3-Pin Microprocessor Reset Circuits
_____________________Pin Description
PIN
NAME
FUNCTION
V
CC
1
GND
Ground
RESET Output remains low while V
is below the reset threshold, and for at
CC
RESET
(MAX803/
MAX809)
MAX809
RESET
least 140ms after V
reset threshold.
rises above the
CC
2
3
R1
RESET Output remains high while V
is below the reset threshold, and for at
CC
100k
RESET
(MAX810)
GND
least 140ms after V
rises above the
CC
reset threshold.
Supply Voltage (+5V, +3.3V, +3.0V, or
+2.5V)
V
CC
Figure 2. RESET Valid to V
= Ground Circuit
CC
Applications Information
400
320
Negative-Going V Transients
CC
T
= +25°C
A
In addition to issuing a reset to the µP during power-up,
power-down, and brownout conditions, the MAX803/
MAX809/MAX810 are relatively immune to short-duration
240
160
80
negative-going V transients (glitches).
CC
Figure 1 shows typical transient duration vs. reset com-
parator overdrive, for which the MAX803/MAX809/
MAX810 do not generate a reset pulse. The graph was
MAX8_ _J/L/M
generated using a negative-going pulse applied to V
,
CC
starting 0.5V above the actual reset threshold and end-
ing below it by the magnitude indicated (reset compara-
tor overdrive). The graph indicates the maximum pulse
MAX8_ _R/S/T/Z
10
0
1
100
1000
width a negative-going V
transient can have without
RESET COMPARATOR OVERDRIVE, V - V (mV)
CC
TH
CC
causing a reset pulse. As the magnitude of the transient
increases (goes farther below the reset threshold), the
maximum allowable pulse width decreases. Typically, for
Figure 1. Maximum Transient Duration Without Causing a
Reset Pulse vs. Reset Comparator Overdrive
the MAX8__L and MAX8__M, a V
transient that goes
CC
100mV below the reset threshold and lasts 20µs or less
will not cause a reset pulse. A 0.1µF bypass capacitor
mounted as close as possible to the V
additional transient immunity.
pin provides
CC
Detailed Description
Ensuring a Valid Reset Output
Down to V = 0
A microprocessor’s (µP’s) reset input starts the µP in a
known state. The MAX803/MAX809/MAX810 assert
reset to prevent code-execution errors during power-
up, power-down, or brownout conditions. They assert a
CC
When V
falls below 1V, the MAX809 RESET output
CC
no longer sinks current—it becomes an open circuit.
Therefore, high-impedance CMOS logic inputs con-
nected to RESET can drift to undetermined voltages.
This presents no problem in most applications since
reset signal whenever the V
supply voltage declines
CC
below a preset threshold, keeping it asserted for at
least 140ms after V has risen above the reset thresh-
CC
most µP and other circuitry is inoperative with V
old. The MAX803 uses an open-drain output, and the
MAX809/MAX810 have a push-pull output stage.
Connect a pull-up resistor on the MAX803’s RESET out-
put to any supply between 0 and 6V.
CC
below 1V. However, in applications where RESET must
be valid down to 0V, adding a pull-down resistor to
RESET causes any stray leakage currents to flow to
_______________________________________________________________________________________
5
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
