PIC18F2331_07 Datasheet, PDF (50/400 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High Performance PWM and A/D

PIC18F2331/2431/4331/4431
4.1 Power-on Reset (POR)
A Power-on Reset pulse is generated on-chip when
VDD rise is detected. To take advantage of the POR
circuitry, just tie the MCLR pin through a resistor (1k to
10 kΩ) to VDD. This will eliminate external RC compo-
nents usually needed to create a Power-on Reset
delay. A minimum rise rate for VDD is specified
(parameter D004). For a slow rise time, see Figure 4-2.
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
conditions are met.
Note:
The following decoupling method is
recommended:
1. A 1 μF capacitor should be connected
across AVDD and AVSS.
2. A similar capacitor should be
connected across VDD and VSS.
FIGURE 4-2:
EXTERNAL POWER-ON
RESET CIRCUIT (FOR
SLOW VDD POWER-UP)
VDD VDD
D
R
R1
MCLR
C
PIC18FXXXX
Note 1: External Power-on Reset circuit is
required only if the VDD power-up slope
is too slow. The diode D helps discharge
the capacitor quickly when VDD powers
down.
2: R < 40 kΩ is recommended to make
sure that the voltage drop across R does
not violate the device’s electrical specifi-
cation.
3: R1 ≥ 1 kΩ will limit any current flowing
into MCLR from external capacitor C, in
the event of MCLR/VPP pin breakdown,
due to Electrostatic Discharge (ESD) or
Electrical Overstress (EOS).
4.2 Power-up Timer (PWRT)
The Power-up Timer (PWRT) of the PIC18F2331/2431/
4331/4431 devices is an 11-bit counter, which uses the
INTRC source as the clock input. This yields a count 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 Configuration bit
PWRTEN.
4.3 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 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.
4.4 PLL Lock Time-out
With the PLL enabled in its PLL mode, the time-out
sequence following a Power-on Reset is slightly
different from other oscillator modes. A portion of the
Power-up 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 Brown-out Reset (BOR)
A Configuration bit, BOREN, can disable (if clear/
programmed) or enable (if set) the Brown-out Reset
circuitry. If VDD falls below VBOR (parameter D005A
through D005K) for greater than TBOR (parameter 35),
the brown-out situation will reset the chip. A Reset may
not occur if VDD falls below VBOR for less than TBOR.
The chip will remain in Brown-out Reset until VDD rises
above VBOR. If the Power-up Timer is enabled, it will be
invoked after VDD rises above VBOR; it then will keep
the chip in Reset for an additional time delay TPWRT
(parameter 33). If VDD drops below VBOR while the
Power-up Timer is running, the chip will go back into a
Brown-out Reset and the Power-up Timer will be
initialized. Once VDD rises above VBOR, the Power-up
Timer will execute the additional time delay. Enabling
the Brown-out Reset does not automatically enable the
PWRT.
DS39616C-page 48
Preliminary
© 2007 Microchip Technology Inc.