X5645 Datasheet, PDF (3/19 Pages) Xicor Inc. – CPU Supervisor with 64Kbit SPI EEPROM

X5643/X5645
PRINCIPLES OF OPERATION
Power On Reset
Application of power to the X5643/X5645 activates a
power on reset circuit. This circuit goes active at about
1V and pulls the RESET/RESET pin active. This signal
prevents the system microprocessor from starting to
operate with insufficient voltage or prior to stabilization
of the oscillator. When V_CC exceeds the device V_TRIP
value for 200ms (nominal) the circuit releases RESET/
RESET, allowing the processor to begin executing code.
Low Voltage Monitoring
During operation, the X5643/X5645 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 microprocessor
activity by monitoring the WDI input. The microproces-
sor must toggle the CS/WDI pin periodically to prevent a
RESET/RESET signal. The CS/WDI pin must be toggled
from HIGH to LOW prior to the expiration of the watchdog
time out period. 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 LOW
and setting the WPEN bit HIGH.
V_CC Threshold Reset Procedure
The X5643/X5645 has a standard V_CC threshold
(V_TRIP) voltage. This value will not change over normal
operating and storage conditions. However, in applica-
tions where the standard V_TRIP is not exactly right, or
for higher precision in the V_TRIP value, the X5643/
X5645 threshold may be adjusted.
Setting the V_TRIP Voltage
This procedure sets 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 directly makes the
change. If the new setting is lower than the current set-
ting, then it is necessary to reset the trip point before
setting the new value.
To set the new V_TRIP voltage, apply the desired V_TRIP
threshold to the Vcc pin and tie the CS/WDI pin and the
WP pin HIGH. RESET/RESET and SO pins are left
unconnected. Then apply the programming voltage V_P
to both SCK and SI and pulse CS/WDI LOW then
HIGH. Remove V_P and the sequence is complete.
Figure 1. Set V_TRIP Voltage
CS
V_P
SCK
V_P
SI
Resetting the V_TRIP Voltage
This procedure sets the V_TRIP to a “native” voltage
level. For example, if the current V_TRIP is 4.4V and the
V_TRIP is reset, the new V_TRIP is something less than
1.7V. This procedure must be used to set the voltage to
a lower value.
To reset the V_TRIP voltage, apply a voltage between 2.7
and 5.5V to the V_CC pin. Tie the CS/WDI pin, the WP
pin, and the SCK pin HIGH. RESET/RESET and SO
pins are left unconnected. Then apply the programming
voltage V_P to the SI pin ONLY and pulse CS/WDI LOW
then HIGH. Remove V_P and the sequence is complete.
Figure 2. Reset V_TRIP Voltage
CS
SCK ^VCC
V_P
SI
REV 1.1.1 3/5/01
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Characteristics subject to change without notice. 3 of 19