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Issue with the application layer from NIOS II IDE while accessing a custom instructio

Honored Contributor II



I have implemented a custom instruction for full adder (just a basic one) in verilog for NIOS II. 

However when my application code (C) from NIOS II tries to access the macro for the custom instruction, I 

see one input operand value being tried up to a strange value, 2047 (!). Any guess what might be causing the issue? 

I have checked almost all the sides, however I could not find the issue. 


I am attaching the verilog modules with this email, I have tested in Modelsim and there it works good. 

I am pasting the valid instructions from the file custom_add_hw.tcl below: 


"add_interface nios_custom_instruction_slave_0 nios_custom_instruction end 

set_interface_property nios_custom_instruction_slave_0 clockCycle 0  

set_interface_property nios_custom_instruction_slave_0 operands 2 

set_interface_property nios_custom_instruction_slave_0 ENABLED true 



add_interface_port nios_custom_instruction_slave_0 clk clk Input 1 

add_interface_port nios_custom_instruction_slave_0 reset reset Input 1 

add_interface_port nios_custom_instruction_slave_0 dataa dataa Input 32 

add_interface_port nios_custom_instruction_slave_0 datab datab Input 32 

add_interface_port nios_custom_instruction_slave_0 n n Input 3 

add_interface_port nios_custom_instruction_slave_0 clk_en clk_en Input 1 

add_interface_port nios_custom_instruction_slave_0 start start Input 1 

add_interface_port nios_custom_instruction_slave_0 done done Output 1 

add_interface_port nios_custom_instruction_slave_0 result result Output 32"  


Now from the NIOS II IDE, I am trying to call the custom instruction using the macro  

sumOutput = ALT_CI_CUSTOM_ADD_INST(4,1,10); as shown in the attached .c file. 

However the output I am getting is, 

Hello from Nios II! 



The sum is 2057  

which is 2047 (for dataa, I donno why) + 10 (for datab, which gets proper value). 


Please help me in solving this issue. 


Thank You, 

0 Kudos
3 Replies
Honored Contributor II



Can someone be of assistance to this post please? I am struck at this point in the simulation, appreciate any inputs. 

If the post is not clear, I can explain the scenario once again. 


Thank You, 

Honored Contributor II

Have you simulated this to make sure it's not the custom instruction itself? i.e. do you know for sure your custom instruction is seeing n = 4, dataa = 2047, datab = 10? 


Also did you read page 11 of this document? By including the clock enable, start, and done signals it is up to your custom instruction to time everything properly.
Honored Contributor II



Sorry, it had been my fault in the verilog and the fault was with respect to the timing. 

Earlier, I had been trying to do the ADD operation and read the result output port in the same clock cycle which was making the module confused 

and I guess the dataa port had been getting unknown value (like 2047) because of that timing issue. I really appreciate for BadOmen who made me 

to look into the custom instruction timing diagram once again (I am just a beginner). 


See what I did now is having separate 'write' and 'read' instructions and added the lines of code in verilog module custom_Component.v 

if(write && chipselect) begin 

cin = 0; 

{cout, output_value} = writedataa + writedatab + cin; 



else if((read == 1) && (chipselect == 1)) 


readdata <= output_value; 



so that before reading a result port, there will be a write latency. 

I tried to access from the application code (in NIOS II IDE) like this: 


/* Write Sum */ dataa = 0XFFFFFFFF; 

datab = 0XFFFFFFFF; 

sumOutput = ALT_CI_CUSTOM_ADD_INST(1,dataa,datab); 

printf("\nThe result line value is %lu",sumOutput); // Expect some unknown value to print 

/* Read Sum */ 

sumOutput = ALT_CI_CUSTOM_ADD_INST(4,dataa,datab); // Expect the earlier SUM value to print 

printf("\nThe SUM of %lu and %lu is %lu",dataa,datab,sumOutput); 


And the sumOutput variable printed an expected value!. 


I appreciate the effort taken by forum admins to sort this out. 


Thank You,