Buttons communication problem NIOS II

It seems like buttons are just for reading. I put the module definition below. Am I right ? Any other idea ?

In the module, I chose "input ports only" and did not specify any input options.

Maybe it comes from my pin assignment, I only added the following assignments in my tcl :

set_location_assignment PIN_F1 -to button[0]

set_location_assignment PIN_F2 -to button[1]

set_location_assignment PIN_A10 -to button[2]

set_location_assignment PIN_B10 -to button[3]

I let the I/O standard to 2.5V and the current strength to 8mA. May it come from this ?

Thanks,

Myriam

Here is my module :

module button_pio (

// inputs:

address,

clk,

in_port,

reset_n,

// outputs:

readdata

)

;

output [ 3: 0] readdata;

input [ 1: 0] address;

input clk;

input [ 3: 0] in_port;

input reset_n;

wire clk_en;

wire [ 3: 0] data_in;

wire [ 3: 0] read_mux_out;

reg [ 3: 0] readdata;

assign clk_en = 1;

//s1, which is an e_avalon_slave

assign read_mux_out = {4 {(address == 0)}} & data_in;

always @(posedge clk or negedge reset_n)

begin

if (reset_n == 0)

readdata <= 0;

else if (clk_en)

readdata <= read_mux_out;

end

assign data_in = in_port;

endmodule