New Features
•
•
•
•
Monitor Voltages: 5V to 1.6V
Memory Security
Battery Switch Backup
V
OUT
5mA to 50mA
X40020/40021
Dual Voltage Monitor with Integrated CPU Supervisor and System Battery Switch
FEATURES
• Dual voltage detection and reset assertion
—Three standard reset threshold settings
(4.6V/2.9V, 4.6V/2.6V, 2.9V/1.6V)
—V
TRIP2
Programmable down to 0.9V
—Adjust low voltage reset threshold voltages
using special programming sequence
—Reset signal valid to V
CC
= 1V
—Monitor two voltages or detect power fail
• Battery Switch Backup
• V
OUT
: 5mA to 50mA from V
CC
; 250µA from V
BATT
• Fault detection register
• Selectable power on reset timeout
(0.05s, 0.2s, 0.4s, 0.8s)
• Selectable watchdog timer interval
(25ms, 200ms, 1.4s, off)
• Debounced manual reset input
• Low power CMOS
—25µA typical standby current, watchdog on
—6µA typical standby current, watchdog off
—1µA battery current in backup mode
• 400kHz 2-wire interface
• 2.7V to 5.5V power supply operation
• Available packages
—14-lead SOIC, TSSOP
APPLICATIONS
• Communications Equipment
—Routers, Hubs, Switches
—Disk arrays
• Industrial Systems
—Process Control
—Intelligent Instrumentation
BLOCK DIAGRAM
V
OUT
V2MON
• Computer Systems
—Desktop Computers
—Network Servers
X40020/21
Standard V
TRIP1
Level Standard V
TRIP2
, Level Suffix
4.6V (+/-1%)
2.9V(+/-1.7%)
-A
4.6V (+/-1%)
2.6V (+/-2%)
-B
2.9V(+/-1.7%)
1.6V (+/-3%)
-C
See “Ordering Information” for more details
For Custom Settings, call Xicor.
DESCRIPTION
The X40020 combines power-on reset control, watch-
dog timer, supply voltage supervision, and secondary
supervision, and manual reset, in one package. This
combination lowers system cost, reduces board space
requirements, and increases reliability.
Applying voltage to V
CC
activates the power on reset
circuit which holds RESET/RESET active for a period of
time. This allows the power supply and system oscillator
to stabilize before the processor can execute code.
Low V
CC
detection circuitry protects the user’s system
from low voltage conditions, resetting the system when
V
CC
falls below the minimum V
TRIP1
point. RESET/
RESET is active until V
CC
returns to proper operating
level and stabilizes. A second voltage monitor circuit
tracks the unregulated supply to provide a power fail
warning or monitors different power supply voltage.
+
V2 Monitor
Logic
V
TRIP2
V2FAIL
-
SDA
WP
Data
Register
Command
Decode Test
& Control
Logic
Fault Detection
Register
Status
Register
Watchdog
and
Reset Logic
V
OUT
WDO
MR
RESET
X40020
SCL
V
OUT
V
CC
(V1MON)
+
V
CC
Monitor
Logic
System
Battery
Switch
V
TRIP1
-
Power on,
Manual Reset
Low Voltage
Reset
Generation
RESET
X40021
BATT-ON
V
OUT
V
BATT
LOWLINE
REV 1.0.9 2/11/04
www.xicor.com
Characteristics subject to change without notice.
1 of 24
X40020/40021
Three common low voltage combinations are available.
However, Xicor’s unique circuits allows the threshold
for either voltage monitor to be reprogrammed to meet
specific system level requirements or to fine-tune the
threshold for applications requiring higher precision.
A manual reset input provides debounce circuitry for
minimum reset component count.
A battery switch circuit compares V
CC
with V
BATT
input
and connects V
OUT
to whichever is higher. This pro-
vides voltage to external SRAM or other circuits in the
event of main power failure. The X40020/21 can drive
50mA from V
CC
to 250µA from V
BATT
. The device only
switches to V
BATT
when V
CC
drops below the low V
CC
voltage threshold and V
BATT
.
