Micrel, Inc.
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, t
SETUP
, is exceeded while the /MR0,
/MR1 pins are driven low. Both the reset outputs remain
asserted for a factory-programmed reset timeout delay
time t
RESET
. Two options are available with 140ms or
240ms minimum t
RESET
duration. The /RST pin is driven
active low while the RSTP pin is driven active high for
the t
RESET
duration. The reset outputs are undefined for
V
IN
< 1.6V.
MIC2786/MIC2787
Figure 3. Voltage Monitor Function
V
IN
Transients
The MIC2786/MIC2787 is relatively immune to small
negative-going V
IN
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 V
IN
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 V
IN
falls below the
reset threshold voltage, V
TH
(V
IN
< V
TH
). The V
IN
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 (t
RESET
) after
V
IN
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 V
IN
as low
as 1.6V. The RSTP output is a compliment of the /RST
output.
Figure 4. V
IN
Transient
October 2011
11
M9999-102711-A
MICREL [ MICREL SEMICONDUCTOR ]
