MIC2774 Micrel Semiconductor, MIC2774 Datasheet - Page 7

• Manufacturer Part Number: MIC2774
• Description: Dual Micro-Power Low Voltage Supervisor Advance Information
• Manufacturer: Micrel Semiconductor

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.
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.
September 29, 2000
MIC2774
Figure 2. Interfacing to Bidirectional Reset Pin
Figure 3a. Typical INPUT Transient Response
1.0V ±5%
2.5V ±5%
V
V
V
CORE
I/O
CC
Figure 1. MIC2774 Example Design
Manual
Reset
R1
R2
RESET COMP. OVERDRIVE, V
R1
676k
1%
40
35
30
25
20
15
10
R2
324k
1%
5
0
0
MIC2774N-XX
VDD
IN
/MR
Transient Response
MIC2774L-23
VDD
IN
/MR
/RST
GND
100
Typical IN
/RST
GND
100k
200
MICROPROCESSOR
MICROPROCESSOR
REF
VCC
/RESET
–V
VCORE
/RESET
VI/O
300
IN
(mV)
GND
GND
7
Ensuring Proper Operation at Low Supply
At levels of V
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
If a given application requires that /RST be valid below V
= 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
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
Figure 3b. Typical V
V
Figure 4. MIC2774L Valid /RST Below 1.2V
Figure 5. MIC2774H Valid RST Below 1.2V
CC
Manual
Reset
Manual
Reset
R1
R2
DD
R1
R2
RESET COMP. OVERDRIVE, V
100
80
60
40
20
below 1.2V, the MIC2774’s reset output
0
0
MIC2774H-XX
VDD
IN
/MR
MIC2774L-XX
VDD
IN
/MR
Transient Response
500
GND
Typical V
RST
/RST
GND
DD
1000
Transient Response
DD
100k
Rpull-up
100k
Rpull-down
DD
1500
≤ 1.2V.)
MICROPROCESSOR
MICROPROCESSOR
REF
VCC
/RESET
VCC
RESET
–V
2000
DD
(mV)
GND
GND
MIC2774
Micrel
DD
DD
<