User Input
User Input consists of 4 bytes, each representing a different device. These bytes can be accessed using the following instructions:
la t0 0x00fffffc
lb t0 t0 device_byteHere, device_byte is a number between 0 and 3.
The keyboard is responsible for reading input and transmitting it in ASCII format.
In Logisim Evolution, it is a native component that stores a string of characters and outputs the first typed character through a 7-bit output. This output is combined with an additional bit that indicates whether there are characters in the buffer. This bit is placed in the most significant position of the byte, effectively representing the sign of the number, which makes it easier to test whether the value is valid.
Whenever the keyboard receives a read signal, the first character is deleted from the buffer. This continues until the buffer is empty. The keyboard also transmits an external interrupt signal to the processor, allowing the currently running program to be interrupted so the available input can be processed immediately.
The joystick is a simple input device that allows directional control within the simulation. In the current version of the project, it is only available in the Logisim simulation.
Joystick data is accessed by reading the third byte of the user input. It returns the X and Y values in the format 0xXY. To use this data effectively, an additional processing step may be required to separate each axis from the byte.
Switches are components that allow individual bit input through the second byte of the User Input. Like the joystick, this device is not available in the SystemVerilog simulation.
Although not a user input device, the Random Number Generator is accessed through the first byte. It generates a new random value every time the user input word is read.
In SystemVerilog, these components cannot be accessed directly. To enable interaction, an external program was developed that facilitates communication with the simulation via a shared text file.
The image below shows this program, with the keyboard component highlighted:
Below is the code responsible for handling user input:
`timescale 1s/1s
module user_input #(
parameter KEYBOARD_FILE = "keyboard.mem"
) (
input read,
input clock,
input reset,
output [31:0] data_out
);
integer file_ptr = 0;
integer char1, char2; // Use integers to handle EOF (-1)
reg [31:0] data_reg;
reg [7:0] random;
reg [7:0] separator; // Explicit register for separator
assign data_out = read ? data_reg : 32'hZ;
function automatic [3:0] hex_to_nibble(input [7:0] c);
case(c)
"0": return 4'h0;
"1": return 4'h1;
"2": return 4'h2;
"3": return 4'h3;
"4": return 4'h4;
"5": return 4'h5;
"6": return 4'h6;
"7": return 4'h7;
"8": return 4'h8;
"9": return 4'h9;
"A","a": return 4'hA;
"B","b": return 4'hB;
"C","c": return 4'hC;
"D","d": return 4'hD;
"E","e": return 4'hE;
"F","f": return 4'hF;
default: return 4'hF;
endcase
endfunction
always @(negedge clock) begin
if (reset) begin
// Close file if already open
if (file_ptr) begin
$fclose(file_ptr);
file_ptr = 0;
end
// Open in read mode
file_ptr = $fopen(KEYBOARD_FILE, "r");
if (file_ptr == 0)
$display("Error: Cannot open %s", KEYBOARD_FILE);
data_reg = 32'hFF000000; // Default reset value
end
else if (read) begin
random = $random; // Get new random value
if (file_ptr == 0) begin
// File not open - use default
data_reg = {8'hFF, 16'h0, random};
end
else begin
char1 = $fgetc(file_ptr); // Read first char
if (char1 == -1) begin // Check EOF
data_reg = {8'hFF, 16'h0, random};
end
else begin
char2 = $fgetc(file_ptr); // Read second char
if (char2 == -1) begin
// Single character case
data_reg = {
{hex_to_nibble(char1[7:0]), 4'h0},
16'h0,
random
};
end
else begin
// Read separator normally
separator = $fgetc(file_ptr); // Discard separator
// Process both characters
data_reg = {
{hex_to_nibble(char1[7:0]), hex_to_nibble(char2[7:0])},
16'h0,
random
};
end
end
end
end
end
endmodule