Hello, Can someone point me out how I can read/write to the following blockram RAM model ? I can read and write to external SRAM, but I am not able to make this work using blockram. Could someone help? ENTITY vram8k IS PORT ( data : IN STD_LOGIC_VECTOR (7 DOWNTO 0); rdaddress : IN STD_LOGIC_VECTOR (12 DOWNTO 0); rdclock : IN STD_LOGIC ; rden : IN STD_LOGIC := '1'; wraddress : IN STD_LOGIC_VECTOR (12 DOWNTO 0); wrclock : IN STD_LOGIC ; wren : IN STD_LOGIC := '1'; q : OUT STD_LOGIC_VECTOR (7 DOWNTO 0) ); END vram8k; Instantiation: vram8k_inst : work.vram8k PORT MAP ( data => v_data_sig, rdaddress => v_address_r_sig(12 downto 0), rdclock => Clk_Z80, wraddress => v_address_w_sig(12 downto 0), wrclock => Clk_Z80, wren => vram_we_sig, rden => vram_re_sig, q => vram_q_sig ); I have tried a lot, and at this time, I have the following assertions for read/writing to the memory: v_address_w_sig <= A - x"2000" when (A >= x"2000" and MReq_n = '0'); v_address_r_sig <= A - x"2000" when (A >= x"2000" and MReq_n = '0'); vram_we_sig <= Wr_n; vram_re_sig <= Rd_n; v_data_sig <= DO_CPU when (Wr_n = '0' and MReq_n = '0'); DI_CPU <= vram_q_sig when (Rd_n = '0' and MReq_n = '0' and A >= x"2000"); What must be changed for this to work? Thanks.

At least, these signals seem to have wrong polarity: vram_we_sig <= Wr_n; vram_re_sig <= Rd_n; The other other point is, that the timing may be inappropriate for your design, but that can't be seen from the shown code. FPGA internal RAM act as synchronous RAM, you should check the timing to see if read and write operation occur at the intended moment. Also registered or non-registered read operation can be choosen. I wonder, why conditional assignment (a multiplexer) is used for the RAM input signals, but it shouldn't have any effect.

--- Quote Start --- At least, these signals seem to have wrong polarity: vram_we_sig <= Wr_n; vram_re_sig <= Rd_n; The other other point is, that the timing may be inappropriate for your design, but that can't be seen from the shown code. FPGA internal RAM act as synchronous RAM, you should check the timing to see if read and write operation occur at the intended moment. Also registered or non-registered read operation can be choosen. I wonder, why conditional assignment (a multiplexer) is used for the RAM input signals, but it shouldn't have any effect. --- Quote End --- Thanks for replying. The timing may be wrong... I will try to check this. However, the polarity in the said signals are ok. they are asserted LOW: vram_we_sig <= Wr_n vram_re_sig <= Rd_n Wr_n is the Z80 write enable signal. vram_we_sig is the RAM write enable signal. The multiplexers are there because it will address differente RAM/ROMS, and this is only some code to debug the block ram access.

I understand that wr_n is active low, as usual with external RAM, but the internal RAM we signal, that's connected to wr_n isn't, thus I think the polarity is wrong.

I think the polarity is wrong also. Appears that you are sending an active low signal to the internal memory that needs an active high signal. Put not(Wr_n) to wren and not(Rd_n) to rden and it should be correct polarity.

I have checked the timing. Read operation needs 3 clock cycle. I have inserted this delay in the logic and will test for functionality. I have also inverted the driving logic for control signals. Thanks guys for pointing that out .

Altera_Forum

Honored Contributor

17 years ago

Blockram: RAM model read/write

Hello,

Can someone point me out how I can read/write to the following blockram RAM model ?

I can read and write to external SRAM, but I am not able to make this work using blockram. Could someone help?


