PIC16C505 Datasheet, PDF (34/80 Pages) Microchip Technology – 14-Pin, 8-Bit CMOS Microcontroller

PIC16C505
FIGURE 7-11: TIME-OUT SEQUENCE ON POWER-UP (MCLR TIED TO VDD): SLOW VDD RISE TIME
V1
VDD
MCLR
INTERNAL POR
TDRT
DRT TIME-OUT
INTERNAL RESET
When VDD rises slowly, the TDRT time-out expires long before VDD has reached its final value. In
this example, the chip will reset properly if, and only if, V1 ≥ VDD min.
7.5 Device Reset Timer (DRT)
In the PIC16C505, the DRT runs any time the device is
powered up. DRT runs from RESET and varies based
on oscillator selection (see Table 7-5.)
The Device Reset Timer (DRT) provides a fixed 18 ms
nominal time-out on reset. 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 oscillator to
stabilize.
Oscillator circuits based on crystals or ceramic
resonators 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 MCLR has reached a logic high (VIHMCLR)
level. Thus, programming RB3/MCLR/VPP as MCLR
and using an external RC network connected to the
MCLR input is not required in most cases, allowing for
savings in cost-sensitive and/or space restricted
applications, as well as allowing the use of the RB3/
MCLR/VPP pin as a general purpose input.
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 (only in HS, XT and LP modes). This is
particularly important for applications using the WDT
to wake from SLEEP mode automatically.
7.6 Watchdog Timer (WDT)
The Watchdog Timer (WDT) is a free running on-chip
RC oscillator which does not require any external
components. This RC oscillator is separate from the
external RC oscillator of the RB5/OSC1/CLKIN pin
and the internal 4 MHz oscillator. That means that the
WDT will run even if the main processor clock has
been stopped, for example, by execution of a SLEEP
instruction. During normal operation or SLEEP, a WDT
reset or wake-up reset generates a device RESET.
The TO bit (STATUS<4>) will be cleared upon a
Watchdog Timer reset.
The WDT can be permanently disabled by
programming the configuration bit WDTE as a '0'
(Section 7.1). Refer to the PIC16C505 Programming
Specifications to determine how to access the
configuration word.
TABLE 7-5: DRT (DEVICE RESET TIMER
PERIOD)
Oscillator
Configuration
IntRC &
ExtRC
HS, XT & LP
POR Reset
18 ms (typical)
18 ms (typical)
Subsequent
Resets
300 µs (typi-
cal)
18 ms (typical)
DS40192A-page 34
Preliminary
© 1998 Microchip Technology Inc.