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PIC18F2525 Datasheet(PDF) 47 Page - Microchip Technology

Part # PIC18F2525
Description  28/40/44-Pin Enhanced Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology
Download  390 Pages
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Manufacturer  MICROCHIP [Microchip Technology]
Direct Link  http://www.microchip.com
Logo MICROCHIP - Microchip Technology

PIC18F2525 Datasheet(HTML) 47 Page - Microchip Technology

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 2004 Microchip Technology Inc.
Preliminary
DS39626B-page 45
PIC18F2525/2620/4525/4620
4.5
Device Reset Timers
PIC18F2525/2620/4525/4620 devices incorporate three
separate on-chip timers that help regulate the Power-on
Reset process. Their main function is to ensure that the
device clock is stable before code is executed. These
timers are:
• Power-up Timer (PWRT)
• Oscillator Start-up Timer (OST)
• PLL Lock Time-out
4.5.1
POWER-UP TIMER (PWRT)
The Power-up Timer (PWRT) of PIC18F2525/2620/
4525/4620 devices is an 11-bit counter which uses
the INTRC source as the clock input. This yields an
approximate time interval of 2048 x 32
µs= 65.6 ms.
While the PWRT is counting, the device is held in
Reset.
The power-up time delay depends on the INTRC clock
and will vary from chip to chip due to temperature and
process variation. See DC parameter 33 for details.
The PWRT is enabled by clearing the PWRTEN
configuration bit.
4.5.2
OSCILLATOR START-UP TIMER
(OST)
The Oscillator Start-up Timer (OST) provides a 1024
oscillator cycle (from OSC1 input) delay after the
PWRT delay is over (parameter 33). This ensures that
the crystal oscillator or resonator has started and
stabilized.
The OST time-out is invoked only for XT, LP, HS and
HSPLL modes and only on Power-on Reset, or on exit
from most power managed modes.
4.5.3
PLL LOCK TIME-OUT
With the PLL enabled in its PLL mode, the time-out
sequence following a Power-on Reset is slightly differ-
ent from other oscillator modes. A separate timer is
used to provide a fixed time-out that is sufficient for the
PLL to lock to the main oscillator frequency. This PLL
lock time-out (TPLL) is typically 2 ms and follows the
oscillator start-up time-out.
4.5.4
TIME-OUT SEQUENCE
On power-up, the time-out sequence is as follows:
1.
After the POR pulse has cleared, PWRT time-out
is invoked (if enabled).
2.
Then, the OST is activated.
The total time-out will vary based on oscillator configu-
ration and the status of the PWRT. Figure 4-3,
Figure 4-4, Figure 4-5, Figure 4-6 and Figure 4-7 all
depict time-out sequences on power-up, with the
Power-up Timer enabled and the device operating in
HS Oscillator mode. Figures 4-3 through 4-6 also
apply to devices operating in XT or LP modes. For
devices in RC mode and with the PWRT disabled, there
will be no time-out at all.
Since the time-outs occur from the POR pulse, if MCLR
is kept low long enough, all time-outs will expire. Bring-
ing MCLR high will begin execution immediately
(Figure 4-5). This is useful for testing purposes or to
synchronize more than one PIC18FXXXX device
operating in parallel.
TABLE 4-2:
TIME-OUT IN VARIOUS SITUATIONS
Oscillator
Configuration
Power-up(2) and Brown-out Reset
Exit from
Power Managed Mode
PWRTEN = 0
PWRTEN = 1
HSPLL
66 ms(1) + 1024 TOSC + 2 ms(2)
1024 TOSC + 2 ms(2)
1024 TOSC + 2 ms(2)
HS, XT, LP
66 ms(1) + 1024 TOSC
1024 TOSC
1024 TOSC
EC, ECIO
66 ms(1)
——
RC, RCIO
66 ms(1)
——
INTIO1, INTIO2
66 ms(1)
——
Note 1:
66 ms (65.5 ms) is the nominal Power-up Timer (PWRT) delay.
2:
2 ms is the nominal time required for the PLL to lock.


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