SDRAM byte (8-bit) write access

[Altera_Forum]

Hi all, 

  I've a NiosII system based on EP1C6144, with an extrernal 2Mb 16-bit wide SDRAM.    While writing SDRAM with 16 or 32-bit wide data gives the expected result, if I try to write a single byte (8-bit) I find 2 adiacent bytes written in SDRAM with the same value.   To check this behaviour I wrote a simple in-line assembler that make me sure I have not compiling problems:     __asm__  ("nop \n\t"                    "movi r6, 0x12 \n\t");                         // Loads data to be written (0x12) in register R6                 __asm__  ("movhi r7, %hi(0x1F7F84) \n\t"                    "ori r7, r7, %lo(0x1F7F84) \n\t");       // Loads address data has to be written to (0x1F7F84) in register R7   __asm__  ("stb r6, 0(r7) \n\t");                           // Writes BYTE in R6 in address pointed by R7+0     Before these instructions I have the memory window displaying    0x001F7F84    0x00 0x00 0x....   and after instructions...    0x001F7F84    0x12 0x12 0x....   I checked all SDRAM's timing parameters and they seems to be right, in effect code runs from SDRAM and 16 and 32-bit accesses are correct, and also any read access.   Any idea?   Thanks all,    Paolo

[Altera_Forum]

The one thing which occurs to me is that your code won't work on processors with a cache. You need to use the STBIO instruction instead. 

 And you should probably use builtin_stbio instead of using inline assembler because the compiler can optimise it much better (or use the HAL macro IOWR_NATIVE8 or something like that which does the same thing).  I assume you've checked your objdump file to make sure the compiler hasn't split up your assembler statements (which is will sometimes do to make optimisation better).

[Altera_Forum]

The processor I use is the "e" version, without any cache... 

 However I'll try to follow all your suggestions, but I think is an hw problem: I just noticed that UDQM & LDQM signals are wired together to Nios' DQM0...   This bring me to think that byte access is not possible on my board...  Thank,   Paolo