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# ISO1541: Isolation circuit Pull up requirement

Part Number: ISO1541

Hi Team,

I am using MSP430 MCU for power measurement unit so thinking to use ISO1541 Ic for programming isolation and referring document SLAU668.

On that i have couple of question:

1. I saw there is pull up on both sides of this Ic ISO1541 but both are different value? On one side it is of 1.5K, 30K, and on other side it is in ohm say 150E and 270E. Why like this?

2. Since i am going to use SPY-BI wire connection for programming so only RESET and SBWTCK will be used. On reset signal already there will be pull up of 47K resistor with one 0.1uF cap to keep MCU always out of reset. But now with this ISO1541 Ic pull up resistor will be like 47K parallel to 150E and that will have equivalent to 149E. So that wont be an issue during programming?

3. Can we replace 150E and 270E resistor with same 47K value?

Best Regards

Sumit Ranjan

• Hi Sumit,

Thanks for bring up this question. It is curious to me too why there are so many different values being used for pull-up resistors here. I believe that the inclusion of such strong pull-ups on side 2 in this design may have been done to allow for a larger capacitive load - such as a long cable or several slave devices - to be present on this side. The variance between same-side pull-ups (difference between SCL and SDA values) is a bit stranger to me. This may have been done to show that for each system there is a range of acceptable resistor values. Because of this and the fact that is sounds like there are other factors contributing to these data lines in your design, I would recommend calculating the range of pull-up values that would fit for your system. The following app report can serve as a guide on how to do this:
I2C Bus Pullup Resistor Calculation

Pull-up resistor values typically fall between around 1k-ohm for low-voltage, fast-mode systems and around 10k-ohm for 5V, standard-mode systems. Measuring your systems' capacitive loads and performing the calculations according system specifications will help make sure that the bus will be compatible with all I2C compliant devices.

I hope this helps explain the values chosen in the reference design. Please let me know if you have any other questions.

Regards,
Eric

• In reply to Eric Schott1:

Hi Eric,

Thanks for detailed explanation and sharing document. I had that and already using in most of my device.

I am not sure, how can i use this Ic ISO1541 for SPY-BI wire communication since we already have 47K pull up with Reset line which i mentioned earlier.

If i will plan to use same 47K resistor pull up same as Reset line then might be this IC will not respond well.

Is there any Isolator IC which can be used for SPY-BI wire communication with any MSP430? Any reference design Ti have?

Best Regards

Sumit Ranjan

• In reply to SUMIT RANJAN:

Hi Sumit,

It sounds like effectively you are planning to have two pull-up resistors on the same data line - one 47k-ohm for passive pull-up when acting as a reset, and one ~1k-10k-ohm for ISO1541 data line pull-up. Including both of these resistors would indeed lower the equivalent resistive path from the data line to Vcc. Because you are choosing the value of the second resistor, you may choose its value to be slightly higher so that the equivalent value falls to the calculated desired pull-up strength. Alternatively, you could choose to omit the 47k-ohm resistor from the design and let the stronger pull-up accomplish both tasks - during normal operation it will act to keep the MSP430 out of reset, and during ski-bi-wire communication it will act as a data line pull-up for ISO1541.

In both cases, ISO1541 will be provided with an adequate pull-up value so it is able to drive data when required.

Let me know if this makes sense or if you would like me to clarify further.

Regards,
Eric

• In reply to Eric Schott1:

Hi Eric,

I wanted to keep single resistor and preference is on 47K pull up resistor.

I am not sure what will be affect on MCU reset side if it will have lower than 47K pull up. As per reference schematic it should have 47K pull up so that is my concern.

Also, if we will have less pull up value then sink current will be more and in that case pin can get damage. Pin can handle +-2mA only.

Best Regards

Sumit Ranjan

• In reply to SUMIT RANJAN:

Hi Sumit,

I understand your concern regarding changing the 47k-ohm resistor on the RST pin. This value, along with suggested values for the pull-down capacitor, are recommended so that the timing requirements for a valid reset pulse can be reliably seen by the pin when configured as a reset input. In order to use this pin as a spi-bi-wire I/O to interface with ISO1541, this line will also need to meet timing requirements for this desired data signal. To achieve this without modifying the 47k-ohm value, I would recommend reducing the data rate of the spi-bi-wire communication so that the slow rise-times due to the weak pull-up are sufficient.

Let me know if you'd like help calculating what data rates would be compatible with this setup and if so, please include the capacitor value included on the RST/NMI/SBWTDIO line.

Regards,
Eric

• In reply to Eric Schott1:

Hi Eric,

I am not planning to keep both resistor on same line. By keeping both resistor, equivalent resistor will be around 149E.

My worries is in that case MSP430 RESET pin sink current will be huge in that case and pin may get damage.

As per datasheet pin current will be limited with +-2mA.

So, how can we handle that?

Best Regards

Sumit Ranjan

• In reply to SUMIT RANJAN:

Hi Sumit,

The maximum diode current at each pin (specified at +-2ma) refers to the maximum allowable current through the ESD protection diode at each pin. Current will only flow through this diode when the voltage at the pin exceeds the the absolute maximum voltage (-0.3v to 4.1v). This does not refer to the output current capabilities of a digital output pin.

When operating as the RESET, the pin will act as a digital input only and will be high impedance, only allowing a small (nA) leakage current while the applied voltage is within the allowable ranges. When operating in spi-bi-wire mode, the pin will also act as a digital output. In this case, it will have the drive characteristics as specified in the Digital I/O section of the datasheet.

Please note the values I've shared are form MSP430FR6989. While the values for other MCU models may vary, the distinction between maximum diode current and maximum drive current remains consistent.

Let me know if you have any other concerns.

Regards,
Eric

• In reply to Eric Schott1:

Hi Eric,

Thanks for detailed explanation. I was confused on digital input side which is clear now after your explanation.

And i think we are good and let's see once we will have the actual board for testing.

Thanks once again for all explanation.

Best Regards

Sumit Ranjan