3
FN8123.0
March 29, 2005
PRINCIPLES OF OPERATION
Power-On Reset
Application of power to the X4643/5 activates a
Power-on Reset Circuit that pulls the RESET
/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 V
CC
exceeds the device V
TRIP
threshold value
for 200ms (nominal) the circuit releases
RESET
/RESET allowing the system to begin operation.
LOW VOLTAGE MONITORING
During operation, the X4643/5 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 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
/RESET signal. The state of two
nonvolatile control bits in the Status Register deter-
mine 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
/RESET goes active as a result of a low
voltage condition or Watchdog Timer Time Out, any in-
progress communications are terminated. While
RESET
/RESET is active, no new communications are
allowed and no nonvolatile write operation can start.
Nonvolatile writes in-progress when RESET
/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.
V
CC
THRESHOLD RESET PROCEDURE
The X4643/5 is shipped with 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 if
higher precision is needed in the V
TRIP
value, the
X4643/5 threshold may be adjusted. The procedure is
described below, and uses the application of a nonvol-
atile control signal.
Figure 1. Set V
TRIP
Level Sequence (V
CC
= desired V
TRIP
values WEL bit set)
01234567
SCL
SDA
A0h
01234567
00h
WP
V
P
= 12-15V
01234567
01h
01234567
00h
X4643, X4645
