PIC17C4X Datasheet, PDF (18/240 Pages) Microchip Technology – High-Performance 8-Bit CMOS EPROM/ROM Microcontroller

PIC17C4X
FIGURE 4-5: OSCILLATOR START-UPTIME
VDD
MCLR
OSC2
TOSC1
OST TIME_OUT
TOST
PWRT TIME_OUT
TPWRT
INTERNAL RESET
This figure shows in greater detail the timings involved
with the oscillator start-up timer. In this example the
low frequency crystal start-up time is larger than
power-up time (TPWRT).
Tosc1 = time for the crystal oscillator to react to an
oscillation level detectable by the Oscillator Start-up
Timer (ost).
TOST = 1024TOSC.
FIGURE 4-6: USING ON-CHIP POR
VDD
VDD
MCLR
PIC17CXX
FIGURE 4-7: BROWN-OUT PROTECTION
CIRCUIT 1
VDD
33k
10k
VDD
MCLR
40 kΩ _PIC17CXX
This circuit will activate reset when VDD goes below
(Vz + 0.7V) where Vz = Zener voltage.
FIGURE 4-8:
PIC17C42 EXTERNAL
POWER-ON RESET CIRCUIT
(FOR SLOW VDD
POWER-UP)
VDD VDD
D
R
C
R1
MCLR
PIC17C42
Note 1: An external Power-on Reset circuit is
required only if VDD power-up time is too
slow. The diode D helps discharge the
capacitor quickly when VDD powers
down.
2: R < 40 kΩ is recommended to ensure
that the voltage drop across R does not
exceed 0.2V (max. leakage current spec.
on the MCLR/VPP pin is 5 µA). A larger
voltage drop will degrade VIH level on the
MCLR/VPP pin.
3: R1 = 100Ω to 1 kΩ will limit any current
flowing into MCLR from external capaci-
tor C in the event of MCLR/VPP pin
breakdown due to Electrostatic Dis-
charge (ESD) or (Electrical Overstress)
EOS.
FIGURE 4-9: BROWN-OUT PROTECTION
CIRCUIT 2
VDD
R1
VDD
Q1
MCLR
R2
40 kΩ _PIC17CXX
This brown-out circuit is less expensive, albeit less
accurate. Transistor Q1 turns off when VDD is below a
certain level such that:
VDD •
R1 _{= 0.7V}
R1 + R2
DS30412C-page 18
© 1996 Microchip Technology Inc.