X4283 Datasheet, PDF (3/21 Pages) Xicor Inc. – CPU Supervisor with 128K EEPROM

X4283, X4285
PRINCIPLES OF OPERATION
Power-on Reset
Application of power to the X4283/85 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 opera-
tion.
LOW VOLTAGE MONITORING
During operation, the X4283/85 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 microproces-
sor 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.
Non-volatile 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 X4283/85 is shipped with a standard V_CC thresh-
old (V_TRIP) voltage. This value will not change over
normal operating and storage conditions. However, in
applications where the standard V_TRIP is not exactly
right, or if higher precision is needed in the V_TRIP
value, the X4283/85 threshold may be adjusted. The
procedure is described below, and uses the applica-
tion of a nonvolatile control signal.
Figure 1. Set V_TRIP Level Sequence (V_CC = desired V_TRIP values WEL bit set)
WP
V_P = 12-15V
01234567 01234567
0 1 23 4 56 7
0123456 7
SCL
SDA
A0h
00h
01h
00h
3
FN8121.0
March 29, 2005