PIC18FXX8 Datasheet, PDF (28/402 Pages) Microchip Technology – 28/40-Pin High-Performance, Enhanced Flash Microcontrollers with CAN Module

PIC18FXX8
3.1 Power-on Reset (POR)
A Power-on Reset pulse is generated on-chip when a
VDD rise is detected. To take advantage of the POR
circuitry, connect the MCLR pin directly (or through a
resistor) to VDD. This eliminates external RC compo-
nents usually needed to create a Power-on Reset
delay. A minimum rise rate for VDD is specified (refer to
parameter D004). For a slow rise time, see Figure 3-2.
When the device starts normal operation (exits the
Reset condition), device operating parameters
(voltage, 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. Brown-out Reset may be used to meet
the voltage start-up condition.
3.2 MCLR
PIC18FXX8 devices have a noise filter in the MCLR
Reset path. The filter will detect and ignore small
pulses.
It should be noted that a WDT Reset does not drive
MCLR pin low.
The behavior of the ESD protection on the MCLR pin
differs from previous devices of this family. Voltages
applied to the pin that exceed its specification can
result in both Resets and current draws outside of
device specification during the Reset event. For this
reason, Microchip recommends that the MCLR pin no
longer be tied directly to VDD. The use of an RC
network, as shown in Figure 3-2, is suggested.
FIGURE 3-2:
EXTERNAL POWER-ON
RESET CIRCUIT (FOR
SLOW VDD POWER-UP)
VDD
D
R
R1
MCLR
C
PIC18FXXX
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 specification.
3: R1 = 100Ω to 1 kΩ will limit any current flow-
ing into MCLR from external capacitor C, in
the event of MCLR/VPP pin breakdown due to
Electrostatic Discharge (ESD) or Electrical
Overstress (EOS).
3.3 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 VDD to rise to an accept-
able level. A configuration bit (PWRTEN in CONFIG2L
register) is provided to enable/disable the PWRT.
The power-up time delay will vary from chip to chip due
to VDD, temperature and process variation. See DC
parameter #33 for details.
3.4 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 additional
delay ensures that the crystal oscillator or resonator
has started and stabilized.
The OST time-out is invoked only for XT, LP, HS and
HS4 modes and only on Power-on Reset or wake-up
from Sleep.
3.5 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
(TPLL) is typically 2 ms and follows the oscillator
start-up time-out (OST).
3.6 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 parameter D005 for greater
than parameter #35, the brown-out situation resets the
chip. A Reset may not occur if VDD falls below param-
eter D005 for less than parameter #35. The chip will
remain in Brown-out Reset until VDD rises above BVDD.
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 initialized. Once
VDD rises above BVDD, the Power-up Timer will
execute the additional time delay.
DS41159D-page 26
 2004 Microchip Technology Inc.