2002 Microchip Technology Inc.
Preliminary
DS30453D-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 VDD to rise above
VDD 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
(VIH) 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 VDD, 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
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
VDD
MCLR
PIC16C5X
VDD
Q1
This brown-out circuit is less expensive, although
less accurate. Transistor Q1 turns off when VDD
is below a certain level such that:
VDD
R1
R1 + R2
= 0.7V
R2
40K
VDD
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
VDD
Vss
RST
MCP809
VDD
bypass
capacitor
VDD