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Can't create workable controller for SRAM 512 kb (ISSI IS61LV25616AL) on Altera DE1

Honored Contributor II

Hello all. 


I have a problem with SRAM 512 kb (ISSI IS61LV25616AL) on Altera DE1. 


My problem is: 


If i set memory address and writing data to memory cell with this address, after if i change address to another address i read last data from that address (which was writed on last adress). And if i change address again, i read this last writed data. I read last writed data from all adresses.  


I writed more simple verilog sram controller (OE CE UB LB is always 0). And i change only WE for control read and write. 


But i have same problems. I read last writed data from all adresses 


qar file in attachement. 


Thanks in advance for any help
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Honored Contributor II

problem is solved. 


We have 16 bidireсtional pins for data writing/reading on SRAM 512 kb (ISSI IS61LV25616AL) on Altera DE1. 


And I define this 16 pins as "reg" type with "inout" direction in my verilog module: 

module new_simple_controller ( ... inout reg data_bidir, ... );  

But as I think - "reg" type can change value only inside verilog module. And this value can't be changed from outside. 


When I writed data to SRAM, this data writed to "reg". But when I readed data from SRAM, i only readed previous writed data from "reg" (but not data which readed from SRAM). 


Thanks to Shawn Gerber (http://www.youtube.com/watch?v=ubsreeqe8hw&list=uu2tvyj0h_2txv9oxqeha7uq - SRAM Controller in Verilog for Altera DE1 board). He sended me source code of his SRAM controller and I finded this mistake in my code. 


solution of this problem is following: 


1) "Inout" direction for 16 SRAM data pins only "wire" type. 

module controller_for_sram_is61lv25616al ( ... inout wire data_from_to_sram_input_output, ... );  


2) Add three registers. 

reg register_for_reading_data; // intermediate buffer register for reading data reg register_for_writing_data; // intermediate buffer register for writing data reg register_for_splitting; // register for disabling last "assign" // if = 0 - disable last "assign" (switch last "assign" to high impedance state) // (when we read data from bidirectional SRAM port - we can't write dada to this port) // if = 1 - enable last "assign" // (when we write data from intermediate buffer register "register_for_writing_data" to bidirectional port)  


3) Reading from SRAM. 

register_for_reading_data<=data_from_to_sram_input_output; // we read data, when last "assign" in high impedance state  


4) Writing to SRAM. 

assign data_from_to_sram_input_output = (register_for_splitting) ? register_for_writing_data : 16'bz; // last "assign"  


Controller source code (controller_for_sram_is61lv25616al.v) and diagram of work (diagram_of_work.pdf) in attachment.
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