The ROM assisted image download sequence is entered by placing the device in flashing mode. Refer to section 3, for further details on the handshake with an external host to receive the image. The communication with the SDF is depicted in Figure 2-3.

Figure 2-3 ROM Assisted Image Download Sequence

Key Points:

• The ROM assisted download should work with all flash variants that allow for “Memory mapped mode” and “Page program command (0x2)” with 1 dummy byte and 24bit addressing.
• The setting of the “Quad Enable” bit varies from one SDF vendor to another. The ROM bootloader supports setting the “Quad Enable” bit for Spansion and Macronix variants (certain specific part variants only) in this flow.

In addition to a checksum based integrity check for every packet received over the UART, a CRC32-based integrity check is performed over the complete image. The CRC32 is computed incrementally as the packets are received and written to the SDF.