20.6.4.2 NVM Write

The NVM Controller requires that an erase must be done before programming. The entire NVM main address space can be erased by a debugger Chip Erase command. Alternatively, rows can be individually erased by the Erase Row command.

After programming the NVM main array, the region that the page resides in can be locked to prevent spurious write or erase sequences. Locking is performed on a per-region basis, and so, locking a region will lock all pages inside the region.

Data to be written to the NVM block are first written to and stored in an internal buffer called the page buffer. The page buffer contains the same number of bytes as an NVM page. Writes to the page buffer must be 16-bit or 32 bit. 8-bit writes to the page buffer are not allowed and will cause a system exception.

Writing to the NVM block through the AHB bus is performed by a load operation to the page buffer. For each AHB bus write, the address is stored in the ADDR register. After the page buffer has been loaded with the required number of bytes, the page can be written to the NVM main array by setting CTRLA.CMD to 'Write Page' and setting the key value to CMDEX. The LOAD bit in the STATUS register indicates whether the page buffer has been loaded or not. Before writing the page to memory, the accessed row must be erased.

Automatic page writes are enabled by writing the manual write bit to zero (CTRLB.MANW = 0). This will trigger a write operation to the page addressed by ADDR when the last location of the page is written.

Because the address is automatically stored in ADDR during the I/O bus write operation, the last given address will be present in the ADDR register. There is no need to load the ADDR register manually, unless a different page in memory is to be written.