PRINCIPLES OF OPERATION
Power On Reset
Application of power to the X40626 activates a Power
On Reset Circuit that pulls the 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 sta-
bilization of the oscillator.
– It allows time for an FPGA to download its conﬁgura-
tion prior to initialization of the circuit.
– It prevents communication to the EEPROM, greatly
reducing the likelihood of data corruption on power up.
When VCC exceeds the device VTRIP threshold value
for tPURST (200ms nominal) the circuit releases
RESET allowing the system to begin operation.
LOW VOLTAGE MONITORING
During operation, the X40626 monitors the VCC level
and asserts RESET if supply voltage falls below a pre-
set minimum VTRIP. The RESET signal prevents the
microprocessor from operating in a power fail or brown-
out condition. The 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 is HIGH (this is a start bit) prior to
the expiration of the watchdog time-out period to prevent
a RESET signal. The state of two nonvolatile control bits
in the Status Register determine the watchdog timer
period. The microprocessor can change these watchdog
bits, or they may be “locked” by tying the WP pin HIGH.
EEPROM INADVERTENT WRITE PROTECTION
When RESET goes active as a result of a low voltage
condition or Watchdog Timer Time-Out, any in-progress
communications are terminated. While RESET is
active, no new communications are allowed and no
non-volatile write operation can start. Non-volatile
writes in-progress when RESET goes active are
allowed to ﬁnish.
Additional protection mechanisms are provided with
memory Block Lock and the Write Protect (WP) pin.
These are discussed elsewhere in this document.
VCC/V2MON THRESHOLD RESET PROCEDURE
The X40626 is shipped with a standard VCC threshold
(VTRIP) voltage. This value will not change over normal
operating and storage conditions. However, in applica-
tions where the standard VTRIP is not exactly right, or if
higher precision is needed in the VTRIP value, the
X40626 threshold may be adjusted. The procedure is
described below, and uses the application of a nonvola-
tile control signal.
Setting the VTRIP Voltage
This procedure is used to set the VTRIP to a higher or
lower voltage value. It is necessary to reset the trip
point before setting the new value.
The VCC and V2MON must be tied together during this
Figure 1. Set VTRIP Level Sequence (VCC/V2MON = desired VTRIP values, WP = 12-15V when WEL bit set)
WP VP = 12-15V
0 1 23 4 56 7
REV 1.1.15 2/11/04
*for VVTRIP2 address is 0DH
for VTRIP address is 01H
Characteristics subject to change without notice. 3 of 23