POWER ON RESET
Application of power to the X1227 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 typically 250ms the circuit releases RESET, allow-
ing the system to begin operation. Recommended slew
rate is between 0.2V/ms and 50V/ms.
WATCHDOG TIMER OPERATION
The watchdog timer is selectable. By writing a value to
WD1 and WD0, the watchdog timer can be set to 3 dif-
ferent time out periods or off. When the Watchdog
timer is set to off, the watchdog circuit is conﬁgured for
low power operation.
Watchdog Timer Restart
The Watchdog Timer is started by a falling edge of
SDA when the SCL line is high and followed by a stop
bit. The start signal restarts the watchdog timer
counter, resetting the period of the counter back to the
maximum. If another start fails to be detected prior to
the watchdog timer expiration, then the RESET pin
becomes active. In the event that the start signal
occurs during a reset time out period, the start will
have no effect. When using a single START to refresh
watchdog timer, a STOP bit should be followed to reset
the device back to stand-by mode.
LOW VOLTAGE RESET OPERATION
When a power failure occurs, and the voltage to the
part drops below a ﬁxed vTRIP voltage, a reset pulse is
issued to the host microcontroller. The circuitry moni-
tors the VCC line with a voltage comparator which
senses a preset threshold voltage. Power up and
power down waveforms are shown in Figure 4. The
Low Voltage Reset circuit is to be designed so the
RESET signal is valid down to 1.0V.
When the low voltage reset signal is active, the operation
of any in progress nonvolatile write cycle is unaffected,
allowing a nonvolatile write to continue as long as possi-
ble (down to the power on reset voltage). The low voltage
reset signal, when active, terminates in progress commu-
nications to the device and prevents new commands, to
reduce the likelihood of data corruption.
Figure 3. Watchdog Restart/Time Out
Note: All inputs are ignored during the active reset period (tRST).
REV 1.1.20 1/13/03
Characteristics subject to change without notice. 8 of 28