MIC6315_14 Datasheet, PDF (6/8 Pages) Micrel Semiconductor – Open-Drain μP Reset Circuit

Micrel, Inc.
Application 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 1V.
The /RESET output is a simple open-drain N-channel
MOSFET 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.
Wire OR’ing the /RESET Output
Because the RESET output is open-drain, several reset
sources can be wire-OR’ed, in parallel, to allow resets
from multiple sources.
MIC6315
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
duration of 20µs 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.
Figure 1. MIC6315 as Used in a Multiple Supply System
October 17, 2014
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Revision 2.0