PIC16F5X
FIGURE 5-2:
EXTERNAL POWER-ON
RESET CIRCUIT (FOR
SLOW VDD POWER-UP)
5.1
Power-on Reset (POR)
The PIC16F5X family of devices incorporate on-chip
Power-on Reset (POR) circuitry which provides an
internal chip Reset for most power-up situations. To
use this feature, the user merely ties the MCLR/VPP pin
to VDD. A simplified block diagram of the on-chip
Power-on Reset circuit is shown in Figure 5-1.
VDD
D
VDD
R
R1
The Power-on Reset circuit and the Device Reset
Timer (Section 5.2) circuit are closely related. On
power-up, the Reset latch is set and the DRT is reset.
The DRT timer begins counting once it detects MCLR
to be high. After the time-out period, which is typically
18 ms, it will reset the Reset latch and thus end the on-
chip Reset signal.
MCLR
PIC16F5X
C
• External Power-on Reset circuit is required
only if VDD power-up is too slow. The diode D
helps discharge the capacitor quickly when
VDD powers down.
A power-up example where MCLR is not tied to VDD is
shown in Figure 5-3. VDD is allowed to rise and stabilize
before bringing MCLR high. The chip will actually come
out of Reset TDRT msec after MCLR goes high.
• R < 40 kΩ is recommended to make sure that
voltage drop across R does not violate the
device electrical specification.
In Figure 5-4, the on-chip Power-on Reset feature is
being used (MCLR and VDD are tied together). The VDD
is stable before the start-up timer times out and there is
no problem in getting a proper Reset. However,
Figure 5-5 depicts a problem situation where VDD rises
too slowly. The time between when the DRT senses a
high on the MCLR/VPP pin and the MCLR/VPP pin (and
VDD) actually reach their full value is too long. In this sit-
uation, when the start-up timer times out, VDD has not
reached the VDD (min) value and the chip is, therefore,
not ensured to function correctly. For such situations,
we recommend that external RC circuits be used to
achieve longer POR delay times (Figure 5-2).
• R1 = 100Ω to 1 kΩ will limit any current
flowing into MCLR from external capacitor C
in the event of MCLR pin breakdown due to
Electrostatic Discharge (ESD) or Electrical
Overstress (EOS).
Note 1: When the device starts normal operation
(exits the Reset condition), device
operating parameters (voltage, fre-
quency, 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.
2: The POR is disabled when the device is
in Sleep.
For more information on the PIC16F5X POR, see
Application Note AN522, “Power-Up Considerations”
at www.microchip.com.
© 2007 Microchip Technology Inc.
DS41213D-page 25
MICROCHIP [ MICROCHIP ]
