Datasheet

PIC16HV540
DS40197B-page 36 Preliminary  2000 Microchip Technology Inc.
7.4 Power-On Reset (POR)
The PIC16HV540 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 sim-
plified block diagram of the on-chip Power-on Reset cir-
cuit is shown in Figure 7-7.
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.
A power-up example where MCLR
is not tied to VDD is
shown in Figure 7-8. 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 7-9, 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 7-10 depicts a problem situation where V
DD
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 cor-
rectly. For such situations, we recommend that external
RC circuits be used to achieve longer POR delay times
(Figure 7-7).
For more information on PIC16HV540 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 7-7: EXTERNAL POWER-ON
RESET CIRCUIT (FOR SLOW
V
DD POWER-UP)
Note: When the device starts normal operation
(exits the reset condition), device operating
parameters (voltage, frequency, tempera-
ture, 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
PIC16HV540
VDDVDD
• External Power-On Reset circuit is required only if
V
DD power-up is too slow. The diode D helps dis-
charge the capacitor quickly when V
DD powers
down.
• R < 40 k
Ω is recommended to make sure that volt-
age drop across R does not violate the device elec-
trical specification.
• R1 = 100
Ω to 1 kΩ will limit any current flowing into
MCLR
from external capacitor C in the event of
MCLR
pin breakdown due to Electrostatic Dis-
charge (ESD) or Electrical Overstress (EOS).