Datasheet

 2010 Microchip Technology Inc. DS39770C-page 53
PIC18F85J90 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 k to 10 k) 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.
Power-on Reset events are captured by the POR
bit
(RCON<1>). The state of the bit is set to ‘0’ whenever
a Power-on Reset 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
Power-on Reset.
5.4 Brown-out Reset (BOR)
The PIC18F85J90 family of devices incorporates a
simple BOR function when the internal regulator is
enabled (ENVREG pin is tied to V
DD). The voltage reg-
ulator will trigger a Brown-out Reset when output of the
regulator to the device core approaches the voltage at
which the device is unable to run at full speed. The
BOR circuit also keeps the device in Reset as V
DD
rises, until the regulator’s output level is sufficient for
full-speed operation.
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 the threshold for full-speed operation
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 to the point where
regulator output is sufficient, 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 Brown-out
Reset or Power-on Reset event. This makes it difficult
to determine if a Brown-out Reset 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 Power-on Reset
event. If BOR
is ‘0’ while POR is ‘1’, it can be reliably
assumed that a Brown-out Reset 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 Brown-out Reset event.
The BOR
bit is still cleared by a Power-on Reset event.
5.5 Configuration Mismatch (CM)
The Configuration Mismatch (CM) Reset register is
designed to detect and attempt to recover from
random, memory corrupting events. This includes
Electrostatic Discharge (ESD) events which can cause
widespread, single bit changes throughout the device
and result in catastrophic failure.
In PIC18FXXXX Flash devices, the device Configura-
tion registers (located in the configuration memory
space) are continuously monitored during operation by
comparing their values to complimentary shadow reg-
isters. If a mismatch is detected between the two sets
of registers, a CM Reset automatically occurs.
These events are captured by the CM
bit (RCON<5>).
Whenever a CM event occurs, this bit is set to ‘0’. For
any other Reset event, this bit does not change.
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 k is recommended to make sure that
the voltage drop across R does not violate
the device’s electrical specification.
3: R1  1 k 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
PIC18F85J90
VDD