Datasheet

© 2007 Microchip Technology Inc. DS41213D-page 27
PIC16F5X
5.2 Device Reset Timer (DRT)
The Device Reset Timer (DRT) provides an 18 ms
nominal time-out on Reset regardless of the oscillator
mode used. The DRT operates on an internal RC
oscillator. 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 chosen 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,
allowing 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 PIC16F5X 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 PIC16F5X 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
PIC16F5X
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:
VDD •
R1
R1 + R2
= 0.7V
R2
40k
VDD
MCLR
PIC16F5X
R1
Q1
VDD
This brown-out protection circuit employs
Microchip 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
selections to accommodate 5V and 3V systems.
MCLR
PIC16F5X
VDD
VSS
RST
MCP809
VDD
Bypass
Capacitor
VDD