PIC16CR7X
DS21993C-page 94
2007 Microchip Technology Inc.
12.4
MCLR
PIC16CR7X 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
has been altered from previous devices of this family.
Voltages applied to the pin that exceed its specification
can result in both MCLR Resets and excessive current
beyond the device specification during the ESD event.
For this reason, Microchip recommends that the MCLR
pin no longer be tied directly to V
DD
. The use of an RC
network, as shown in Figure 12-5, is suggested.
FIGURE 12-5:
RECOMMENDED MCLR
CIRCUIT
12.5
Power-on Reset (POR)
A Power-on Reset pulse is generated on-chip when
V
DD
rise is detected (in the range of 1.2V-1.7V). To take
advantage of the POR, tie the MCLR pin to V
DD
as
described in
Section 12.4 “MCLR”
. A maximum rise
time for V
DD
is specified. See the Electrical Specifica-
tions for details.
When the device starts normal operation (exits the
Reset condition), device operating parameters (volt-
age, frequency, temperature,...) 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. For additional information, refer to Application
Note AN607, “
Power-up Trouble Shooting
” (DS00607).
12.6
Power-up Timer (PWRT)
The Power-up Timer provides a fixed 72 ms nominal
time-out on power-up only 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 acceptable
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 parameters for details (T
PWRT
, parameter #33).
12.7
Oscillator Start-up Timer (OST)
The Oscillator Start-up Timer (OST) provides 1024 oscil-
lator cycles (from OSC1 input) delay after the PWRT
delay is over (if enabled). This helps to ensure 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.
12.8
Brown-out Reset (BOR)
The Configuration bit, BODEN, can enable or disable
the Brown-out Reset circuit. If V
DD
falls below V
BOR
(parameter D005, about 4V) for longer than T
BOR
(parameter #35, about 100
μ
S), the brown-out situation
will reset the device. If V
DD
falls below V
BOR
for less
than T
BOR
, a Reset may not occur.
Once the brown-out occurs, the device will remain in
Brown-out Reset until V
DD
rises above V
BOR
. The
Power-up Timer then keeps the device in Reset for
T
PWRT
(parameter #33, about 72 mS). If V
DD
should fall
below V
BOR
during T
PWRT
, the Brown-out Reset pro-
cess will restart when V
DD
rises above V
BOR
, with the
Power-up Timer Reset. The Power-up Timer is always
enabled when the Brown-out Reset circuit is enabled,
regardless of the state of the PWRT Configuration bit.
12.9
Time-out Sequence
On power-up, the time-out sequence is as follows: the
PWRT delay starts (if enabled) when a POR Reset
occurs. Then, OST starts counting 1024 oscillator
cycles when PWRT ends (LP, XT, HS). When the OST
ends, the device comes out of Reset.
If MCLR is kept low long enough, all delays will expire.
Bringing MCLR high will begin execution immediately.
This is useful for testing purposes or to synchronize
more than one PIC16CR7X device operating in
parallel.
Table 12-5 shows the Reset conditions for the
STATUS, PCON and PC registers, while Table 12-6
shows the Reset conditions for all the registers.
C1
0.1
μ
F
(optional, not critical)
R1
1 k
Ω
(or greater)
V
DD
MCLR
PIC16CR7X