
1995 Microchip Technology Inc.
DS30390B-page 127
PIC16C7X
14.4
Power-on Reset (POR), Power-up
Timer (PWRT) and Oscillator Start-up
Timer (OST), Brown-out Reset (BOR)
Applicable Devices
70 71 71A 72 73 73A 74 74A
14.4.1
POWER-ON RESET (POR)
A Power-on Reset pulse is generated on-chip when
V
DD
rise is detected (in the range of 1.5V - 2.1V). To
take advantage of the POR, just tie the MCLR pin
directly (or through a resistor) to V
DD
. This will elimi-
nate external RC components usually needed to create
a Power-on Reset. A maximum rise time for V
DD
is
specified. See Electrical Specifications for details.
When the device starts normal operation (exits the
reset condition), device operating parameters (voltage,
frequency, temperature, ...) must be meet 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"
14.4.2
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 and
due to V
DD
, temperature, and process variation. See
DC parameters for details.
14.4.3
OSCILLATOR START-UP TIMER (OST)
The Oscillator Start-up Timer (OST) provides 1024
oscillator cycle (from OSC1 input) delay after the
PWRT delay is over. This ensures that the crystal oscil-
lator 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.
14.4.4
BROWN-OUT RESET (BOR)
Applicable Devices
70 71 71A 72 73 73A 74 74A
A configuration bit, BODEN, can disable (if clear/pro-
grammed) or enable (if set) the Brown-out Reset cir-
cuitry. If V
DD
falls below 4.0V (3.8V - 4.2V range) for
greater than parameter #35, the brown-out situation
will reset the chip. A reset may not occur if V
DD
falls
below 4.0V for less than parameter #35. The chip will
remain in Brown-out Reset until V
DD
rises above BV
DD
.
The Power-up Timer will now be invoked and will keep
the chip in RESET an additional 72 ms. If V
DD
drops
below BV
DD
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 BV
DD
, the Power-up Timer will execute a 72 ms
reset. The Power-up Timer should always be enabled
when Brown-out Reset is enabled. Figure 14-11 shows
typical brown-out situations.
FIGURE 14-11: BROWN-OUT SITUATIONS
72 ms
BV
DD
Max.
BV
DD
Min.
V
DD
Internal
Reset
BV
DD
Max.
BV
DD
Min.
V
DD
Internal
Reset
72 ms
<72 ms
72 ms
BV
DD
Max.
BV
DD
Min.
V
DD
Internal
Reset