Datasheet

 1997-2013 Microchip Technology Inc. Preliminary DS30453E-page 21
PIC16C5X
5.1 Power-On Reset (POR)
The PIC16C5X family incorporates on-chip Power-On
Reset (POR) circuitry which provides an internal chip
RESET for most power-up situations. To use this fea-
ture, 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.
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.
A power-up example where MCLR
is not tied to VDD is
shown in Figure 5-3. V
DD is allowed to rise and stabilize
before bringing MCLR
high. The chip will actually come
out of reset T
DRT msec after MCLR goes high.
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 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, V
DD
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 cir-
cuits be used to achieve longer POR delay times
(Figure 5-2).
For more information on PIC16C5X POR, see Power-
Up Considerations - AN522 in the Embedded Control
Handbook.
The POR circuit does not produce an internal RESET
when V
DD declines.
FIGURE 5-2: EXTERNAL POWER-ON
RESET CIRCUIT (FOR
SLOW VDD POWER-UP)
Note: When the device starts normal operation
(exits the RESET condition), device oper-
ating parameters (voltage, frequency, tem-
perature, 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.
C
R1
R
D
MCLR
PIC16C5X
VDDVDD
• External Power-On Reset circuit is required
only if V
DD power-up is too slow. The diode D
helps discharge the capacitor quickly when
VDD powers down.
•R < 40k is recommended to make sure that
voltage drop across R does not violate the
device electrical specification.
•R1 = 100 to 1 k will limit any current flow-
ing into MCLR from external capacitor C in the
event of MCLR
pin breakdown due to Electro-
static Discharge (ESD) or Electrical Over-
stress (EOS).