X4005S8IZ-27 Datasheet, PDF (4/16 Pages) Intersil Corporation – Selectable watchdog timer

X4003, X4005
Principles of Operation
Power-on Reset
Application of power to the X4003/X4005 activates a power-on
reset circuit that pulls the RESET/RESET pin active. This signal
provides several benefits:
• It prevents the system microprocessor from starting to
operate with insufficient voltage.
• It prevents the processor from operating prior to
stabilization of the oscillator.
• It allows time for an FPGA to download its configuration
prior to initialization of the circuit.
When V_CC exceeds the device V_TRIP threshold value for
200ms (nominal) the circuit releases RESET/RESET, allowing
the system to begin operation.
Low Voltage Monitoring
During operation, the X4003/X4005 monitors the V_CC level
and asserts RESET/RESET if supply voltage falls below a
preset minimum V_TRIP. The RESET/RESET signal prevents
the microprocessor from operating in a power fail or
brownout condition. The RESET/RESET signal remains
active until the voltage drops below 1V. It also remains active
until V_CC returns and exceeds V_TRIP for 200ms.
Watchdog Timer
The watchdog timer circuit monitors the microprocessor
activity by monitoring the SDA and SCL pins. The
microprocessor must toggle the SDA pin HIGH to LOW
periodically, while SCL also toggles from HIGH to LOW (this
is a start bit) followed by a stop condition prior to the
expiration of the watchdog time-out period to prevent a
RESET/RESET signal. The state of two nonvolatile control
bits in the control register determine the watchdog timer
period. The microprocessor can change these watchdog
bits, or they may be “locked” by tying the WP pin HIGH.
SCL
0.6µs
0.6µs
SDA
START
CONDITION
RESTART
STOP
CONDITION
FIGURE 1. WATCHDOG RESTART
V_CC Threshold Reset Procedure
The X4003/X4005 is shipped with a standard V_CC threshold
(V_TRIP) voltage. This value will not change over normal
operating and storage conditions. However, in applications
where the standard V_TRIP is not exactly right, or if higher
precision is needed in the V_TRIP value, the X4003/X4005
threshold may be adjusted. The procedure is described in
the following and uses the application of a nonvolatile control
signal.
Setting the V_TRIP Voltage
This procedure is used to set the V_TRIP to a higher voltage
value. For example, if the current V_TRIP is 4.4V and the new
V_TRIP is 4.6V, this procedure will directly make the change. If
the new setting is to be lower than the current setting, then it
is necessary to reset the trip point before setting the new
value.
To set the new V_TRIP voltage, apply the desired V_TRIP
threshold voltage to the V_CC pin and tie the WP pin to the
programming voltage V_P. Then write data 00hto address
01h. The stop bit following a valid write operation initiates the
V_TRIP programing sequence. Bring WP LOW to complete the
operation.
WP
V_P = 15V TO 18V
SCL
0 1 2 34 56 7
0 1 23 4 56 7
0 1 23 4 56 7
SDA
A0h
01h
00h
FIGURE 2. SET V_TRIP LEVEL SEQUENCE (V_CC = DESIRED V_TRIP VALUE)
4
FN8113.2
June 30, 2008