Principles of Operation
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 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 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
FIGURE 1. WATCHDOG RESTART
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. However, 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 in the
following and uses the application of a nonvolatile control
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.
To set the new VTRIP voltage, apply the desired VTRIP threshold
voltage to the VCC pin and tie the WP pin to the programming
voltage VP. Then write data 00hto address 01h. The stop bit
following a valid write operation initiates the VTRIP programing
sequence. Bring WP LOW to complete the operation.
WP VP = 15V TO 18V
0 1 2 34 56 7
0 1 23 4 56 7
0 1 23 4 56 7
FIGURE 2. SET VTRIP LEVEL SEQUENCE (VCC = DESIRED VTRIP VALUE)
FN8113 Rev 2.00
June 30, 2008
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