Writing 16-bit data to bram

Started by gundamz2001 6 years ago6 replieslatest reply 6 years ago770 views


I am currently working on a Xilinx development board that has PowerPC and virtex 5. I used Xilinx core generator to instantiate a bram with data width size of 2.

From a C program running on the PowerPC, I have no trouble accessing the bram if the memory address is multiple of 4. However, I cannot write to the memory if the address is a multiple of 2 (but no multiple of 4, for example 0x00000002, 0x00000006).

If I run the program along with this vhdl file, and look at the memory location using mrd, I get the following:

mrd 0x41000000 10

41000000:0000F0F0  [ I would expect this to be F2F2F0F0]




Any help is greatly appreciated. Thank you very much.user_logic.vhd

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Reply by david_daysApril 26, 2018

I am just getting into this exact topic, myself, so I probably don't have any real insights.

However, I noticed that on line 263 of the VHDL, and 290-295, you have the "0100..." operations commented out.  If I'm reading it correctly, this is 2's place operations.  

Could this be related?  If not, why are those lines commented? 

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Reply by gundamz2001April 26, 2018

I assume you are talking about the lines:

----when "01000000000000000000" =>

----for byte_index in 0 to (C_SLV_DWIDTH/8)-1 loop

----if ( Bus2IP_BE(byte_index) = '1' ) then

----slv_reg1(byte_index*8 to byte_index*8+7) <= Bus2IP_Data(byte_index*8 to byte_index*8+7);

----end if;

Those lines are used to write to the register slv_reg1 through the PLB. I commented those lines out because I was using slv_reg1 as status register for the dividedby2 module's output. If I don't comment these lines out, I would get an error because of multiple sources.

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Reply by david_daysApril 26, 2018

Ok--thanks.  Was just wondering (since I'm new).  What about the C-code for the host machine?  Have you confirmed that there's no problem there?

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Reply by gundamz2001April 26, 2018

It's good to discuss this along with someone since I may miss something obvious.

The C code is just the following:

short *addr = 0x41000000; *addr = 1; // This will trigger writing F0F0 to address 0x41000000 since I hardcoded the F0F0 in the vhdl code.

addr = 0x41000002; *add = 2; // This supposed to trigger writing F2F2 to address 0x41000002 but never got written

addr = 0x41000004; *add = 3; // This will trigger writing F4F4 to address 0x41000004 since I hardcoded the F4F4 in the vhdl code.

These were verified by the xmd command mrd (memory read).

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Reply by rajkeerthy18April 26, 2018

In C the short data type is usually 16 bits. In the above code it seems it is 32 bits. May be addr could be declared as int.

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Reply by gundamz2001April 26, 2018

The address itself should be 32-bit and for PLB, the Bus2IP_Addr is 32-bit also.

Inside the vhdl code, it trims the 32-bit address to 17-bit address needed by the memory (need 17-bit address for 80k memory depth).

I have tested that if I set addr = 0x41000001 (in C), the Bus2IP_Addr will get the same number, and when I trim it down to 00....01 as input to bram, the write doesn't seem to happen.