SCPA062 July   2021 TCA9517

 

  1.   Trademarks
  2. 1Introduction
  3. 2SDA Signal During Acknowledge
  4. 3SDA Signal During Acknowledge With Propagation Delay (TCA9517 A to B)
    1. 3.1 Example Waveform in Actual System
  5. 4System Impact
  6. 5SDA Handoff Delay After the Acknowledge
  7. 6Concern With Rise Time Accelerators
  8. 7Conclusion

4 System Impact

Although adding the TCA9517 to the system can create a circumstance in which the signals generated by the controller are delayed by the TCA9517, there is no impact to the signal integrity of the communication. The I2C-bus specification and user manual shows in Section 3.1.3, Figure 4 that the data line must be stable when the SCL line is high, but is allowed to change states when the SCL line is low. The time in which the delay due to the TCA9517 occurs after the SCL line goes low and only happens while the SCL is low.

The I2C-bus specification does not specify the Acknowledgment timing for the controller and target devices. It does not state when the controller must release the bus for Acknowledgment and when the target must take control of the SDA line after the controller releases. However, it does specify a data set-up time requirement, which is found in Table 10 of the I2C standard. Looking at the requirement, each different operating mode requires a specific set-up time for the SDA line to be stable before the SCL line is at 30% of its rising voltage. Therefore, as long as the target device has control of the SDA line adequately before SCL line rises to 30% of its maximum voltage based upon the set-up time specification, the target is able to send a proper Acknowledge to the controller. Any voltage changes on the SDA line before the set-up time are simply ignored.