Datasheet

© 2009 Microchip Technology Inc. DS39663F-page 49
PIC18F87J10 FAMILY
5.2 Master Clear (MCLR)
The MCLR pin provides a method for triggering a hard
external Reset of the device. A Reset is generated by
holding the pin low. PIC18 extended microcontroller
devices have a noise filter in the MCLR
Reset path
which detects and ignores small pulses.
The MCLR
pin is not driven low by any internal Resets,
including the WDT.
5.3 Power-on Reset (POR)
A Power-on Reset condition is generated on-chip
whenever V
DD rises above a certain threshold. This
allows the device to start in the initialized state when
V
DD is adequate for operation.
To take advantage of the POR circuitry, tie the MCLR
pin through a resistor (1 kΩ to 10 kΩ) to VDD. 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 5-2.
When the device starts normal operation (i.e., 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
conditions are met.
POR events are captured by the POR
bit (RCON<1>).
The state of the bit is set to ‘0’ whenever a POR occurs;
it does not change for any other Reset event. POR is
not reset to ‘1’ by any hardware event. To capture
multiple events, the user manually resets the bit to ‘1’
in software following any POR.
5.4 Brown-out Reset (BOR)
The PIC18F87J10 family of devices incorporate a
simple BOR function when the internal regulator is
enabled (ENVREG pin is tied to V
DD). Any drop of VDD
below VBOR (parameter D005) for greater than time
T
BOR (parameter 35) will reset the device. A Reset may
or may not occur if V
DD falls below VBOR for less than
T
BOR. The chip will remain in Brown-out Reset until
V
DD rises above VBOR.
Once a BOR has occurred, the Power-up Timer will
keep the chip in Reset for T
PWRT (parameter 33). If
V
DD 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 V
DD
rises above VBOR, the Power-up Timer will execute the
additional time delay.
FIGURE 5-2: EXTERNAL POWER-ON
RESET CIRCUIT (FOR
SLOW V
DD POWER-UP)
5.4.1 DETECTING BOR
The BOR bit always resets to ‘0’ on any BOR or POR
event. This makes it difficult to determine if a BOR
event has occurred just by reading the state of BOR
alone. A more reliable method is to simultaneously
check the state of both POR
and BOR. This assumes
that the POR
bit is reset to ‘1’ in software immediately
after any POR event. If B
OR is ‘0’ while POR is ‘1’, it
can be reliably assumed that a BOR event has
occurred.
If the voltage regulator is disabled, Brown-out Reset
functionality is disabled. In this case, the BOR
bit
cannot be used to determine a BOR event. The BOR
bit is still cleared by a POR event.
Note 1: External Power-on Reset circuit is required
only if the V
DD power-up slope is too slow.
The diode D helps discharge the capacitor
quickly when V
DD 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 ≥ 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).
C
R1
R
D
V
DD
MCLR
PIC18F87J10
VDD