MIC2774
Typical V
DD
MAX. TRANSIENT DURATION (µs)
Micrel
Transient Response
V
CORE
1.0V
±5%
V
I/O
2.5V
±5%
R1
676k
1%
R2
324k
1%
MIC2774L-23
VDD /RST
IN
MICROPROCESSOR
VCORE
VI/O
/RESET GND
100
80
60
40
20
/MR
Manual
Reset
GND
0
0
500
1000
1500
2000
RESET COMP. OVERDRIVE, V
REF
–V
DD
(mV)
Figure 1. MIC2774 Example Design
Interfacing to Processors With Bidirectional Reset Pins
Some microprocessors have reset signal pins that are bidi-
rectional, 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
MICROPROCESSOR
VCC
MIC2774N-XX
VDD /RST
R1
IN
R2
/MR
GND
100k
/RESET 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
cannot 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.)
If a given application requires that /RST be valid below V
DD
= 1.2V, this can be accomplished by adding a pull-down
resistor to the /RST output. A value of 100kΩ is recom-
mended 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
VCC
MIC2774L-XX
VDD /RST
IN
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.
Typical IN
Transient Response
MAX. TRANSIENT DURATION (µs)
R1
R2
/RESET GND
100k
Rpull-down
/MR
Manual
Reset
GND
40
35
30
25
20
15
10
5
0
0
100
200
300
Figure 4. MIC2774L Valid /RST Below 1.2V
MICROPROCESSOR
VCC
R1
MIC2774H-XX
RST
VDD
IN
100k
Rpull-up
RESET
GND
V
CC
RESET COMP. OVERDRIVE, V
REF
–V
IN
(mV)
R2
/MR
Manual
Reset
GND
Figure 3a. Typical INPUT Transient Response
Figure 5. MIC2774H Valid RST Below 1.2V
September 29, 2000
7
MIC2774
MICREL [ MICREL SEMICONDUCTOR ]
