ISO-SPI Communication Issue with LTC6802-1 and LTC6820

Hello,

I am currently working on an HV battery system using the LTC6802-1 in combination with the LTC6820 for isolated SPI communication.

Previously, I used a typical non-isolated daisy-chain configuration, which worked without any issues. Using the exact same software, I changed only the hardware to incorporate isolation via the LTC6820.

I understand that the LTC6802-1 is not a recommended-for-new-design part anymore, but due to internal project constraints, I must use this IC. Therefore, I implemented the following ISO-SPI configuration:

The problems are as follows:

1. CPOL/CPHA inconsistency
   This has been a longstanding question. Although the LTC6802-1 datasheet specifies CPOL = 1, CPHA = 1, it only works in CPOL = 0 setting in practice. I have verified this with the DC590B EVM as well, where the SPI waveform clearly shows CPOL = 0 operation.

2. ISO-SPI Communication Failure
   The isolated communication is currently not working. I probed the signals with an oscilloscope and observed the following:

   • Yellow: CLK signal received by the target(slv) via isolated SPI

   • Red: IP/IM differential signal between LTC6820s

   • Mint/Green: CLK signal generated from the MCU (controller(mst) side)

   From the waveforms, it seems that the LTC6802 itself is functioning and responding in some way, but we are not getting valid data transmission.

I am attaching the schematics of the custom LTC6820 modules I made for this experiment. I am using two such modules, one on the MCU side and one on the battery monitoring IC side, as shown in the diagram above.

The LTC6820 configurations are as follows:

• SLOW = 0

• MSTR = 1 on the MCU side, MSTR = 0 on the target(slv) side

• CPOL = 1

• CPHA = 1

Any insights on what might be causing this communication issue or guidance on debugging would be greatly appreciated.

Thank you.