July, 2004

5

M9999-071504

MIC6315

Micrel

Applications Information

Microprocessor Reset

The /RESET pin is asserted whenever V

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

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

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

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

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-ORed in parallel to allow resets from

multiple sources.

V

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 25

µs 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.

VCC

RESET

5V SYSTEM

RESET

VCC

MIC6315

+5.0V

MR

10k

+3.3V

MIC6315 Used in a Multiple Supply System