ISO6741: Question about minimum SPI Frequency

Carlos Ravelo

Part Number: ISO6741
Other Parts Discussed in Thread: ISO7741

Hello,

I am selecting a device to isolate the SPI signals between two MCUs on different boards and I was thinking of using the ISO6741. My design will run at a maximum clock speed of 12MHz at 3.3V and while in the ISO6741 datasheet I could not find any information on minimum SPI clock frequency, in a document called Isolating SPI Signals there is a table in which it is stated that for ISO6741 the minimum SPI clock frequency supported (at 5V) is 13.9MHz. Does this means it is not adequate for my design? What would be an valid alternative in this case?

over 3 years ago

0 Aaditya V. over 3 years ago

TI__Mastermind 21395 points

Hello,

Thank you for your question!

Please refer to the FAQ post below to calculate the SPI speed of a digital isolator.

https://e2e.ti.com/support/isolation-group/isolation/f/isolation-forum/991536/faq-how-to-calculate-the-maximum-spi-speed-supported-by-a-digital-isolator

Considering ISO6741 that has a tpd(max) of 18ns for VCC1 = VCC2 = 5V. The maximum SPI speed that can be achieved on ISO6741 is, fSCLK(max) = 13.9 MHz. If you'd like more margin for SPI frequency, you can also consider ISO7741 which supports a frequency of 15.6 MHz.

Regards,

Aaditya Vittal

0 Carlos Ravelo over 3 years ago in reply to Aaditya V.

Prodigy 10 points

Hello

Thank you very much for your response. I am aware of how to calculate the maximum SPI frequency, my question is related to an information in a document called "Isolating SPI Signals" that mentions minimum SPI frequency supported. In fact the value in the table is the same as the fSCLK(max) by your calculations, but I am confused by the use of the word minimum here.

Shouldn't this be the maximum value? Or is it in fact minimum?

Best regards,

Carlos

+1 Aaditya V. over 3 years ago in reply to Carlos Ravelo

TI__Mastermind 21395 points

Hello,

Thank you for your question!

Since we have used tpd(max), this would be the minimum SPI frequency supported.

When tpd(typical) is used, you will see higher SPI speeds. In order to make sure the SPI speed is supported across temperature, you should use a SPI frequency that is below the minimum SPI speed, which can be achieved with ISO6741 or ISO7741.

Regards,

Aaditya Vittal

Isolation

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ISO6741: Question about minimum SPI Frequency