====== LED Toggle ======

===== Project Creation =====

Create a new RTL project named ''00338_led_toggle'', select ''xc7k480tffg1156-2'' part (or the actual device using)

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===== Add Design Source =====

Add new design source ''top.v'' to the project, with the content below. 

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<code verilog top.v [enable_line_numbers="true"]>
module top (
    input  wire sw,
    output wire led
);

assign led = ~sw;

endmodule
</code>

There is a **module** (not the filename) named ''top''. A module is a block of circuit that is able to perform specific function. Here, it defines an input signal ''sw'' and an output signal ''led''. Both of the signal have the type of ''wire'', which represents a net (connection) between hardware elements. It does not store a value by itself; its value is determined by whatever drives it. The ''assign'' describes continuous combinational logic. ''assign led = ~sw;'' means ''led'' continuously follows the inverse of ''sw''. ''~'' operator is bitwise invert of a signal, like some programming languages. That is, if ''sw'' is ''0''(low), ''led'' will be ''1''(high), and if ''sw'' is ''1'', ''led'' will be ''0''. ''module'' need to be completed by ''endmodule'' phrase. 

===== Add Constraint =====

The module ''top'' only describes how the circuit behaves, but we don't know where the signals connected to. Constraint file is used to specify the connection from our signal to physical IO pins. To create a constraint file, similarly we can create a ''top.xdc'' file to the project. 

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<code - top.xdc [enable_line_numbers="true"]>
set_property PACKAGE_PIN R28 [get_ports sw]
set_property IOSTANDARD LVCMOS18 [get_ports sw]

set_property PACKAGE_PIN P30 [get_ports led]
set_property IOSTANDARD LVCMOS18 [get_ports led]
</code>

Now we telling that the signal ''sw'' is physically connected to pin ''R28'' using IO standard ''LVCMOS18'', and also ''led'' is ''P30'' with ''LVCMOS18''. The choice of ''LVCMOS18'' is not a random option, it must follow the design of peripherals that the pin connected to, and must be compatible to the VCCO supply of the IO bank. 

For your information, the ''LVCMOS18'' standard specify the voltage as below: 

^ $V_\rm{IL,min}$ ^ $V_\rm{IL,max}$ ^ $V_\rm{IH,min}$ ^ $V_\rm{IH,max}$ ^ $V_\rm{OL,max}$ ^ $V_\rm{OH,min}$ ^ 
| $-0.3 \rm{V}$ | $0.35 \times 1.8 = 0.63 \rm{V}$ | $0.65 \times 1.8 = 1.17 \rm{V}$ | $1.9 \rm{V}$ | $0.45 \rm{V}$ | $1.8 - 0.45 = 1.35 \rm{V}$ |

===== Program Device =====

Now click "Generate Bitstream", which will perform tasks like a compiler, to perform a series operation including synthesis and implementation, to convert our project to a FPGA-readable content. Once completed, it can be downloaded to the FPGA device via the "Hardware Manager". 

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The device is expected to behave like what above codes describe: when pressing SW2, red LED light up, and the LED blow once release SW2. Be careful not to touch SW1 which will clear the currently downloaded FPGA chip configuration. If SW1 get pressed accidentally, re-download the bitstream.