Datasheet

PIC16C505
DS40192D-page 34 1999-2012 Microchip Technology Inc.
FIGURE 7-10: TIME-OUT SEQUENCE ON POWER-UP (MCLR TIED TO VDD): SLOW VDD RISE TIME
VDD
MCLR
INTERNAL POR
DRT TIME-OUT
INTERNAL RESET
TDRT
V1
Note: 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 V
DD 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 and reset type (see Table 7-5).
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 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 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 from SLEEP
mode automatically.
Reset sources are POR, MCLR
, WDT time-out and
Wake-up on pin change. (See Section 7.9.2, Notes 1,
2, and 3, page 37.)
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
POR Reset
Subsequent
Resets
IntRC &
ExtRC
18 ms (typical) 300 µs
(typical)
HS, XT & LP 18 ms (typical) 18 ms (typical)