ADM706P/ADM706R/ADM706S/ADM706T, ADM708R/ADM708S/ADM708T Data Sheet
Rev. E | Page 10 of 16
CIRCUIT INFORMATION
WATCHDOG
TRANSITION
DETECTOR
WATCHDOG
INPUT (WDI)
POWER-FAIL
INPUT (PFI)
POWER-FAIL
OUTPUT (PFO)
WATCHDOG
OUTPUT (WDO)
RESET,
(P = RESET)
*
VOLTAGE REFERENCE = 2.63V (P/R), 2.93V (S), 3.08V (T)
RESET AND
WATCHDOG
TIMEBASE
RESET
GENERATOR
MR
V
CC
70μA
V
CC
WATCHDOG
TIMER
V
REF
*
1.25V
06435-006
ADM706P/ADM706R/
ADM706S/ADM706T
Figure 13. ADM706P/ADM706R/ADM706S/ADM706T Functional Block
Diagram
1.25V
POWER-FAIL
INPUT (PFI)
RESET
* VOLTAGE REFERENCE = 2.63V (R), 2.93V (S), 3.08V (T)
RESET
GENERA
TOR
MR
V
CC
70μ
A
V
CC
RESET
06435-007
V
REF
*
POWER-FAIL
OUTPUT (PFO)
ADM708R/ADM708S/
ADM708T
Figure 14. ADM708R/ADM708S/ADM708T Functional Block Diagram
POWER FAIL RESET
The reset output provides a reset (RESET or
RESET
) output
signal to the microprocessor whenever the V
CC
input is below
the reset threshold. The actual reset threshold voltage is dependent
on whether a P, R, S, or T suffix device is used. An internal timer
holds the reset output active for 200 ms after the voltage on V
CC
rises above the threshold. This is intended as a power-on reset
signal for the microprocessor. It allows time for both the power
supply and the microprocessor to stabilize after power-up. If a
power supply brownout or interruption occurs, the reset line is
similarly activated and remains active for 200 ms after the supply
recovers. If another interruption occurs during an active reset
period, the reset timeout period continues for an additional 200 ms.
The reset output is guaranteed to remain valid with V
CC
as low
as 1 V. This ensures that the microprocessor is held in a stable
shutdown condition as the power supply starts up.
The ADM706P provides an active high RESET signal; the
ADM706R/ADM706S/ADM706T provide an active low
RESET
signal; and the ADM708R/ADM706S/ADM706T provide both
RESET and
RESET
.
MANUAL RESET
The
MR
input allows other reset sources, such as a manual reset
switch, to generate a processor reset. The input is effectively
debounced by the timeout period (200 ms typical). The
MR
input is TTL-/CMOS-compatible; it can also be driven by any
logic reset output. If unused, the
MR
input can be tied high or
left floating.
V
CC
RESET
MR
WDO
V
RT
MR EXTERNALLY
DRIVEN LOW
06435-008
V
RT
t
RS
t
RS
NOTES
RESET = COMPLEMENT OF RESET
Figure 15.
RESET
,
MR
, and
WDO
Timing
WATCHDOG TIMER (ADM706P/ADM706R/
ADM706S/ADM706T)
The watchdog timer circuit monitors the activity of the
microprocessor to check that it is not stalled in an indefinite loop.
An output line on the processor is used to toggle the watchdog
input (WDI) line. If this line is not toggled within the timeout
period (1.6 sec), the watchdog output (
WDO
) is driven low. The
WDO
output is connected to a nonmaskable interrupt (NMI) on
the processor. Therefore, if the watchdog timer times out, an
interrupt is generated. The interrupt service routine is used to
rectify the problem.
The watchdog timer is cleared either by a high to low or by a
low to high transition on WDI. Pulses as narrow as 50 ns are
detected. The timer is also cleared by RESET/
RESET
going
active. Therefore, the watchdog timeout period begins after
reset goes inactive.
When V
CC
falls below the reset threshold,
WDO
is forced low
whether or not the watchdog timer has timed out. Normally,
this generates an interrupt, but it is overridden by RESET/
RESET
going active.
t
WP
WDI
WDO
RESET
t
RS
RESET EXTERNALLY
TRIGGERED BY MR
t
WD
t
WD
t
WD
06435-009
Figure 16. Watchdog Timing
