15.1 Program Flash Memory (PFM)

The Program Flash Memory (PFM) is readable, writable and erasable over the entire VDD range.

PFM consists of the following regions:
  • User program memory (read/write)
  • Configuration Words (read/write)
  • Device ID (read-only)
  • Revision ID (read-only)
  • User ID (read-write)
  • Device Information Area (read-only)
  • Device Configuration Information (read-only)
PFM can be read and/or written to through:
  • CPU instruction fetch (read-only)
  • FSR/INDF indirect access (read-only)
  • NVMREG access (read-write)
  • In-Circuit Serial Programming™ (ICSP™) (external read-write)

It is important to understand the program memory structure for erase and programming operations. Program memory is arranged in rows. A row consists of 32 14-bit program memory words. A row is the minimum size that can be erased by user software. A Bulk Erase command cannot be issued from user code.

Read operations return a single word of memory. Write and erase operations are done on a row basis. Program memory will erase to a logic ‘1’ and program to a logic ‘0’.

All or a portion of a row can be programmed. Data to be written into the program memory row is written to 14-bit wide data write latches. These latches are not directly accessible, but may be loaded via sequential writes to the NVMDATH:NVMDATL register pair.
Important: To modify only a portion of a previously programmed row, the contents of the entire row must be read. Then, the new data and retained data can be written into the write latches to reprogram the row of program memory. However, any unprogrammed locations can be written without first erasing the row. In this case, it is not necessary to save and rewrite the other previously programmed locations.

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.