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 VCC exceeds the device VTRIP 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 VCC
level and asserts RESET/RESET if supply voltage falls
below a preset minimum VTRIP. 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 VCC returns and exceeds
VTRIP for 200ms.
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
Set VTRIP Level Sequence (VCC = desired VTRIP value)
WP VP = 15-18V
0 1 2 34 56 7
0 1 23 4 56 7
0 1 23 4 56 7
VCC THRESHOLD RESET PROCEDURE
The X4003/X4005 is shipped with a standard VCC
threshold (VTRIP) voltage. This value will not change
over normal operating and storage conditions. How-
ever, in applications where the standard VTRIP is not
exactly right, or if higher precision is needed in the
VTRIP value, the X4003/X4005 threshold may be
adjusted. The procedure is described below, and uses
the application of a nonvolatile control signal.
Setting the VTRIP Voltage
This procedure is used to set the VTRIP to a higher
voltage value. For example, if the current VTRIP is 4.4V
and the new VTRIP 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.
REV 1.1.3 4/30/02
Characteristics subject to change without notice. 3 of 18