SPRUJ28E November 2021 – September 2024 AM68 , AM68A , TDA4AL-Q1 , TDA4VE-Q1 , TDA4VL-Q1
Per the OSPI protocol, the FLASH device drives DQS while CS is asserted. When CS is not asserted the FLASH device presents HiZ on DQS. When configured to use DQS, the controller uses the DQS as a clock, which samples the incoming data into a FIFO. Noise on the DQS when it is HiZ can cause spurious false triggering of the FIFO and filling it with invalid data. There is no way to clear this data except to reset the OSPI module.
To avoid this issue, it is recommended to add a pull down on the DQS line.
During device wakeup, before the IO ring is configured properly, the CS to the FLASH device is HiZ. Depending on the actual level of the CS line the FLASH device might drive the DQS High, Low or HiZ. A pull down on DQS forces the DQS input to Low, but the DQS might still be High or in the presence of noise there might be transitions between Low and High. This again can cause the same issue of capturing garbage data in the Controller FIFO.
To avoid this issue it is recommended to release the OSPI from reset only after the IO ring is configured properly.