The Great I2C Mystery

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The Great I2C Mystery
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The mystery

• Occasionally, when accessing the I2C bus, the
  APVs are found not to respond correctly to I2C
  commands, actually they are stuck
• This is confirmed by Resetting the APV, after
  which they respond correctly

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The theoretical waveforms
Ack Cycle
D0
D1
SCL
SDA ? must occur after SCL ?
SDA
Driven by master
Driven by slave
Example for an I2C write cycle
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The actual waveforms
Ack Cycle
D0
D1
SCL
SCL ? before SDA ? by a few ns
SDA
Driven by master
Driven by slave
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on scope
SDA
SCL on FE-Hybrid
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What we believe the APV believes
Ack Cycle
D0
D1
SCL
ARRRGGGHHHH!!!!!
SDA
Driven by master
Driven by slave
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Present Electrical circuit
FE-Hybrid
82W
Parasitic on FE Hybrid CH
SCL
SDA
AOH
CH gt CA
Parasitic on AOH CA
82W
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Proposed Electrical circuit
FE-Hybrid
82W
SCL
SDA
AOH
gt 3 KW
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Solution

• Replace the 82 ohm protection resistor to AOH
  (ONLY the one to the AOH !) with something like
  3.3 K (Precise value to be confirmed after full
  Temp cycle tests !)

10
82-gt500 W and 47 pF on AOH
SDA
SCL on AOH
SCL on FE-Hybrid