U6808B ATMEL Corporation, U6808B Datasheet - Page 3

• Manufacturer Part Number: U6808B
• Description: Special Fail-safe Ic
• Manufacturer: ATMEL Corporation

3. Fail-safe Functions
4. Truth Tables
4707B–AUTO–10/05
A fail-safe IC has to maintain its monitoring function even if there is a fault condition at one of the
pins (e.g., short circuit). This ensures that a microcontroller system is not brought into a critical
status. A critical status is reached if the system is not able to switch off the relay and to give a
signal to the microcontroller via the ENABLE and RESET outputs. The following table shows the
fault conditions for the pins.
Table 3-1.
Table 4-1.
Table 4-2.
Supply Voltage
RELAY
Watchdog Input
OSC
WDI
RIN
Pin
Too high
Normal
Too low
Too slow
(V
Too fast
Normal
(WDI)
S
)
Function
Digital input to
activate the
fail-safe relay
Watchdog trigger
input
Capacitor and
resistor of
watchdog
Driver of the fail-
safe relay
Table of Fault Conditions
Truth Table for Over and Undervoltage Conditions
Truth Table for Watchdog Failures (Reset Output Do Not Care)
Relay Input (RIN)
High
High
High
Low
Low
Low
Relay Input
Short to V
Relay on
Watchdog reset
Watchdog reset
(RIN)
High
High
High
Low
Low
Low
Relay Output Driver
S
(RELAY)
Off
On
Off
Off
Off
Off
Relay Output Driver
Watchdog reset
Watchdog reset
Short to V
Relay on
(RELAY)
Off
On
Off
Off
Off
Off
Bat
RESET Output
(RESET)
Short to GND
Relay off
Watchdog reset
Watchdog reset
Relay on
High
High
Low
Low
Low
Low
Enable Output Driver
(ENABLE)
Driver (ENABLE)
Enable Output
Open Circuit
Relay off
Watchdog reset
Watchdog reset
Relay off
Off
Off
On
On
On
On
U6808B
Off
Off
On
On
On
On
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