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ISO1541: Clarification regarding voltage levels

Venugopal_S
Prodigy 125 points
Part Number: ISO1541
Other Parts Discussed in Thread: ISO1540

Hi, 

I am having trouble understanding the difference between VIH1 and VIHT1. 

As can be seen from the recommended operation conditions (Table 6.3), for VCC1 = 3.3V,

2.31V < VIH1 < 3.3V

From electrical characteristics (Table 6.9),

0.54V < VIHT1 < 0.7V

Same doubt goes for the other voltage levels too, namely VIL1- VILT1, VIL2 - VILT2, VIH2 - VIHT2. 

Which of these tables should I consider for checking my logic level compatibilies while interfacing with I2C or SMBus devices

  • 0
    TI__Mastermind 49545 points

    Hi Venugopal,

    Thank you for showing your interest in ISO1541. I will try to clarify your doubts regarding the two tables that you see in the datasheet.

    Please note that all the thresholds (like VILT1, VIHT1, etc.) listed in table 6.9 are the actual device thresholds. This is make sure you aware of the actual device thresholds. While the voltages (like VIL1, VIH1, etc.) listed in table 6.3 are the recommendations. This means that, we recommend you to apply voltage levels according to VIL and and VIH to make sure device operates the way it is expected to and follows all datasheet conditions.

    Hence, for compatibility check you should consider the recommended operating conditions as that is what we recommend for reliable operation of device. Let me know if you have questions, thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 125 points

    Hello Koteshwar,

    Thanks for the prompt response. 

    Also, are there any guidelines on choosing SIDE1 and SIDE2.

    Say, I am planning to use the isolator on a point-to-point SMBus line, with single device on either side. Assuming that the voltage logic levels are satisfied, is there a guideline to choose between SIDE 1 and SIDE2 for interfacing with the host and slave?

  • 0 in reply to Venugopal_S
    TI__Mastermind 49545 points

    Hi Venugopal,

    SIDE2 has higher driving strength (35mA) and supports higher load capacitance (400pF). Hence SIDE2 allows connects to I2C bus where multiple devices are connected. While SDIE1 only supports 3.5mA of driving current and a max load of 40pF, hence to be connected to only one device.

    In your case, you only have one device on each side. Hence, it shouldn't matter which side is connected to which device when ISO1540 (with bidirectional clock) is used. But since you are planning to use ISO1541 which has unidirectional clock, SIDE1 to be connected to Master while SIDE2 to the slave.

    Since SIDE1 has very tight logic thresholds, please make sure SIDE1 of ISO154x is placed as close as possible to the I2C device. Thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 125 points

    Got it. Thank you!