Electronic – Trying to get the head around RTSP and failing miserably

flashpic

I am trying to write a system which stores 1024 bytes of data in the program space (PSV) – arranged in 128 blocks of 8 bytes.

I have pored through the data sheet, the flash programming manual, the "Data EEPROM Emulation" application note and source code, but I still just don't understand the workings of it.

So, my questions that I could really do with being answered are:

1) How do you reliably allocate the memory within the PSV space so that it uses one entire table page so that erasing only affects that allocated data?

At the moment I have:

unsigned char __attribute__((space(psv),aligned(1024))) nvram[128][8];

From what I can see that is then giving me a variable address of 0x00C400, a table page of 0x000000, and a table offset of 0x004400. When I erase the table page using these values, as per the example code, it nicely corrupts my code.

2) How do the values relate to each other?

From reading the data sheets etc I understood that the table page should be the page number – being a multiple of 512 rows of 64 program words. That to my mind is 1024 bytes of low-order storage. So how is the table page from above 0? Surely it would be 0x000031 (0x32 * 1024 = 0x00C400) or something similar?

Further, I understood that the table offset is the number of bytes from the start of the table to the address of the object you want to reference – so for a properly aligned variable that should be 0. How can you have an offset of 0x004400 in a block of memory that is only 0x0400 bytes long?

I am using the __builtin_tblpage() and __builtin_tbloffset() functions to obtain the values in question.

3) Is there anywhere that explains, in plain English how to work with the internal flash?

Yes, it's all very well having these data sheets and stuff, but they present way too much information in an overly complex way. Even the example code I can find is ridiculously complex and convoluted. Is there anywhere that can explain it better?

Extra info:

The PIC in question is a dsPIC33FJ128GP802

I am using MPLAB C30 V3.30b

Best Answer

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
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