Datasheet

© 2005 Microchip Technology Inc. Preliminary DS41236B-page 49
PIC12F508/509/16F505
7.5 Device Reset Timer (DRT)
On the PIC12F508/509/16F505 devices, 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 7-6).
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 resona-
tors require a certain time after power-up to establish a
stable oscillation. The on-chip DRT keeps the devices in
a Reset condition for approximately 18 ms after MCLR
has reached a logic high (VIH MCLR) level.
Programming (GP3/RB3)/MCLR
/VPP as MCLR and
using an external RC network connected to the MCLR
input is not required in most cases. This allows savings
in cost-sensitive and/or space restricted applications, as
well as allowing the use of the (GP3/RB3)/MCLR/VPP
pin as a general purpose input.
The Device Reset Time delays 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 from Sleep. 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 “Wake-up
from Sleep”, Notes 1, 2 and 3.
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 (GP5/RB5)/OSC1/CLKIN
pin and the internal 4 MHz oscillator. This 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 program-
ming the configuration WDTE as a ‘0’ (see Section 7.1
“Configuration Bits”). Refer to the PIC12F508/509/
16F505 Programming Specifications to determine how
to access the Configuration Word.
TABLE 7-6: DRT (DEVICE RESET TIMER
PERIOD)
7.6.1 WDT PERIOD
The WDT has a nominal time-out period of 18 ms, (with
no prescaler). If a longer time-out period is desired, a
prescaler with a division ratio of up to 1:128 can be
assigned to the WDT (under software control) by
writing to the OPTION register. Thus, a time-out period
of a nominal 2.3 seconds can be realized. These peri-
ods vary with temperature, V
DD and part-to-part pro-
cess variations (see DC specs).
Under worst case conditions (VDD = Min., Temperature
= Max., max. WDT prescaler), it may take several
seconds before a WDT time-out occurs.
7.6.2 WDT PROGRAMMING
CONSIDERATIONS
The CLRWDT instruction clears the WDT and the
postscaler, if assigned to the WDT, and prevents it from
timing out and generating a device Reset.
The SLEEP instruction resets the WDT and the
postscaler, if assigned to the WDT. This gives the
maximum Sleep time before a WDT wake-up Reset.
Oscillator
Configuration
POR Reset
Subsequent
Resets
INTOSC, EXTRC 18 ms (typical) 10 μs (typical)
HS
(1)
, XT, LP 18 ms (typical) 18 ms (typical)
EC
(1)
18 ms (typical) 10 μs (typical)
Note 1: PIC16F505 only.