1 turns off when V
DD
is below a certain level such that:
V
DD •
R1
R1 + R2
= 0.7V
R2
40K
V
DD
MCLR
PIC16C5X
R1
Q1
VDD
This brown-out protection circuit employs Micro-
chip Technology’s MCP809 microcontroller
supervisor. The MCP8XX and MCP1XX families
of supervisors provide push-pull and open collec-
tor outputs with both "active high and active low"
RESET pins. There are 7 different trip point selec-
tions to accommodate 5V and 3V systems.
MCLR
PIC16C5X
V
DD
Vss
RST
MCP809
VDD
bypass
capacitor
VDD