New component in SOPC builder

i can't give you and an VHDL example as i use verilog HDL

but have a look at avalon memory-mapped slave template (http://www.altera.com/support/examples/nios2/exm-avalon-memory-slave.html?gsa_pos=1&wt.oss_r=1&wt.oss=avalon slave template)

to understand how you can include your own functionality as an sopc component.

mainly you need some signals like chipselect, write, writedata, read, readdata and clock & reset

you need to code a register that holds your signals you want to control via nios

this register is read via the read signal and delivers its content via readdata

and vice versa, write & writedata to store the information.

use chipselect to enable the access

now you can use one of these bits from that register as enable for your functionality