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U6813B Datasheet(PDF) 6 Page - ATMEL Corporation

Part No. U6813B
Description  Fail-safe IC with High-side and Relay Driver
Download  16 Pages
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Maker  ATMEL [ATMEL Corporation]
Homepage  http://www.atmel.com
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U6813B Datasheet(HTML) 6 Page - ATMEL Corporation

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6
4543B–AUTO–10/05
U6813B
Figure 4-3.
Watchdog State Diagram
4.6
Explanation
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.
4.7
Watchdog Window Calculation
Example with recommended values
C
osc = 6.8 nF (should be preferably 10%, NPO)
R
osc = 36 kΩ (can be 5%, Rosc < 200 kΩ due to leakage current and humidity)
RC oscillator
t
WDC (s) = 10
-3 [C
osc (nF) [(0.00078 Rosc (kΩ)) + 0.0005]]
f
WDC (Hz) = 1/(tWDC)
Watchdog WDI
f
WDI (Hz) = 0.01 fWDC
t
WDC = 200 µs → fWDC = 5 kHz
f
WDI = 50 Hz → tWDI = 20 ms
WDI pulse width for fault detection after 3 pulses:
Upper watchdog window
Minimum: 169/ f
WDC = 33.8 ms→ fWDC/169 = 29.55 Hz
Maximum: 170/ f
WDC = 34 ms → fWDC/170 = 29.4 Hz
Lower watchdog window
Minimum: 79/ f
WDC = 15.8 ms → fWDC/79 = 63.3 Hz
Maximum: 80/ f
WDC = 16 ms → fWDC/80 = 62.5 Hz
1/NF
2/NF
O/F
1/F
2/F
3/NF
bad
good
bad
good
bad
good
good
bad
good
good
bad
bad
Initial status


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