U6813B ATMEL Corporation, U6813B Datasheet - Page 6

• Manufacturer Part Number: U6813B
• Description: U6813b Fail-safe Ic With High-side And Relay Driver
• Manufacturer: ATMEL Corporation

4.6
4.7
6
Explanation
Watchdog Window Calculation
U6813B
Figure 4-3.
In each block, the first character represents the state of the counter. The second notation indi-
cates the fault status of the counter. A fault status is indicated by an “F” and a no-fault status is
indicated by an “NF”. When the watchdog is powered up initially, the counter starts at the 0/F
block (initial state). “Good” indicates that a pulse has been received whose width resides within
the timing window. “Bad” indicates that a pulse has been received whose width is either too
short or too long.
Example with recommended values
C
R
RC oscillator
t
f
Watchdog WDI
f
t
f
WDI pulse width for fault detection after 3 pulses:
Upper watchdog window
Minimum: 169/ f
Maximum: 170/ f
Lower watchdog window
Minimum: 79/ f
Maximum: 80/ f
WDC
WDC
WDI
WDC
WDI
osc
osc
(Hz) = 0.01 f
= 50 Hz
= 36 k (can be 5%, R
= 6.8 nF (should be preferably 10%, NPO)
= 200 µs
(s) = 10
(Hz) = 1/(t
-3
Watchdog State Diagram
[C
WDC
WDC
t
WDC
WDI
WDC
WDC
WDC
f
osc
WDC
)
= 15.8 ms
= 20 ms
= 16 ms
(nF) [(0.00078 R
= 33.8 ms
= 34 ms
= 5 kHz
bad
Initial status
osc
good
O/F
< 200 k due to leakage current and humidity)
f
WDC
f
f
WDC
f
WDC
WDC
bad
/80 = 62.5 Hz
osc
/170 = 29.4 Hz
bad
/79 = 63.3 Hz
/169 = 29.55 Hz
1/NF
1/F
(k )) + 0.0005]]
good
good
bad
bad
2/NF
2/F
bad
good
good
3/NF
good
4543B–AUTO–10/05