The Watchdog Timer provides an independent protec-
tion mechanism for microcontrollers. When the micro-
controller fails to restart a timer within a selectable time
out interval, the device activates the WDO signal. The
user selects the interval from three preset values. Once
selected, the interval does not change, even after
cycling the power.
The device features an 2-wire interface and software
protocol allowing operation on a two-wire bus.
The device utilizes Xicor’s proprietary Direct Write
™
cell, providing a minimum endurance of 100,000 cycles
and a minimum data retention of 100 years.
PIN CONFIGURATION
X40020
14-Pin SOIC, TSSOP
V2FAIL
V2MON
LOWLINE
WDO
MR
RESET
V
SS
1
2
3
4
5
6
7
14
13
12
11
10
9
8
V
CC
BATT-ON
V
OUT
V
BATT
WP
SCL
SDA
V2FAIL
V2MON
LOWLINE
WDO
MR
RESET
V
SS
X40021
14-Pin SOIC, TSSOP
1
2
3
4
5
6
7
14
13
12
11
10
9
8
V
CC
BATT-ON
V
OUT
V
BATT
WP
SCL
SDA
PIN DESCRIPTION
Pin
1
2
Name
V2FAIL
V2MON
Function
V2 Voltage Fail Output.
This open drain output goes LOW when V2MON is less than V
TRIP2
and
goes HIGH when V2MON exceeds V
TRIP2
. There is no power up reset delay circuitry on this pin.
V2 Voltage Monitor Input.
When the V2MON input is less than the V
TRIP2
voltage, V2FAIL goes
LOW. This input can monitor an unregulated power supply with an external resistor divider or can
monitor a second power supply with no external components. Connect V2MON to V
SS
or
V
CC
when not used.
Early Low V
CC
Detect.
This open drain output signal goes LOW when
V
CC
< V
TRIP1
.
When
V
CC
> V
TRIP1
, this pin is pulled high with the use of an external pull up resistor.
WDO Output.
WDO is an active LOW, open drain output which goes active whenever the watch-
dog timer goes active.
Manual Reset Input.
Pulling the MR pin LOW initiates a system reset. The RESET/RESET pin will
remain HIGH/LOW until the pin is released and for the t
PURST
thereafter. It has an internal pull up
resistor.
3
4
5
LOWLINE
WDO
MR
REV 1.0.9 2/11/04
www.xicor.com
Characteristics subject to change without notice.
2 of 24
X40020/40021
PIN DESCRIPTION
(Continued)
Pin
6
Name
RESET/
RESET
Function
RESET Output.
(X40021) This open drain pin is an active LOW output which goes LOW whenever
V
CC
falls below V
TRIP1
voltage or if manual reset is asserted. This output stays active for the pro-
grammed time period (t
PURST
) on power up. It will also stay active until manual reset is released
and for t
PURST
thereafter.
RESET Output.
(X40020) This pin is an active HIGH open drain output which goes HIGH when-
ever V
CC
falls below V
TRIP1
voltage or if manual reset is asserted. This output stays active for the
programmed time period (t
PURST
) on power up. It will also stay active until manual reset is released
and for t
PURST
thereafter.
Ground
Serial Data.
SDA is a bidirectional pin used to transfer data into and out of the device. 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 toggled from HIGH to LOW
and followed by a stop condition) restarts the Watchdog timer. The absence of this transition within
the watchdog time out period results in WDO going active.
Serial Clock.
The Serial Clock controls the serial bus timing for data input and output.
Write Protect.
WP HIGH prevents writes to any location in the device (including all the registers).
It has an internal pull down resistor. (>10M
Ω
typical)
Battery Supply Voltage.
This input provides a backup supply in the event of a failure of the
primary V
CC
voltage. The V
BATT
voltage typically provides the supply voltage necessary to
maintain the contents of SRAM and also powers the internal logic to “stay awake.” If the battery is
not used, connect V
BATT
to ground.
