ISO7761: C2000 F28P650 — MISO signal collapses when routed through ISO7761 isolators; works when bypassed --->AD7490

Manikandan Velayudham

Prodigy 200 points

Part Number: ISO7761
Other Parts Discussed in Thread: SN74LVC1G08, SN74LVC1G125

Tool/software:

Hi Team,

We are facing an issue with the ISO7761 isolator used between the MCU and ADC. Please find the details below:

Platform / Parts

MCU: TMS320F28P650 (3.3 V I/O), SPI-B
ADC: AD7490, VDRIVE = 3.3 V (DOUT/DIN referenced to 3.3 V)
Isolation: Two TI ISO7761 devices on the SPI lines (one for the analog section, one for the discrete/control section).

Problem:

With either isolator connected to MISO/POCI (ADC→MCU), the logic-high level attenuates and SPI readback fails.
When both isolators are removed from the MISO path (direct ADC→MCU connection), the signal is a clean ~3.3 V and communication is stable.
The attenuation is observed on both sides of the barrier:
- At the ADC DOUT pin when tied into the isolator input.
- At the MCU POCI pin when driven by the isolator output.
  → This suggests the isolator channel is loading or clamping the line.

Test performed:

Initially, I removed U69 and connected MISO directly to the MCU, but the issue persisted.
Later I found that the same MISO net was also connected to another isolator (pin 7 / pin 10 to ADC output).
After lifting that pin on the PCB, the MISO signal became clean and reached the full 3.3 V, and communication worked properly.

Questions for TI:

For ISO77xx in SPI applications, what is the recommended channel assignment for POCI (peripheral→controller) to avoid loading/clamping?
Is it supported to fan the MISO net into two separate isolator channels?

An image of the connection details is attached for reference.

Kindly review and suggest how we can resolve this issue.

Thanks,
Manikandan V

6 months ago

0 Manikandan Velayudham 6 months ago

Prodigy 200 points

Hi Team,

Please respond for the above quires.

Thanks,
MANIKANDAN V

0 Aaditya V. 6 months ago in reply to Manikandan Velayudham

TI__Mastermind 21205 points

Hi Manikandan,

Thanks for reaching out.

I understand that you are experiencing issues with SPI communication when the ISO7761 device is used. To help understand the communication failures mentioned above, can you please comment on the below points?

Can you please provide a full schematic, showing MCU, isolators, and peripheral devices?
You mentioned that the waveforms were not accurate to the signal from MCU, can you please show the input and output waveforms of the isolator? please also include the Much waveform for reference.
As you have assessed and mentioned, the issue could happen when the signal drive strength is not enough for both isolator inputs. The above information will help us understand if this is a potential issue. If this is an issue, using a buffer between the MCU and isolator can help redrive the signal to the isolator.

Regards,
Aaditya V

0 Clemens Ladisch 6 months ago

Guru 321710 points

The two SPI slaves disable their MISO pins when they are not active. But the ISO7761's OUTF pin is always active, so the two MISO signals get mixed.

You can add three-state buffers (e.g., SN74LVC1G125) that are controlled with the respective /CS signals between the isolators and the MCU. Alternatively, combine the two MISO signals with an AND gate like the SN74LVC1G08.

0 Manikandan Velayudham 6 months ago in reply to Clemens Ladisch

Prodigy 200 points

Hi Aditya and Clemens,

Thanks for the clarification. I will add a buffer between the MCU and the isolator as suggested, test the setup, and share the results once available.

Thanks,
Manikandan V

0 Manikandan Velayudham 6 months ago in reply to Manikandan Velayudham

Prodigy 200 points

Hi Team,

Our plan is to add a three-state buffer (e.g., SN74LVC1G125) on each isolated MISO line, with the buffer’s OE pin controlled by the corresponding /CS signal. This should ensure that only the selected slave drives the MCU MISO line.

Since we have 10 slaves on the same SPI bus, could you please advise on the best way to connect and configure the /CS signals to the OE pins in this setup?