ENTITY vram8k IS
	PORT
	(
		data		: IN STD_LOGIC_VECTOR (7 DOWNTO 0);
		rdaddress		: IN STD_LOGIC_VECTOR (12 DOWNTO 0);
		rdclock		: IN STD_LOGIC ;
		rden		: IN STD_LOGIC  := '1';
		wraddress		: IN STD_LOGIC_VECTOR (12 DOWNTO 0);
		wrclock		: IN STD_LOGIC ;
		wren		: IN STD_LOGIC  := '1';
		q		: OUT STD_LOGIC_VECTOR (7 DOWNTO 0)
	);
END vram8k;

Instantiation:


	vram8k_inst : work.vram8k PORT MAP (
		data	 	=> v_data_sig,
		rdaddress 	=> v_address_r_sig(12 downto 0),
		rdclock	 	=> Clk_Z80,
		wraddress	=> v_address_w_sig(12 downto 0),
		wrclock	 	=> Clk_Z80,
		wren	 	=> vram_we_sig,
		rden	 	=> vram_re_sig,
		q			=> vram_q_sig
	);

I have tried a lot, and at this time, I have the following assertions for read/writing to the memory:


	v_address_w_sig <= A - x"2000" when (A >= x"2000" and MReq_n = '0');
	v_address_r_sig <= A - x"2000" when (A >= x"2000" and MReq_n = '0');
	vram_we_sig <= Wr_n;
	vram_re_sig <= Rd_n;
	v_data_sig <= DO_CPU when (Wr_n = '0' and MReq_n = '0');
        DI_CPU <= vram_q_sig when (Rd_n = '0' and MReq_n = '0' and A >= x"2000");

What must be changed for this to work?

Thanks.

15 Replies

Altera_Forum
Honored Contributor
17 years ago
It would be easier to have a design example, that contains the complete path from generating addresses, write data and control signals up to reading the data. From your post, I also don't see clearly, which step of the RAM access doesn't work as intended. This could easily be seen from a simulation, also a simulation would help us to understand the timing as achieved in the present design.

I'm pretty sure that the intended operation can be performed with internal RAM if used correct.
Altera_Forum
Honored Contributor
17 years ago
These seems to have worked very well:

-- Write into RAM vram_wraddress_sig <= A - x"2000"; vram_wren_sig <= not Wr_n when (A >= x"2000" and A <= x"4000"); vram_data_sig <= DO_CPU; -- Read from RAM vram_rden_sig <= Rd_n; vram_rdaddress_sig <= A - x"2000"; DI_CPU <= vram_q_sig when (A >= x"2000" and A <= x"4000") else D_ROM when (Rd_n = '0' and MReq_n = '0' and A < x"2000") else .....

Also, I am now able to write using a 3.58Mhz clock, and read at 25Mhz.

However, I am very confused. The LPM model for ram have control signals asserted low:
ENTITY vram8k IS
PORT
(
data : IN STD_LOGIC_VECTOR (7 DOWNTO 0);
rdaddress : IN STD_LOGIC_VECTOR (12 DOWNTO 0);
rdclock : IN STD_LOGIC ;
rden : IN STD_LOGIC := '1';
wraddress : IN STD_LOGIC_VECTOR (12 DOWNTO 0);
wrclock : IN STD_LOGIC ;
wren : IN STD_LOGIC := '1';
q : OUT STD_LOGIC_VECTOR (7 DOWNTO 0)
);
END vram8k;

In my code, asserting write HIGH will write data. Asserting READ LOW will read data. something is very wrong here.... Is it me?
Altera_Forum
Honored Contributor
17 years ago
I can't say anything about lpm_ram Megafunctions, I used altsyncram since Quartus V2, if I remember right. It has active high control signals, I would have thought the same of lpm_ram function, cause it's actually a wrapper of altsyncram, not inverting any signals as far as I see. Thus I guess, there is a misunderstanding of the basic operation of your design.
Altera_Forum
Honored Contributor
17 years ago
You might want to change this snipet of code...

(A >= x"2000" and A <= x"4000")
to
(A >= x"2000" and A < x"4000")

otherwise you will be writing to ram when the address is x"2000" and x"4000".
Altera_Forum
Honored Contributor
17 years ago
That's right, thank you...
:)

Forum Discussion

Blockram: RAM model read/write

15 Replies

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