SPRUIU1C July 2020 – February 2024 DRA821U , DRA821U-Q1
The aim of the indirect mode of operation is to perform bulk transfer of data from the processor into a FLASH memory in the most efficient manner. The fewest possible write cycles inside the FLASH device will be carried out for the indirect transfer, thus maximizing the life of the device. Indirect write operation can be thought of from a software perspective as the inverse of the indirect read. It is controlled and triggered by software via specific control/configuration Indirect Write Transfer registers (for more information see the following registers: OSPI_INDIRECT_WRITE_XFER_CTRL_REG, OSPI_INDIRECT_WRITE_XFER_WATERMARK_REG, OSPI_INDIRECT_WRITE_XFER_START_REG, and OSPI_INDIRECT_WRITE_XFER_NUM_BYTES_REG). This block will await delivery of the write data via the external data interface controller, placing it in the local SRAM before communicating with the existing legacy SPI core to perform an efficient and optimized FLASH write burst.
By default, the indirect write controller is disabled. Before enabling it, the software must configure how much data is required and the start address. The start address and total number of bytes to be written is defined in OSPI_INDIRECT_WRITE_XFER_START_REG and OSPI_INDIRECT_WRITE_XFER_NUM_BYTES_REG registers, respectively. Up to two indirect operations can be programmed at any one time. The second operation can be triggered while the first is in progress. Supporting two indirect operations allows a short turnaround time between the completion of one indirect operation and the start of the second. The Indirect write queuing is very similar to indirect read queuing. For more information refer to Section 12.3.2.4.10.3, Indirect Access Queuing.
The total number of bytes to write in an indirect operation is not limited by the size of the SRAM. The size of SRAM will only limit the amount of data that can be accepted from the external controller. In the case of an SRAM overrun, the controller will back pressure the data interface with wait states. Note the fill level of the SRAM is readable via programmable OSPI_SRAM_FILL_REG register and this can be used to avoid this situation.
An external controller will provide the write data and will transfer this to the OSPI module by issuing data interface writes. The address of the incoming write access must be in the range of Indirect trigger address programmed via the OSPI_IND_AHB_ADDR_TRIGGER_REG register to Indirect trigger address + 2**(Indirect trigger address range) - 1. Default value of the range is equal to 16 locations. This allows a 16-beat burst to be applied starting from the Indirect trigger address. The smaller bursts are possible to handle effectively as well with this approach. Furthermore it is not strict requirement to push consecutive address sequence. Actual address just has to be in the Indirect Range to grant SRAM as source. Each write will cause the internal SRAM to be pushed, thereby decoupling the incoming write access address from the FLASH address – that is not direct mapped. Therefore Indirect trigger address does not have any relationship with FLASH address. It is just to indicate that data should take SRAM as source instead of FLASH Memory array after triggering of any valid Indirect Write. The FLASH address for Indirect Write is taken from the OSPI_INDIRECT_WRITE_XFER_START_REG register. Assuming the SRAM is not full at the point the data interface access is received by the OSPI module, then the data will be pushed to the SRAM with minimum latency.
If a write access is received whose address is not within the range described above then that access will not be completed using the indirect controller. It will instead be serviced by the direct access controller.
If a write access is received whose address is within the range described above but the SRAM is full then wait states will be applied until some or all of the data has been pushed from the SRAM to the FLASH.
If a write burst is received whose access elements traverse the Indirect trigger range, then the accesses within the Indirect trigger range will be processed by the indirect controller, and the rest will be taken by the direct access controller. This is likely to be a software configuration error.
The external controller is only permitted to issue 32-bit data interface writes until the last word of an indirect transfer. This helps keep the SRAM control logic less complex. On the final write, the external controller may issue a 32-bit word, 16-bit (halfword) or a byte access to complete the transfer. If the number of bytes to write is less than 4 on the last transfer, the controller is still permitted to issue a 32-bit transfer. In these cases, the extra bytes are discarded by the controller.
When the SRAM holds a number of bytes equal to or greater than the size of a FLASH page (which itself is programmed into the OSPI module, with a default of 256 bytes) or when the SRAM holds all remaining bytes of the currently executing indirect transfer, the OSPI module will initiate a write burst to the flash command generator.
An indirect operation may be cancelled at any time by setting 1 to the OSPI_INDIRECT_WRITE_XFER_CTRL_REG[1] CANCEL_FLD bit.
Any bus controller should be allowed to initiate an indirect access. The OSPI module provide software access mechanism to the SRAM fill-level directly via the configuration registers and then decide for itself when the data should be written to the local SRAM. The fill level watermark register (see OSPI_INDIRECT_WRITE_XFER_WATERMARK_REG register) is provided. When the SRAM fill level falls below this watermark, an interrupt is generated.
Two further interrupt sources are provided to help understand the status of an indirect operation. Firstly, an interrupt is generated when an indirect operation has completed. Secondly, an interrupt is generated if an indirect write operation was requested but could not be accepted due to the fact 2 indirect operations have already been buffered by the OSPI module.
Setting the OSPI_INDIRECT_WRITE_XFER_CTRL_REG[0] START_FLD bit starts an indirect write operation. The OSPI_INDIRECT_WRITE_XFER_CTRL_REG[2] WR_STATUS_FLD bit is available to check the status.