Thanks,
Manikandan V

0 Aaditya V. 6 months ago in reply to Manikandan Velayudham

TI__Mastermind 21205 points

Hi Manikandan,

Manikandan Velayudham said:
Since we have 10 slaves on the same SPI bus, could you please advise on the best way to connect and configure the /CS signals to the OE pins in this setup?

Clemens mentioned how the buffer can be added between the MCU and the isolators. For a better understanding of your system, please attach your schematic showing the 10 peripheral devices, MCU and isolator to provide better connection input.

Regards,
Aaditya V

0 Manikandan Velayudham 6 months ago in reply to Aaditya V.

Prodigy 200 points

Hi Aaditya,

Please find the reference images below. Since our schematics are confidential, I am sharing only understandable partial snippets.

MCU SPI Pin Assignments
- Device: F28P650DK6PZPR
Digital-to-Analog Isolator
- Four devices connected on the output side of the isolator
- All four chip select pins also routed through the isolator
- Analog Slave Devices:
  - 2 × ADS124S08IPBS
  - 2 × AD7490
Additional Isolator (sharing same SPI channels)
- Five slave devices connected on the output side
- All five chip select pins also routed through the GPIO isolator
- Slave DIO Section:
  - 5 × HI-8435PQIF

since our schematics are confidential ,so sharing the understandable partial schematics snips.

1. MCU SPI pin assignments --> F28P650DK6PZPR
/resized-image/__size/640x480/__key/communityserver-discussions-components-files/1013/pastedimage1758689863708v1.png

2. Digital to analog isolator - 4 devices connected in the output side of the isolator. (All 4 chip select pins also routed through the Isolator)

/resized-image/__size/640x480/__key/communityserver-discussions-components-files/1013/pastedimage1758689939343v2.png

Analog salve device - 2 ( ADS124S08IPBS)

/resized-image/__size/640x480/__key/communityserver-discussions-components-files/1013/pastedimage1758690390983v5.png

Another 2 Analog devices - AD7490

/resized-image/__size/640x480/__key/communityserver-discussions-components-files/1013/pastedimage1758690441216v6.png

3. Another Isolator using the same SPI channels, and 5 slave devices are connected in the isolator output side. (All 5 chip select pins also routed through the additional GPIO Isolator )

/resized-image/__size/640x480/__key/communityserver-discussions-components-files/1013/pastedimage1758690187218v3.png

HI-8435PQIF (5 Devices) connection details:

/resized-image/__size/640x480/__key/communityserver-discussions-components-files/1013/pastedimage1758690249964v4.png

Thanks,

Manikandan V

0 Aaditya V. 6 months ago in reply to Manikandan Velayudham

TI__Mastermind 21205 points

Hi Manikandan,

It is a bit difficult to read the schematic since it is all different pictures. Please allow me some more time to look at the information and pictures you have provided so I can provide a simple drawing on using the buffer with your system.

Regards,
Aaditya V

0 Manikandan Velayudham 6 months ago in reply to Aaditya V.

Prodigy 200 points

Hi Aaditya,

Hope you’re doing well. Just wanted to check if there’s any update on the buffer connection drawing you were preparing based on the schematics shared earlier.

Thanks,
Manikandan V

0 Aaditya V. 6 months ago in reply to Manikandan Velayudham

TI__Mastermind 21205 points

Hi Manikandan,

Sorry for the delay and thank you for your patience. I have looked at the schematic snippets you have sent and I was able to slightly understand the setup better. As mentioned before, since they are independent pics, it was a bit difficult to grasp a full idea.

Considering the schematic and understanding, I have included an example drawing of how the tri-state buffer should be interfaced with the digital isolator. To put it into words, the tri-state buffer is connected to the output of the digital isolator, allowing for the output from the buffer to be at HIGH-Z so that the MISO pins on the MCU are not always active.

Regards,
Aaditya V

0 Manikandan Velayudham 6 months ago in reply to Aaditya V.

Prodigy 200 points

Hi Aaditya,

Thank you for your support. We’ll proceed to connect a tristate buffer between the MCU and the isolator on the MISO line.

Thanks,
Manikandan V

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ISO7761: C2000 F28P650 — MISO signal collapses when routed through ISO7761 isolators; works when bypassed --->AD7490