PRINCIPLES OF OPERATION
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 VCC exceeds the device VTRIP1 threshold value
for tPURST (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
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
tPURST or till the push-button is released and for tPURST
thereafter. A weak pull up resistor is connected to the
Low Voltage V1 Monitoring
During operation, the X40020/21 monitors the VCC
level and asserts RESET if supply voltage falls below
a preset minimum VTRIP1. 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 VCC returns and exceeds VTRIP1 for tPURST.
Low Voltage V2 Monitoring
The X40020/21 also monitors a second voltage level and
asserts V2FAIL if the voltage falls below a preset mini-
mum VTRIP2. The V2FAIL signal is either ORed with
RESET to prevent the microprocessor from operating 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 VCC
drops below 1V (VCC falling). It also remains active until
V2MON returns and exceeds VTRIP2.
V2MON voltage monitor is powered by VOUT. If VCC
and VBATT go away, V2MON cannot be monitored.
Figure 2. Two Uses of Multiple Voltage Monitoring
Resistors selected so 3V appears on V2MON when unregulated
supply reaches 6V.
Notice: No external components required to monitor two voltages.
The Watchdog Timer circuit monitors the microprocessor
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 sig-
nal 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 microproces-
sor can change these watchdog bits by writing to the
X40020/21 control register (also refer to page 21).
May 17, 2006