Rainbow Electronics DS28DG02 - Manual (Page 15)

DS28DG02: 2kb SPI EEPROM with PIO, RTC, Reset, Battery Monitor, and Watchdog
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MONITORING FUNCTIONS
The DS28DG02 has two voltage monitors: one for the V
CC
supply voltage and another one for the battery that
supplies the RTC and associated registers if V
CC
is switched off. If V
CC
falls below the V
TRIP
threshold the V
CC
monitor activates the open-drain RSTZ output, as shown in Figure 6. There is a delay of t
DEL
between crossing the
trip point and RSTZ going LOW. As long as V
CC
is above V
POR
or the device has a functioning battery backup, the
logic level at RSTZ does not exceed V
OLmax
. Without battery support, the state of the RSTZ output is undefined for
V
CC
values below V
POR
. When V
CC
ramps up, RSTZ remains at LOW until the V
TRIP
threshold is reached. As V
TRIP
is
crossed, the voltage at RSTZ rises until it reaches V
TRMS
, the manual reset release threshold. This activates the
debounce circuit, which holds RSTZ low for t
RST
. After t
RST
is expired, the voltage at RSTZ ramps up to the value of
the applied pullup voltage.
Figure 6. RSTZ Power-Fail Reset
V
CC
V
TRIP
V
POR
V
CC
*
RSTZ
With the V
BAT
pin tied to V
CC
, the RSTZ behavior for
V
CC
< V
POR
is undefined.
* V
CC
or the applicable pullup voltage for the RSTZ pin.
t
DEL
t
RST
As V
TRIP
is crossed, the voltage at
RSTZ starts rising, which triggers
the manual switch debounce
circuit and activates RSTZ for t
RST
.
The RSTZ pin is internally connected to a debounce circuit, which allows using a manually operated switch to
generate a reset signal. Figure 7 illustrates the timing of the manual reset. As the switch closes, it forces the
voltage at RSTZ to fall below V
ILmax
, which triggers the debounce circuit. Now the voltage at RSTZ is held at logic
LOW by both, the manual switch and the debounce circuit. When the manual switch is opened or t
DEB
is over,
(whichever occurs later) the voltage at RSTZ rises until it reaches V
TRMS
. This again triggers the debounce circuit,
which holds RSTZ low for t
RST
, after which the voltage at RSTZ ramps up to the pullup voltage. The minimum LOW
time of a manually generated reset is t
DEB
+ t
RST
.
Figure 7. RSTZ Manual Switch Debounce
RSTZ held low by
DS28DG02
Open
Manual
Switch
closed
V
CC
*
RSTZ
For t
RST
to start, the voltage at RSTZ has to cross V
TRMS
after t
DEB
is expired.
* V
CC
or the applicable pullup voltage for the RSTZ pin.
V
TRMS
t
RST
t
DEB
RSTZ held low
by manual switch