PIC16C62B/72A
10.4
Power-On Reset (POR)
10.5
Power-up Timer (PWRT)
A Power-on Reset pulse is generated on-chip when
VDD 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 VDD. This will elimi-
nate external RC components usually needed to create
a Power-on Reset. A maximum rise time for VDD is
specified (SVDD, parameter D004). For a slow rise
time, see Figure 10-6.
The Power-up Timer provides a fixed nominal time-out
(TPWRT, parameter #33) 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 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 VDD, temperature and process variation. See DC
parameters for details.
When the device starts normal operation (exits the
reset condition), device operating parameters (voltage,
frequency, temperature,...) must be met to ensure oper-
ation. If these conditions are not met, the device must
be held in reset until the operating conditions are met.
Brown-out Reset may be used to meet the start-up con-
ditions.
10.6
Oscillator Start-up Timer (OST)
The Oscillator Start-up Timer (OST) provides a delay of
1024 oscillator cycles (from OSC1 input) after the
PWRT delay is over (TOST, parameter #32). This
ensures that the crystal oscillator or resonator has
started and stabilized.
FIGURE 10-6: EXTERNAL POWER-ON
RESET CIRCUIT (FOR SLOW
VDD POWER-UP)
The OST time-out is invoked only for XT, LP and HS
modes and only on Power-on Reset or wake-up from
SLEEP.
VDD
Note: The OST delay may not occur when the
device wakes from SLEEP.
D
R
R1
10.7
Brown-Out Reset (BOR)
MCLR
PIC16CXX
The configuration bit, BODEN, can enable or disable
the Brown-Out Reset circuit. If VPP falls below Vbor
(parameter #35, about 100µS), the brown-out situation
will reset the device. If VDD falls below VBOR for less
than TBOR, a reset may not occur.
C
Note 1: External Power-on Reset circuit is required
only if VDD power-up slope is too slow. The
diode D helps discharge the capacitor
quickly when VDD powers down.
Once the brown-out occurs, the device will remain in
brown-out reset until VDD rises above VBOR. The
power-up timer then keeps the device in reset for
TPWRT (parameter #33, about 72mS). If VDD should fall
below VBOR during TPWRT, the brown-out reset pro-
cess will restart when VDD rises above VBOR 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.
2: R < 40 kΩ is recommended to make sure
that voltage drop across R does not violate
the device’s electrical specification.
3: R1 = 100Ω to 1 kΩ will limit any current
flowing into MCLR from external capacitor
C in the event of MCLR/VPP pin break-
down due to Electrostatic Discharge
(ESD) or Electrical Overstress (EOS).
1999 Microchip Technology Inc.
Preliminary
DS35008B-page 59
MICROCHIP [ MICROCHIP ]
