November 2005 7 MIC2776
MIC2776 Micrel, Inc.
IN
/RST
VDD
/MR GND
MIC2776
R1
676k
1%
R2
324k
1%
/RESET
VCORE
GND
V
CORE
1.0V 5%
Manual
Reset
MICROPROCESSOR
VI/O
V
I/O
2.5V 5%
Figure 1. MIC2776 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 MIC2776N’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.
IN
/RST
VDD
/MR GND
MIC2776N
R1
R2
/RESET
VCC
GND
V
CC
MICROPROCESSOR
100k
Figure 2. Interfacing to Bidirectional Reset Pin
Transient Response
The MIC2776 is inherently immune to very short negative-
going “glitches.” Very brief transients may exceed the voltage
threshold without tripping the output.
As shown in Figure 3, the narrower the transient, the deeper the
threshold overdrive that will be ignored by the MIC2776. The
graph represents the typical allowable transient duration for a
given amount of threshold overdrive that will not generate a reset.
0
5
10
15
20
25
30
35
40
0 100 200 300
RESET COMP. OVERDRIVE, V
REF
–V
IN
(mV)
Typical INPUT
Transien
t Response
Figure 3. Typical INPUT Transient Response
Ensuring Proper Operation at Low Supply
At levels of V
DD
below 1.2V, the MIC2776L’s /RST output
driver cannot turn on sufficiently to produce a valid logic-low
on the /RST output. In this situation, other circuits driven by
/RST could be allowed to float, causing undesired opera-
tion. (In most cases, however, it is expected that the circuits
driven by the MIC2776L will be similarly inoperative at V
DD
≤ 1.2V.)
If a given application requires that /RST be valid below V
DD
=
1.2V, this can be accomplished by adding a pull-down resis-
tor to the /RST output. A value of 100kΩ is recommended as
this is usually an acceptable compromise of leakage current
and pull-down current. The resistor’s value is not critical,
however. See Figure 4.
The statements above also apply to the MIC2776H’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.
IN
/RST
VDD
/MR GND
MIC2776L
R1
R2
/RESET
VCC
GND
V
CC
Manual
Reset
MICROPROCESSOR
100k
Rpull-down
Figure 4. MIC2776L Valid /Reset Below 1.2V
IN
RST
VDD
/MR GND
MIC2776H
R1
R2
RESET
VCC
GND
V
CC
Manual
Reset
MICROPROCESSOR
100k
Rpull-up
Figure 5. MIC2776H Valid Reset Below 1.2V
