Addressing the Flash During Self-Programming

The Z-pointer is used to address the SPM commands.

Since the Flash is organized in pages, the Program Counter can be treated as having two different sections. One section, consisting of the least significant bits, is addressing the words within a page, while the most significant bits are addressing the pages. This is shown in the following figure. The Page Erase and Page Write operations are addressed independently. Therefore it is of major importance that the Boot Loader software addresses the same page in both the Page Erase and Page Write operation. Once a programming operation is initiated, the address is latched and the Z-pointer can be used for other operations.

The only SPM operation that does not use the Z-pointer is Setting the Boot Loader Lock bits. The content of the Z-pointer is ignored and will have no effect on the operation. The LPM instruction does also use the Z-pointer to store the address. Since this instruction addresses the Flash byte-by-byte, also the LSB (bit Z0) of the Z-pointer is used.

Figure 1. Addressing the Flash During SPM(1)
  1. 1.Fo the different variables used in the figure, see the table of the different variables used and the Mapping to the Z-pointer in the boot loader parameters section.
  2. 2.PCPAGE and PCWORD are listed in table Number of Words in a Page and number of Pages in the Flash in the Signal Names section.