Micrel, Inc.
MIC2786/MIC2787
Asserting /RST and RSTP Outputs
a) When Functioning as a Manual Reset
The reset outputs /RST and RSTP are asserted when
the setup delay time, tSETUP, is exceeded while the /MR0,
/MR1 pins are driven low. Both the reset outputs remain
asserted for a factory-programmed reset timeout delay
time tRESET. Two options are available with 140ms or
240ms minimum tRESET duration. The /RST pin is driven
active low while the RSTP pin is driven active high for
the tRESET duration. The reset outputs are undefined for
VIN < 1.6V.
Figure 3. Voltage Monitor Function
VIN Transients
The MIC2786/MIC2787 is relatively immune to small
negative-going VIN glitches below the reset threshold. As
shown in Figure 4, the overdrive voltage is the difference
between the threshold voltage and the minimum point of
the VIN glitch. Typically, an overdrive of 100mV, with
duration of 20μs or less will not cause a reset.
Figure 2. Manual Reset Function
b) When Functioning as a Voltage Monitor
The /RST pin is asserted whenever VIN falls below the
reset threshold voltage, VTH (VIN < VTH). The VIN pin
circuitry includes hysteresis to prevent /RST or RSTP pin
chattering due to noise. The /RST pin remains asserted
for the duration of the reset timeout delay (tRESET) after
VIN has risen above the reset threshold voltage plus the
hysteresis. The reset function ensures a microprocessor
is properly reset and powers up in a known condition
after a power failure. /RST remains valid with VIN as low
as 1.6V. The RSTP output is a compliment of the /RST
output.
Figure 4. VIN Transient
M9999-102711-A
October 2011
11
MICREL [ MICREL SEMICONDUCTOR ]
