10.3 Program Flash Memory (PFM)

The Program Flash Memory is readable, writable and erasable over the entire V_DD range.

A 128-word PFM page is the only size that can be erased by user software. A Bulk Erase operation cannot be issued from user code. A read from program memory is executed either one byte, one word or a 128-word page at a time. A write to program memory can be executed as either 1 or 128 words at a time.

Writing or erasing program memory will cease instruction fetches until the operation is complete. The program memory cannot be accessed during the write or erase, so code cannot execute. An internal programming timer controls the write time of program memory writes and erases.

A value written to program memory does not need to be a valid instruction. Executing a program memory location that forms an invalid instruction results in a NOP.

It is important to understand the PFM memory structure for erase and programming operations. Program memory word size is 16 bits wide.

After a page has been erased, all or a portion of this page can be programmed. Data can be written directly into PFM one 16-bit word at a time using the NVMADR, NVMDAT and NVMCON1 controls or as a full page from the buffer RAM. The buffer RAM is directly accessible as any other SFR/GPR register and also may be loaded via sequential writes using the TABLAT and TBLPTR registers.

Important: To modify only a portion of a previously programmed page, the contents of the entire page must be read and saved in the buffer RAM prior to the page erase. The Read Page operation is the easiest way to do this. The page needs to be erased so that the new data can be written into the buffer RAM to reprogram the page of PFM. However, any unprogrammed locations can be written using the single word Write operation without first erasing the page.