X51638
PRINCIPLES OF OPERATION
To set the new V
voltage, apply the desired V
TRIP TRIP
threshold to the Vcc pin and tie the CS/WDI pin and the
WP pin HIGH. 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.
POWER ON RESET
Application of power to the X51638 activates a Power On
Reset Circuit. This circuit goes active at V
sense level
CC
(V
) and pulls the RESET pin LOW. This signal pre-
TRIP
vents the system microprocessor from starting to operate
with insufficient voltage or prior to stabilization of the oscil-
Figure 1. Set V
Voltage
TRIP
lator. When Vcc exceeds the device V
value for
TRIP
800ms (nominal) the circuit releases RESET, allowing the
processor to begin executing code.
CS
Vp
SCK
LOW VOLTAGE MONITORING
During operation, the X51638 monitors the V level and
asserts RESET if supply voltage falls below a preset mini-
CC
Vp
SI
mum V
. The RESET signal prevents the microproces-
TRIP
sor from operating in a power fail or brownout condition.
The RESET signal remains active until the voltage drops
below 1V. It also remains active until Vcc returns and
Resetting the V
Voltage
TRIP
exceeds V
for 800ms.
TRIP
This procedure sets the V
to a “native” voltage level.
TRIP
For example, if the current V
is 4.4V and the V
is
TRIP
TRIP
WATCHDOG TIMER
reset, the new V
is something less than 1.7V. This
TRIP
The Watchdog Timer circuit monitors the microprocessor
activity by monitoring the WDI input. The microprocessor
must toggle the CS/WDI pin periodically to prevent a
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.
procedure must be used to set the voltage to a lower
value.
To reset the V
voltage, apply a voltage between 2.7
TRIP
and 5.5V to the Vcc pin. Tie the CS/WDI pin, the WP pin,
AND THE SCK pin HIGH. 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 V
Voltage
TRIP
VCC THRESHOLD RESET PROCEDURE
The X51638 is offered with one of several standard Vcc
CS
threshold (V
) voltages. This value will not change
TRIP
Vcc
over normal operating and storage conditions. However,
in applications where the standard V is not exactly
SCK
TRIP
right, or for higher precision in the V
X51638 threshold may be adjusted.
value, the
TRIP
Vp
SI
Setting the V
Voltage
TRIP
This procedure sets the V
to a higher voltage value.
TRIP
For example, if the current V
is 4.4V and the new
TRIP
V
is 4.6V, this procedure directly makes the change. If
TRIP
the new setting is lower than the current setting, then it is
necessary to reset the trip point before setting the new
value.
3
ETC [ ETC ]
