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 VCC exceeds the device VTRIP
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 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 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.
VCC Threshold Reset Procedure
The X5643/X5645 has 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
for higher precision in the VTRIP value, the X5643/
X5645 threshold may be adjusted.
Setting the VTRIP Voltage
This procedure sets 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 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 VTRIP voltage, apply the desired VTRIP
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 VP
to both SCK and SI and pulse CS/WDI LOW then
HIGH. Remove VP and the sequence is complete.
Figure 1. Set VTRIP Voltage
Resetting the VTRIP Voltage
This procedure sets the VTRIP to a “native” voltage
level. For example, if the current VTRIP is 4.4V and the
VTRIP is reset, the new VTRIP is something less than
1.7V. This procedure must be used to set the voltage to
a lower value.
To reset the VTRIP voltage, apply a voltage between 2.7
and 5.5V to the VCC 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 VP to the SI pin ONLY and pulse CS/WDI LOW
then HIGH. Remove VP and the sequence is complete.
Figure 2. Reset VTRIP Voltage
REV 1.1.1 3/5/01
Characteristics subject to change without notice. 3 of 19