PIC16CR54C
7.4
Power-On Reset (POR)
FIGURE 7-8: EXTERNAL POWER-ON
RESET CIRCUIT (FOR SLOW
VDD POWER-UP)
The PIC16CR54C incorporates 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 7-7.
VDD
VDD
D
R
R1
MCLR
The Power-On Reset circuit and the Device Reset
Timer (Section 7.5) 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.
PIC16CR54C
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 7-9. 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.
• 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).
In Figure 7-10, 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 7-11 depicts a problem situation where VDD
rises too slowly. The time between when the DRT
senses a high on the MCLR/VPP pin, and when the
MCLR/VPP pin (and VDD) actually reach their full value,
is too long. In this situation, when the start-up timer
times out, VDD has not reached the VDD (min) value
and the chip is, therefore, not guaranteed to function
correctly. For such situations, we recommend that
external RC circuits be used to achieve longer POR
delay times (Figure 7-8).
Note: When the device starts normal operation
(exits the reset condition), device operat-
ing parameters (voltage, frequency, tem-
perature, etc.) 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 more information on PIC16CR54C POR, see
Power-Up Considerations - AN522 in the Embedded
Control Handbook.
The POR circuit does not produce an internal reset
when VDD declines.
DS40191A-page 30
Preliminary
1998 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