Output Voltage. (V)
V
OUT
= V
CC
if V
CC
> V
TRIP1
.
IF V
CC
< V
TRIP1
then V
OUT
= V
CC
if V
CC
> V
BATT
+ 0.03V
else V
OUT
= V
BATT
(ie if V
CC
< V
BATT
– 0.03V)
Note:
There is hysteresis around V
BATT
± 0.03V point to avoid oscillation at or near the
switchover voltage. A capacitance of 0.1µF must be connected to V
OUT
to ensure stability.
Battery On.
This CMOS output goes HIGH when the V
OUT
switches to V
BATT
and goes LOW
when V
OUT
switches to V
CC
. It is used to drive an external PNP pass transistor when V
CC
= V
OUT
and current requirements are greater than 50mA.
The purpose of this output is to drive an external transistor to get higher operating currents when
the V
CC
supply is fully functional. In the event of a V
CC
failure, the battery voltage is applied to the
V
OUT
pin and the external transistor is turned off. In this “backup condition,” the battery only needs
to supply enough voltage and current to keep SRAM devices from losing their data–there is no
communication at this time.
7
8
V
SS
SDA
9
10
11
SCL
WP
V
BATT
12
V
OUT
13
BATT-ON
14
V
CC
Supply Voltage
REV 1.0.9 2/11/04
www.xicor.com
Characteristics subject to change without notice.
3 of 24
X40020/40021
PRINCIPLES OF OPERATION
Power On Reset
Applying power to the X40020/21 activates a Power On
Reset Circuit that pulls the RESET/RESET pins 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
TRIP1
threshold value
for t
PURST
(selectable) the circuit releases the RESET
(X40021) and RESET (X40020) pin allowing the system
to begin operation.
Figure 1. Connecting a Manual Reset Push-Button
X40020
System
Reset
RESET
MR
Manual
Reset
Unreg.
Supply
5V
Reg
3V
Reg
Low Voltage V2 Monitoring
The X40020/21 also monitors a second voltage level
and asserts V2FAIL if the voltage falls below a preset
minimum V
TRIP2
. The V2FAIL signal is either ORed
with RESET to prevent the microprocessor from oper-
ating in a power fail or brownout condition or used to
interrupt the microprocessor with notification of an
impending power failure. The V2FAIL signal remains
active until the V
CC
drops below 1V (V
CC
falling). It
also remains active until V2MON returns and exceeds
V
TRIP2
.
V2MON voltage monitor is powered by V
OUT.
If V
CC
and V
BATT
go away, V2MON cannot be monitored.
Figure 2. Two Uses of Multiple Voltage Monitoring
V
OUT
X40020
Unreg.
Supply
R
R
5V
Reg
V
CC
RESET
V2MON
V2FAIL
System
Reset
Resistors selected so 3V appears on V2MON when unregulated
supply reaches 6V.
V
OUT
X40021
V
CC
RESET
V2MON
V2FAIL
System
Reset
Manual Reset
By connecting a push-button directly from MR to
ground, the designer adds manual system reset capa-
bility. The MR pin is LOW while the push-button is
closed and RESET/RESET pin remains LOW for
t
PURST
or till the push-button is released and for t
PURST
thereafter. A weak pull up resistor is connected to the
MR pin.
Low Voltage V1 Monitoring
During operation, the X40020/21 monitors the V
CC
level and asserts RESET if supply voltage falls below a
preset minimum V
TRIP1
. The RESET signal prevents
the microprocessor from operating in a power fail or
brownout condition. The V1FAIL signal remains active
until the voltage drops below 1V. It also remains active
until V
CC
returns and exceeds V
TRIP1
for
t
PURST
.
Notice:
No external components required to monitor two voltages.
WATCHDOG TIMER
The Watchdog Timer circuit monitors the microproces-
sor activity by monitoring the SDA and SCL pins. A
standard read or write sequence to any slave address
byte restarts the watchdog timer and prevents the
WDO signal to go active. A minimum sequence to
reset the watchdog timer requires four microprocessor
instructions namely, a Start, Clock Low, Clock High and
Stop. The state of two nonvolatile control bits in the
Status Register determine the watchdog timer period.
