PIC12F508T-E/MS View Datasheet(PDF) - Microchip Technology
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PIC12F508T-E/MS Datasheet PDF : 110 Pages
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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 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 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 VDD, 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
programming 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)
Oscillator
Configuration
POR Reset
Subsequent
Resets
INTOSC, EXTRC 18 ms (typical)
HS(1), XT, LP
18 ms (typical)
EC(1)
18 ms (typical)
Note 1: PIC16F505 only.
10 μs (typical)
18 ms (typical)
10 μs (typical)
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
periods vary with temperature, VDD and part-to-part
process 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 post-
scaler, if assigned to the WDT. This gives the maximum
Sleep time before a WDT wake-up Reset.
DS41236E-page 52
© 2009 Microchip Technology Inc.
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