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Preliminary Information
64K X4643/5
CPU Supervisor with 64K EEPROM
8K x 8 Bit
FEATURES
• Selectable watchdog timer
• Low VCC detection and reset assertion
—Four standard reset threshold voltages
—Adjust low VCC reset threshold voltage using
special programming sequence
—Reset signal valid to VCC = 1V
• Low power CMOS
—<20µA max standby current, watchdog on
—<1µA standby current, watchdog off
—3mA active current
• 64Kbits of EEPROM
—64-byte page write mode
—Self-timed write cycle
—5ms write cycle time (typical)
• Built-in inadvertent write protection
—Power-up/power-down protection circuitry
• 400kHz 2-wire interface
• 2.7V to 5.5V power supply operation
• Available packages
—8-lead SOIC
—8-lead TSSOP
DESCRIPTION
The X4643/5 combines four popular functions, Power-on
Reset Control, Watchdog Timer, Supply Voltage
Supervision, and Serial EEPROM Memory in one
package. This combination lowers system cost,
reduces board space requirements, and increases reli-
ability.
Applying power to the device activates the power on
reset circuit which holds RESET/RESET active for a
period of time. This allows the power supply and oscilla-
tor to stabilize before the processor can execute code.
The Watchdog Timer provides an independent protection
mechanism for microcontrollers. When the microcon-
troller fails to restart a timer within a selectable time out
interval, the device activates the RESET/RESET signal.
The user selects the interval from three preset values.
Once selected, the interval does not change, even after
cycling the power.
The device’s low VCC detection circuitry protects the
user’s system from low voltage conditions, resetting the
system when VCC falls below the set minimum VCC trip
point. RESET/RESET is asserted until VCC returns to
proper operating level and stabilizes. Four industry
BLOCK DIAGRAM
WP
SDA
SCL
S0
S1
Watchdog Transition
Detector
Data
Register
Command
Decode &
Control
Logic
VCC Threshold
Reset Logic
Protect Logic
Status
Register
EEPROM Array
VCC
VTRIP
+
-
Watchdog
Timer Reset
Reset &
Watchdog
Timebase
Power on and
Low Voltage
Reset
Generation
RESET (X4643/5)
RESET (X4645)
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X4643/5 – Preliminary Information
standard VTRIP thresholds are available, however,
Xicor’s unique circuits allow the threshold to be
reprogrammed to meet custom requirements or to fine-
tune the threshold for applications requiring higher
precision.
PIN CONFIGURATION
8-Pin JEDEC SOIC
S0
S1
RESET/RESET
VSS
1
2
3
4
8 VCC
7 WP
6 SCL
5 SDA
8 Pin TSSOP
WP
VCC
S0
S1
1
2
3
4
8
7
6
5
SCL
SDA
VSS
RESET/RESET
PIN FUNCTION
Pin
(SOIC)
1
2
3
Pin
(TSSOP)
3
4
5
46
57
68
71
82
Name
S0
S1
RESET/
RESET
VSS
SDA
SCL
WP
VCC
Function
Device Select Input
Device Select Input
Reset Output. RESET/RESET is an active LOW/HIGH, open drain output which
goes active whenever VCC falls below the minimum VCC sense level. It will remain
active until VCC rises above the minimum VCC sense level for 250ms. RESET/
RESET goes active if the Watchdog Timer is enabled and SDA remains either
HIGH or LOW longer than the selectable Watchdog time out period. A falling edge
on SDA, while SCL is HIGH, resets the Watchdog Timer. RESET/RESET goes
active on power up and remains active for 250ms after the power supply stabilizes.
Ground
Serial Data. SDA is a bidirectional pin used to transfer data into and out of the de-
vice. It has an open drain output and may be wire ORed with other open drain or
open collector outputs. This pin requires a pull up resistor and the input buffer is
always active (not gated).
Watchdog Input. A HIGH to LOW transition on the SDA (while SCL is HIGH) restarts
the Watchdog timer. The absence of a HIGH to LOW transition within the watchdog
time out period results in RESET/RESET going active.
Serial Clock. The Serial Clock controls the serial bus timing for data input and output.
Write Protect. WP HIGH used in conjunction with WPEN bit prevents writes to the
control register.
Supply Voltage
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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.
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 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)
WP
SCL
SDA
VP = 12-15V
01234567 01234567
0 1 23 4 56 7
0123456 7
A0h
00h 01h
00h
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X4643/5 – Preliminary Information
Setting the VTRIP Voltage
This procedure is used to set the VTRIP to a higher or
lower voltage value. It is necessary to reset the trip
point before setting the new value.
To set the new VTRIP voltage, start by setting the WEL
bit in the control register, then apply the desired VTRIP
threshold voltage to the VCC pin and the programming
voltage, VP, to the WP pin and 2 byte address and 1
byte of “00” data. The stop bit following a valid write
operation initiates the VTRIP programming sequence.
Bring WP LOW to complete the operation.
Resetting the VTRIP Voltage
This procedure is used to set the VTRIP to a “native”
voltage level. For example, if the current VTRIP is 4.4V
and the new VTRIP must be 4.0V, then the VTRIP must
be reset. When VTRIP is reset, the new VTRIP is some-
thing less than 1.7V. This procedure must be used to
set the voltage to a lower value.
To reset the new VTRIP voltage start by setting the
WEL bit in the control register, apply VCC and the pro-
gramming voltage, VP, to the WP pin and 2 byte
address and 1 byte of “00” data. The stop bit of a valid
write operation initiates the VTRIP programming
sequence. Bring WP LOW to complete the operation.
Figure 2. Reset VTRIP Level Sequence (VCC > 3V. WP = 12-15V, WEL bit set)
WP
SCL
SDA
VP = 12-15V
01234567 01234567
0 1 23 4 56 7
0123456 7
A0h
00h 03h
00h
Figure 3. Sample VTRIP Reset Circuit
VTRIP
Adj.
4.7K
RESET
SOIC
18
27
3 X4643 6
45
VP
Adjust
Run
µC
SCL
SDA
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X4643/5 – Preliminary Information
Figure 4. VTRIP Programming Sequence
VTRIP Programming
Execute
Reset VTRIP
Sequence
Set VCC = VCC Applied =
Desired VTRIP
New VCC Applied =
Old VCC Applied + Error
Execute
Set VTRIP
Sequence
Apply 5V to VCC
Decrement VCC
(VCC = VCC - 50mV)
New VCC Applied =
Old VCC Applied - Error
Execute
Reset VTRIP
Sequence
NO RESET pin
goes active?
YES
Error –Emax
Emax = Maximum Allowed VTRIP Error
Measured VTRIP -
Desired VTRIP
Error Emax
–Emax < Error < Emax
DONE
Control Register
The Control Register provides the user a mechanism
for changing the Block Lock and Watchdog Timer set-
tings. The Block Lock and Watchdog Timer bits are
nonvolatile and do not change when power is removed.
The Control Register is accessed at address FFFFh. It
can only be modified by performing a byte write opera-
tion directly to the address of the register and only one
data byte is allowed for each register write operation.
Prior to writing to the Control Register, the WEL and
RWEL bits must be set using a two step process, with
the whole sequence requiring 3 steps. See "Writing to
the Control Register" below.
The user must issue a stop after sending this byte to the
register to initiate the nonvolatile cycle that stores WD1,
and WD0. The X4643/5 will not acknowledge any data
bytes written after the first byte is entered.
The state of the Control Register can be read at any
time by performing a random read at address FFFFh.
Only one byte is read by each register read operation.
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