MIC6315 Datasheet, PDF (5/6 Pages) Micrel Semiconductor – Open-Drain UP Reset Circuit

MIC6315
Applications Information
Microprocessor Reset
The /RESET pin is asserted whenever V_CC falls below the
reset threshold voltage or if /MR (manual reset) is forced low.
The /RESET pin remains asserted for the duration of the reset
timeout period after V_CC has risen above the reset threshold
or /MR has returned high. The reset function ensures the
microprocessor is properly reset and powers up in a known
condition after a power failure. /RESET will remain valid with
V_CC as low as 1.4V.
The /RESET output is a simple open-drain N-channel MOS-
FET structure. A pull-up resistor must be used to pull this
output up to some voltage. For most applications, this voltage
will be the same power supply that supplies V_CC to the
MIC6315. It is possible however to tie this resistor to some
other voltage. This will allow the MIC6315 to monitor one
voltage while level-shifting the /RESET output to some other
voltage. The pull-up voltage must be limited to 6.0V or less
(absolute maximum) to avoid damage to the MIC6315. The
resistor must be small enough to supply current to the inputs
and leakage paths that are driven by the /RESET output.
/RESET Valid at Low Voltage
As V_CC drops to 0V, the MIC6315 will no longer be able to pull
the /RESET output low. At this point, the pull-up resistor will
pull the output high. The value of the pull-up resistor and the
voltage it is connected to will affect the point at which this
happens.
Micrel
Wire OR'ing The /RESET Output
Because the RESET output is open-drain, several reset
sources can be wire-ORed in parallel to allow resets from
multiple sources.
V_CC Transients
The MIC6315 is relatively immune to negative-going V_CC
glitches below the reset threshold. Typically, a negative-
going transient 125mV below the reset threshold with a
duration of 25µs or less will not cause an unwanted reset. If
additional transient immunity is needed, a bypass capacitor
can be placed a close as possible to the MIC6315.
+3.3V
+5.0V
VCC
10k
MIC6315
RESET
MR
VCC
5V SYSTEM
RESET
MIC6315 Used in a Multiple Supply System
July, 2004
5
M9999-071504