98 lines
2.5 KiB
Verilog
98 lines
2.5 KiB
Verilog
/////////////////////////////////////////////////////////////////////////
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/// TinyAdder.v
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/// Author: K. Isom
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/// Created: 2018-12-27
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/////////////////////////////////////////////////////////////////////////
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/// TinyAdder is a 4-bit adder that displays calculations on a pair of
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/// 7-segment LCDs. The EXEC button doubles as a plus and equals button:
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/// the first time it is pressed, entry begins with the second number. The
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/// second time it is pressed, the sum of the two numbers is shown on the
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/// display.
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module TinyAdder (
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input PIN_1, // A[0]
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input PIN_2, // A[1]
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input PIN_3, // A[2]
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input PIN_4, // A[3]
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input PIN_5, // EXEC
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input PIN_6, // CLR
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// There are two 7-segment displays: 'low' shows the 4 LSBs, and 'hi'
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// shows the 4 MSBs. That is, given 0x12, 'low' would display '2' and
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// 'hi' would show '1'.
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output PIN_9, // LOW/A
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output PIN_10, // LOW/B
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output PIN_11, // LOW/C
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output PIN_12, // LOW/D
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output PIN_13, // LOW/E
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output PIN_14, // LOW/F
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output PIN_15, // LOW/G
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output PIN_16, // HI/A
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output PIN_17, // HI/B
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output PIN_18, // HI/C
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output PIN_19, // HI/D
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output PIN_20, // HI/E
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output PIN_21, // HI/F
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output PIN_22, // HI/G
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output LED, // User/boot LED next to power LED
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output USBPU // USB pull-up resistor
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);
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// Drive USB pull-up resistor to '0' to disable USB. This prevents us
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// from falling back into the bootloader, I think.
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assign USBPU = 0;
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wire [7:0] sum = 8'b00000000;
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wire [3:0] switches;
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assign switches = {PIN_4, PIN_3, PIN_2, PIN_1};
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// state determines what happens when the button is pressed.
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reg [1:0] state = 2'b00;
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SegmentLCD low (
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.number(sum[3:0]),
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.out({PIN_15, PIN_14, PIN_13, PIN_12, PIN_11, PIN_10, PIN_9})
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);
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SegmentLCD hi (
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.number(sum[7:4]),
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.out({PIN_22, PIN_21, PIN_20, PIN_19, PIN_18, PIN_17, PIN_16})
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);
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// When EXEC is pressed, the adder's state is advanced. There are
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// three states:
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// + state 00: entry of the first number.
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// + state 01: entry of the second number.
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// + state 10: the sum of the two numbers is displayed. Pressing
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// exec again resets the display.
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always @(negedge PIN_5)
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begin
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case (state)
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2'b00: begin
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sum = switches;
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state <= 2'b01;
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end
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2'b01: begin
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sum = sum + switches;
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state = 2'b10;
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LED <= 1;
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end
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2'b10: begin
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sum = 8'b00000000;
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state = 2'b00;
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LED <= 0;
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end
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endcase
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end
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// When CLEAR is pressed, the adder should go back to its initial
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// state.
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always @(negedge PIN_6)
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begin
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state <= 2'b00;
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sum = 8'b0000000;
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LED <= 0;
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end
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endmodule
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