Rainbow Electronics DS1680 - Manual (Page 10)

DS1680
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MICROPROCESSOR MONITOR
The DS1680 monitors three vital conditions for a microprocessor: power supply, software execution, and
external override.
First, a precision temperature-compensated reference and comparator circuit monitors the status of V
CC
.
When an out-of-tolerance condition occurs, an internal power-fail signal is generated that forces the RST
pin to the active state, thus warning a processor-based system of impending power failure. When V
CC
returns to an in- tolerance condition upon power-up, the reset signal is kept in the active state for t
RST
to
allow the power supply and microprocessor to stabilize. Note, however, that if the EOSC bit is set to a
logic 1 (to disable the oscillator during battery-backup mode), the RST signal will be kept in an active
state for t
RST
plus the start-up time of the oscillator.
The second monitoring function is pushbutton reset control. The DS1680 provides for a pushbutton
switch to be connected to the
RST output pin. When the DS1680 is not in a reset cycle, it continuously
monitors the RST signal for a low-going edge. If an edge is detected, the DS1680 will debounce the
switch by pulling the RST line low. After the internal timer has expired, the DS1680 will continue to
monitor the RST line. If the line is still low, the DS1680 will continue to monitor the line looking for a
rising edge. Upon detecting release, the DS1680 will force the RST line low and hold it low for t
RST
.
The third microprocessor monitoring function provided by the DS1680 is a watchdog timer. The
watchdog timer function forces
RST to the active state when the ST input is not stimulated within the
predetermined time period. The time period is set by the time delay (TD) bits in the watchdog register.
The time delay can be set to 250ms, 500ms, or 1000ms. If TD0 and TD1 are both set to zero, the
watchdog timer is disabled. When enabled, the watchdog timer starts timing out from the set time period
as soon as
RST is inactive. The default setting is for the watchdog timer to be enabled with 1000ms time
delay. If a high-to- low transition occurs on the ST input pin prior to time-out, the watchdog timer is reset
and begins to time-out again. If the watchdog timer is allowed to time-out, the RST signal is driven to the
active state for t
RST
. The ST input can be derived from microprocessor address signals, data signals,
and/or control signals. To guarantee that the watchdog timer does not time-out, a high-to- low transition
must occur at or less than the minimum period.
WATCHDOG REGISTER – 0Dh
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
0 0 0 0 0 0 TD1 TD0
WATCHDOG TIME-OUT BITS Table 2
TD1 TD0 WATCHDOG TIME-OUT
0 0 Watchdog Disabled
0 1 250ms
1 0 500ms
1 1 1000ms