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ManualsBrandsMICROCHIP TECHNOLOGY ManualsMicrocontrollers, Microprocessors & QuartzesEmbedded microcontroller PLCC 44 (16.59x16.59) 8-Bit 40 MHz I/O number 33
21222324252627282930
PIC18CXX2
DS39026D-page 26 1999-2013 Microchip Technology Inc.
3.1 Power-on Reset (POR)
A Power-on Reset pulse is generated on-chip when
V
DD rise is detected. To take advantage of the POR cir-
cuitry, just tie the MCLR
pin directly (or through a resis-
tor) to V
DD. This will eliminate external RC components
usually needed to create a Power-on Reset delay. A
minimum rise rate for V
DD is specified (parameter
D004). For a slow rise time, see Figure 3-2.
When the device starts normal operation (i.e., exits the
RESET condition), device operating parameters (volt-
age, frequency, temperature, etc.) must be met to
ensure operation. If these conditions are not met, the
device must be held in reset until the operating condi-
tions are met.
FIGURE 3-2: EXTERNAL POWER-ON
RESET CIRCUIT (FOR
SLOW V
DD POWER-UP)
3.2 Power-up Timer (PWRT)
The Power-up Timer provides a fixed nominal time-out
(parameter #33) only on power-up from the POR. The
Power-up Timer operates on an internal RC oscillator.
The chip is kept in reset as long as the PWRT is active.
The PWRT’s time delay allows V
DD to rise to an accept-
able level. A configuration bit is provided to enable/
disable the PWRT.
The power-up time delay will vary from chip-to-chip due
to V
DD, temperature and process variation. See DC
parameter #33 for details.
3.3 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 #32). This ensures that
the crystal oscillator or resonator has started and
stabilized.
The OST time-out is invoked only for XT, LP and HS
modes and only on Power-on Reset or wake-up from
SLEEP.
3.4 PLL Lock Time-out
With the PLL enabled, the time-out sequence following
a Power-on Reset is different from other oscillator
modes. A portion of the Power-up Timer is used to pro-
vide a fixed time-out that is sufficient for the PLL to lock
to the main oscillator frequency. This PLL lock time-out
(T
PLL) is typically 2 ms and follows the oscillator start-
up time-out (OST).
3.5 Brown-out Reset (BOR)
A configuration bit, BOREN, can disable (if clear/
programmed), or enable (if set) the Brown-out Reset
circuitry. If V
DD falls below parameter D005 for greater
than parameter #35, the brown-out situation will reset
the chip. A RESET may not occur if VDD falls below
parameter D005 for less than parameter #35. The chip
will remain in Brown-out Reset until VDD rises above
BV
DD. The Power-up Timer will then be invoked and
will keep the chip in RESET an additional time delay
(parameter #33). If VDD drops below BVDD while the
Power-up Timer is running, the chip will go back into a
Brown-out Reset and the Power-up Timer will be initial-
ized. Once V
DD rises above BVDD, the Power-up Timer
will execute the additional time delay.
3.6 Time-out Sequence
On power-up, the time-out sequence is as follows:
First, PWRT time-out is invoked after the POR time
delay has expired. Then, OST is activated. The total
time-out will vary based on oscillator configuration and
the status of the PWRT. For example, in RC mode with
the PWRT disabled, there will be no time-out at all.
Figure 3-3, Figure 3-4, Figure 3-5, Figure 3-6 and
Figure 3-7 depict time-out sequences on power-up.
Since the time-outs occur from the POR pulse, if MCLR
is kept low long enough, the time-outs will expire.
Bringing MCLR
high will begin execution immediately
(Figure 3-5). This is useful for testing purposes or to
synchronize more than one PIC18CXXX device oper-
ating in parallel.
Table 3-2 shows the RESET conditions for some
Special Function Registers, while Table 3-3 shows the
RESET conditions for all the registers.