Power-Up Considerations
Keywords:microchip technology? pic16c5x? microcontroller? harvard architecture? power management?
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2
? 1993 Microchip Technology Inc. DS00522C-page 1
Power-Up Considerations
2
AN522
INTRODUCTION
When powering up all microcontrollers it is necessary for
the power supply voltage to traverse voltage ranges
where the device is not guaranteed to operate before the
power supply voltage reaches its final state. Since some
circuits on the device (logic) will start operating at
voltage levels lower than other circuits on the chip
(memory), the device may power-up in an unknown
state. To guarantee that the device starts up in a known
state, it is necessary that it contain a power-up reset
circuit. PIC16C5X microcontrollers are equipped with
on-chip power-on reset circuitry, which eliminates the
need for external reset logic. This circuit will function in
most power-up situations where VCC rise time is fast
enough (50 ms or less). This application note describes
the typical power-up sequence for PIC16C5X microcon-
trollers. Methods of assuring reset on power-up and
after a brownout are discussed and simple, low cost
external solutions are discussed for power-up situations
where the PIC16C5X's internal circuitry cannot provide
the reset.
POWER-UP SEQUENCE
The PIC16C5X incorporates complex power-on reset
(POR) circuitry on-chip which provides solid, reliable
internal chip reset for most power-up situations. To use
this feature, the user merely needs to tie MCLR to VDD.
A simplified block diagram of the on-chip reset circuitry
is shown in Figure 1. On power-up, the reset latch and
the start-up timer are reset to appropriate states by the
power-on reset (POR). The start-up timer will begin
counting once it detects MCLR to be high (i.e., external
chip reset goes inactive). After the time-out period,
which is typically 18 ms long, the timer will reset the reset
latch and thus end the on-chip reset signal.
Figures 2 and 3 are two power-up situations with relative
fast rise time on VDD. In Figure 1, VDD is stable when
MCLR is brought high (i.e., reset pulse is being provided
by external source). The chip actually comes out reset
about tOST ms after that, where tOST = oscillator start-up
timer. (The timer is called oscillator start-up timer
becausethetime-outwasincorporatedprimarilytoallow
the crystal oscillator to stabilize on power-up.) In
Figure 3, the MCLR and VDD are tied together and
clearly the on-chip rest mechanism is being utilized. The
VDD is stable before the start-up timer expires and there
is no problem with proper reset.
Figure 4, where VDD rise time is much greater than tOST
(typically 18 ms) clearly is the potentially problematic
situation. The POR (power-on reset) pulse comes when
VDD isabout1.5V. MostCMOSlogic,includingthestart-
up timer starts functioning between 1.5V to 2.0V. When
the start-up timer starts times out, the chip reset is ended
and the chip attempts to execute. If by this time the VDD
has reached VDDMIN value, then all circuits are guaran-
teed to function correctly and power-up reset is success-
ful. If, however, the VDD slope was too slow and had not
reached VDDMIN, then the chip may or may not function
properly.
FIGURE 1 - PIC16C5X INTERNAL RESET CIRCUIT
Power-Up Considerations
2-11
RC
OSC
POWER-UP
DETECT
8-BIT ASYNCH
RIPPLE COUNTER
(START-UP TIMER)
CLEAR WDT
RESET
S
R
Q
Q
CHIP_RESET
VDD
POR (POWER-ON RESET)
MCLR
PIN
DS00522C-page 2 ? 1993 Microchip Technology Inc.
Power-Up Considerations
FIGURE 2 - EXTERNAL RESET PULSE
FIGURE 3 - INTERNAL RESET (VDD AND MCLR TIED TOGETHER)
FIGURE 4 - INTERNAL RESET (VDD AND MCLR TIED TOGETHER): SLOW VDD RISE TIME
VDD
INTERNAL POR
MCLR
OST TIME-OUT
INTERNAL RESET
0 V V1
5 V
When VDD rises slowly, the internal time-out period expires long before VDD has reached its final value.
In this example, the chip will reset properly if, and only if, V1 VDDMIN.
tOST
VDD
INTERNAL POR
MCLR
OST TIME-OUT
INTERNAL RESET
tOST
VDD
INTERNAL POR
MCLR
OST TIME-OUT
INTERNAL RESET
tOST
2-12
2
? 1993 Microchip Technology Inc. DS00522C-page 3
Power-Up Considerations
2
EXTERNAL POWER-ON RESET
CIRCUIT
To use power supplies with slow rise times it is neces-
sarytouseanexternalpower-onresetcircuitsuchasthe
one shown in Figure 5. This circuit uses an external RC
to generate the reset pulse. The time constant of the RC
should be long enough to guarantee that the reset pulse
is still present until VDD has reached VDD min. R should
be 40K or less to guarantee that the MCLR will pull to
within 0.2 volts of VDD. (since the leakage spec on
MCLR is ?5 ?A, a resistor larger than 40K may cause
input high voltage on this pin to be less than VDD ? 0.2V,
the required spec). The diode D is used to rapidly
discharge the capacitor on power-down. This is very
important as a power-up reset pulse is needed after a
short power-down (less than the time constant of RC) or
after a power spike. The resistor R1 protects against
high current flowing into MCLR pin from fully charged
capacitorCintheeventMCLRpinbreakdown is induced
through ESD or EOS. The circuit, however, does not
protect against brown-out situations where the power
does not drop to zero, but merely dips below VDD MIN.
In such a situation, voltage at the MCLR pin will not go
low enough (i.e., below VIL) to guarantee a reset pulse.
The following section presents an example circuit to
protect against such brown-outs.
FIGURE 5 - EXTERNAL POWER-ON RESET
CIRCUIT
BROWNOUT PROTECTION
In many applications it is necessary to guarantee a reset
pulse whenever VDD is less than VDD min. This can be
accomplished using a brownout protection circuit such
as the one shown in Figure 6. This is a simple circuit that
causes a reset pulse whenever VDD drops below the
zener diode voltage plus the Vbe of Q1. A 3.3 volt zener
will produce a reset pulse whenever VDD drops below
about 4 volts. This circuit has a typical accuracy of about
?100 mV. A less expensive, albeit less precise, brown-
out circuit is shown in Figure 7. Transistor Q1 turns off
when Vbe = VDD? R1/(R1+R2) falls below 0.7 V allowing
R3 to pull down MCLR input.
FIGURE 6 - BROWNOUT PROTECTION
CIRCUIT
IN4684
VDD
Q1
MCLR
PIC16C5X
100K
10K
VDD
33K
FIGURE 7 - BROWNOUT PROTECTION
CIRCUIT
VDD
Q1
MCLR
PIC16C5X
R3
R3
DS00522C-page 4 ? 1993 Microchip Technology Inc.
Power-Up Considerations
NOTES:
2-14
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All rights reserved. ? 1995, Microchip Technology Incorporated, USA.
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