X4643/5 – Preliminary Information
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 VCC exceeds the device VTRIP 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 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 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 non-
volatile 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 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.
VCC THRESHOLD RESET PROCEDURE
The X4643/5 is shipped with 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 if
higher precision is needed in the VTRIP 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 VTRIP Level Sequence (VCC = desired VTRIP values WEL bit set)
VP = 12-15V
0 1 23 4 56 7
REV 1.26 4/30/02
Characteristics subject to change without notice. 3 of 22
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