Electronic – Rewriting PIC configuration bytes at runtime

I suspect you are a C language programmer. IC datasheets are generally written to a target audience of assembly language programmers, who need to be aware of many quirky little details. Often C language programmers are happy to let pre-written library functions take care of most of those details, rather than re-writing everything from scratch. Alas, the people who write those libraries often let some of the quirky little details show through.

There are two popular ways to store data to flash memory: let some library functions handle the quirky bits for you, or write your own functions to handle the quirky bits.

Using the library functions

Using "Data EEPROM Emulation" library that you linked. There are several ways of using its functions to read and write your data, to store your 1024 bytes of data, such as "8 virtual EEPROM banks with 128 bytes in each bank."

Check out the "PIC24/dsPIC33F/dsPIC33E Emulation Checklist" in AN1095. In principle, it explains how to use that library to store stuff in flash in relatively clear English. You edit the "DEE Emulation 16-bit.h" file, add that file and a few other library files to your project. When your program runs, it calls the DataEEInit() function during boot-up initialization. Later your program calls DataEERead() to read the latest version of your data values from flash, or calls DataEEWrite() to write new version of your data values to flash, or both.

Since it does wear-leveling, the latest version of the data is stored at different addresses at different times -- it allocates the memory for you, and keeps track of the address of the latest version of your data. So there's no point in creating your own variable "nvram" at some fixed address to refer to that data, since even if that happens to point to the correct address at one time, sooner or later that data will move to some other address, and that variable will be left pointing to old stale data.

writing your own library functions

The __builtin_tblpage() gives the "high part" of an address when divided up in the right way for the TBLRD and TBLWT instructions to read and write flash.

The __builtin_psvpage() gives the "high part" of an address when divided up in the right way for PSV to read flash. (My understanding is that the only way for a program running on that chip to write values to its program flash is with the TBLWT instruction; those values can later be read with either TBLRD or PSV).

The slight difference between these two ways of dividing an address into a "high part" and a "low part" is implied in the "dsPIC33FJ32GP302/304, dsPIC33FJ64GPX02/X04, and dsPIC33FJ128GPX02/X04 Data Sheet" that you linked to, in 4 pages of the datasheet starting with "4.6 Interfacing Program and Data Memory Spaces" and "TABLE 4-39: PROGRAM SPACE ADDRESS CONSTRUCTION".

1) How do you reliably allocate the memory [at some specific address] ? Alas, this is different for every programming language, and is different even between different C compilers. The "MPLAB C30 C Compiler User's Guide" and its documentation updates would be a good place to look for this information. I think you will also be interested in the documentation for

• void _erase_flash(_prog_addressT dst)
• void _write_flash16(_prog_addressT dst, int *src)
• _PROGRAM_END

You'd start at 0x0, because the stuff there (IVT, etc) are part of what you want to upload. PIC addressing is always confusing because of their insistence on weird architectures with 14 or 24 bit words, but I believe the destination address would increment by 0x100 for 128 instructions -- this is supported by the comment in 5.2.9 of the programming spec, "The destination address should be a multiple of 0x100", as well as the flow chart shown in figure 3-4.