PIC16F5X
FIGURE 5-7:
EXTERNAL BROWN-OUT
PROTECTION CIRCUIT 2
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 VDD to rise above
VDD min. and for the chosen oscillator to stabilize.
VDD
VDD
R1
R2
Q1
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
(VIH) 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.
MCLR
40k
PIC16F5X
This brown-out circuit is less expensive, although
less accurate. Transistor Q1 turns off when VDD is
below a certain level such that:
The device Reset time delay will vary from chip-to-chip
due to VDD, temperature and process variation. See
AC parameters for details.
R1
= 0.7V
VDD •
R1 + R2
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.
FIGURE 5-8:
EXTERNAL BROWN-OUT
PROTECTION CIRCUIT 3
5.3
Reset on Brown-Out
VDD
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.
VDD
Bypass
Capacitor
VDD
MCP809
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.
RST
MCLR
PIC16F5X
VSS
FIGURE 5-6:
EXTERNAL BROWN-OUT
PROTECTION CIRCUIT 1
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.
VDD
33k
VDD
Q1
10k
MCLR
40k
PIC16F5X
This circuit will activate Reset when VDD goes below
Vz + 0.7V (where Vz = Zener voltage).
© 2007 Microchip Technology Inc.
DS41213D-page 27
MICROCHIP [ MICROCHIP ]
