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TCA9802: Problem when connected to ISO1641: low state is not correctly transferred from side B to A

Wojtek Jodlowski
Prodigy 50 points
Part Number: TCA9802
Other Parts Discussed in Thread: ISO1641, TCA9509

Tool/software:

We have processor driving side A of TCA9802 (2.2k pullups).

Then side B of TCA9802 is connected to ISO1641 (side 1) - no other I2C device.

Side 2 of ISO1641 is connected to EEPROM 24LC64.

Processor starts I2C transfer addressing EEPROM, everything goes well until ACK stage: EEPROM pulls SDA down to 0V, ISO1641 pulls down to ist low around 0.5V (side 1). But TCA9802 responds in unstable way, getting into sort of oscillations.

Do you have any suggestions what causing it? 

Does TCA9802 (side B) can be interfaced to ISO1641 (side 1)?

Regards

Wojtek

When EPROM device is addressed correctly and responds with ACK but TCA9802 is not responding correctly to this ACK:

When no device is addressed so NACK is correctly generated:

  • 0
    Guru 316640 points

    The ISO1641 has a voltage offset on side 1. This cannot be used with the current source on the TCA9802's side B. Swap the two sides of the ISO1641 or the TCA9802 or both.

  • 0
    TI__Mastermind 47188 points

    Hi Wojtek,

    TCA9802 B-side cannot connect to a static voltage offset of another buffering device, of which ISO1641 is. ISO1641 I believe uses SVO on side 1, where VOL1 = 570mV to 710 mV offset. The SVO interferes with the internal current source ICS on B-side of the TCA9802, which can result in oscillations. 

    This only occurs when the B-side is being driven LOW from B-side to A-side of the TCA9802 (during the ACK pulse). NACK works because communication is only from A-side to B-side, and the downstream target does not pull the bus LOW. 

    Consider using TCA9509 in the place of TCA9802 for a p2p fix. You would need PU resistors on B-side of the TCA9509, and removal of any PU resistors on A-side for the device to work. 

    Regards,

    Tyler