MIC2774
T ypic al V
MAX. TRANSIENT DURATION (µs)
Micrel
MICROPROCESSOR
OCESSO
OCESSOR
VCORE
VI/O
MIC2774L-23
/RST
VDD
IN
/RESET
GND
100
80
60
40
20
0
0
500
1000
1500
2000
RESET COMP. OVERDRIVE, V
REF
–V
DD
(mV)
V
CORE
1.0V 5%
V
I/O
2.5V 5%
R1
676k
1%
R2
324k
1%
Manual
Reset
ese
eset
T rans ient R es pons e
DD
/MR
GND
Figure 1. MIC2774 Example Design
Interfacing to Processors With Bidirectional Reset Pins
Some microprocessors have reset signal pins that are bi-
directional, rather than input only. The Motorola 68HC11
family is one example. Because the MIC2774N’s output is
open-drain, it can be connected directly to the processor’s
reset pin using only the pull-up resistor normally required.
See Figure 2.
V
CC
100k
/RESET
GND
MICROPROCESSOR
OCESSO
OCESSOR
VCC
MIC2774N-
MIC2774N-XX
/RST
VDD
R1
IN
R2
/MR
GND
Figure 3b. Typical V
DD
Transient Response
Ensuring Proper Operation at Low Supply
At levels of V
DD
below 1.2V, the MIC2774’s reset output can-
not turn on sufficiently to produce a valid logic-low on /RST.
In this situation, circuits driven by /RST could be allowed to
float, causing undesired operation. (In most cases, however,
it is expected that the circuits driven by the MIC2774L will be
similarly inoperative at V
DD
≤ 1.2V.)
Figure 2. Interfacing to Bidirectional Reset Pin
Transient response
The MIC2774 is inherently immune to very short negative-
going “glitches.” Very brief transients may exceed the voltage
thresholds without tripping the output.
As shown in Figure 3a and 3b, in general the narrower the
transient, the deeper the threshold overdrive that will be
ignored by the MIC2774. The graphs represent the typical
allowable transient duration for a given amount of threshold
overdrive that will not generate a reset.
MAX. TRANSIENT DURATION (µs)
If a given application requires that /RST be valid below V
DD
= 1.2V, this can be accomplished by adding a pull-down re-
sistor to the /RST output. A value of 100kΩ is recommended
as this is usually an acceptable compromise of quiescent
current and pull-down current. The resistor’s value is not
critical, however. See Figure 4.
The statements above also apply to the MIC2774H’s RST
output. That is, to ensure valid RST signal levels at V
DD
<
1.2V, a pull-up resistor (as opposed to a pull-down) should
be added to the RST output. A value of 100kΩ is typical for
this application as well. See Figure 5.
V
CC
MICROPROCESSOR
OCESSO
OCESSOR
VCC
MIC2774L-
MIC2774L-XX
/RST
VDD
IN
R1
R2
/RESET
100k
Rpull-d
Rpull-down
GND
/MR
Manual
Reset
ese
eset
GND
40
35
30
25
20
15
10
5
0
0
T ypic al IN
T rans ient R es pons e
Figure 4. MIC2774L Valid /RST Below 1.2V
V
CC
100k
Rpull-up
RESET
GND
MICROPROCESSOR
OCESSO
OCESSOR
VCC
R1
100
200
300
MIC2774H-
MIC2774H-XX
RST
VDD
IN
R2
/MR
Manual
Reset
ese
eset
GND
RESET COMP. OVERDRIVE, V
REF
–V
IN
(mV)
Figure 3a. Typical INPUT Transient Response
Figure 5. MIC2774H Valid RST Below 1.2V
October 2005
7
M9999-102605
MICREL [ MICREL SEMICONDUCTOR ]