The microprocessor can change these watchdog bits
by writing to the X40020/21 control register (also refer
to page 21).
Characteristics subject to change without notice.
REV 1.0.9 2/11/04
www.xicor.com
4 of 24
X40020/40021
Figure 3. V
TRIPX
Set/Reset Conditions
V
TRIPX
(X = 1, 2)
V
CC
/V2MON
V
P
WDO
SCL
0
7
0
7
0
7
SDA
A0h
00h
t
WC
Figure 4. Watchdog Restart
.6µs
SCL
SDA
Start
WDT Reset
Stop
1.3µs
Data Byte in order to program V
TRIPx
. The STOP bit
following a valid write operation initiates the programming
sequence. Pin WDO must then be brought LOW to
complete the operation.
To check if the V
TRIPX
has been set, set VXMON to a
value slightly greater than V
TRIPX
(that was previously
set). Slowly ramp down VXMON and observe when the
corresponding outputs (LOWLINE and V2FAIL) switch.
The voltage at which this occurs is the V
TRIPX
(actual).
C
ASE
A
Now if the desired V
TRIPX
is greater than the V
TRIPX
(actual), then add the difference between V
TRIPX
(desired) – V
TRIPX
(actual) to the original V
TRIPX
desired.
This is your new V
TRIPX
that should be applied to
VXMON and the whole sequence should be repeated
again (see Figure 5).
C
ASE
B
V1 AND V2 THRESHOLD PROGRAM PROCEDURE
(OPTIONAL)
The X40020/21 is shipped with standard V1 and V2
threshold (V
TRIP1,
V
TRIP2
) voltages. These values will not
change over normal operating and storage conditions.
However, in applications where the standard thresholds
are not exactly right, or if higher precision is needed in the
threshold value, the X40020 trip points may be adjusted.
The procedure is described below, and uses the applica-
tion of a high voltage control signal.
Setting a V
TRIPx
Voltage (x=1, 2)
There are two procedures used to set the threshold volt-
ages (V
TRIPx
), depending if the threshold voltage to be
stored is higher or lower than the present value. For
example, if the present V
TRIPx
is 2.9 V and the new
V
TRIPx
is 3.2 V, the new voltage can be stored directly
into the V
TRIPx
cell. If however, the new setting is to be
lower than the present setting, then it is necessary to
“reset” the V
TRIPx
voltage before setting the new value.
Setting a Higher V
TRIPx
Voltage (x=1, 2)
To set a V
TRIPx
threshold to a new voltage which is higher
than the present threshold, the user must apply the
desired V
TRIPx
threshold voltage to the corresponding
input pin (Vcc(V1MON) or V2MON). Then, a program-
ming voltage (Vp) must be applied to the WDO pin before
a START condition is set up on SDA. Next, issue on the
SDA pin the Slave Address A0h, followed by the Byte
Address 01h for V
TRIP1
, and 09h for V
TRIP2
, and a 00h
REV 1.0.9 2/11/04
Now if the V
TRIPX
(actual), is higher than the V
TRIPX
(desired), perform the reset sequence as described in the
next section. The new V
TRIPX
voltage to be applied to
VXMON will now be: V
TRIPX
(desired) – (V
TRIPX
(actual)
– V
TRIPX
(desired)).
Note:
1. This operation does not corrupt the memory
array.
2. Set V
CC
= 5V, when V
TRIP2
is being pro-
grammed
Setting a Lower V
TRIPx
Voltage (x=1, 2)
In order to set V
TRIPx
to a lower voltage than the
present value, then V
TRIPx
must first be “reset” accord-
ing to the procedure described below. Once V
TRIPx
has been “reset”, then V
TRIPx
can be set to the desired
voltage using the procedure described in “Setting a
Higher V
TRIPx
Voltage”.
Characteristics subject to change without notice.
www.xicor.com
5 of 24
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