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 configura-
tion 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 microproces-
sor 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 pre-
vent a RESET/RESET signal. The state of two nonvol-
atile 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.
Figure 1. Watchdog Restart
.6µs
SCL
.6µs
SDA
Start
Condition
Stop
Condition
Restart
Set V
TRIP
Level Sequence (V
CC
= desired V
TRIP
value)
WP
V
P
= 15-18V
0 1 2 3 4 5 6 7
SCL
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
SDA
A0h
01h
00h
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. How-
ever, 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 below, and uses
the application of a nonvolatile control signal.
REV 1.1.3 4/30/02
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.
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Characteristics subject to change without notice.
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