sandbox/fpga/tinyfpga/TinyAdder/TinyAdder.v

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`include "SegmentLCD.v"
/////////////////////////////////////////////////////////////////////////
/// TinyAdder.v
/// Author: K. Isom
/// Created: 2018-12-27
/////////////////////////////////////////////////////////////////////////
/// TinyAdder is a 4-bit adder that displays calculations on a pair of
/// 7-segment LCDs. The EXEC button doubles as a plus and equals button:
/// the first time it is pressed, entry begins with the second number. The
/// second time it is pressed, the sum of the two numbers is shown on the
/// display.
module TinyAdder (
input PIN_1, // A[0]
input PIN_2, // A[1]
input PIN_3, // A[2]
input PIN_4, // A[3]
input PIN_5, // EXEC
input PIN_6, // CLR
// There are two 7-segment displays: 'low' shows the 4 LSBs, and 'hi'
// shows the 4 MSBs. That is, given 0x12, 'low' would display '2' and
// 'hi' would show '1'.
output PIN_9, // LOW/A
output PIN_10, // LOW/B
output PIN_11, // LOW/C
output PIN_12, // LOW/D
output PIN_13, // LOW/E
output PIN_14, // LOW/F
output PIN_15, // LOW/G
output PIN_16, // HI/A
output PIN_17, // HI/B
output PIN_18, // HI/C
output PIN_19, // HI/D
output PIN_20, // HI/E
output PIN_21, // HI/F
output PIN_22, // HI/G
output LED, // User/boot LED next to power LED
output USBPU // USB pull-up resistor
);
// Drive USB pull-up resistor to '0' to disable USB. This prevents us
// from falling back into the bootloader, I think.
assign USBPU = 0;
wire [7:0] sum = 8'b00000000;
wire [3:0] switches;
assign switches = {PIN_4, PIN_3, PIN_2, PIN_1};
// state determines what happens when the button is pressed.
reg [1:0] state = 2'b00;
segment7 low (
.number(sum[3:0]),
.out({PIN_15, PIN_14, PIN_13, PIN_12, PIN_11, PIN_10, PIN_9})
);
segment7 hi (
.number(sum[7:4]),
.out({PIN_22, PIN_21, PIN_20, PIN_19, PIN_18, PIN_17, PIN_16})
);
// When EXEC is pressed, the adder's state is advanced. There are
// three states:
// + state 00: entry of the first number.
// + state 01: entry of the second number.
// + state 10: the sum of the two numbers is displayed. Pressing
// exec again resets the display.
always @(negedge PIN_5)
begin
case (state)
2'b00: begin
sum = switches;
state <= 2'b01;
end
2'b01: begin
sum = sum + switches;
state = 2'b10;
LED <= 1;
end
2'b10: begin
sum = 8'b00000000;
state = 2'b00;
LED <= 0;
end
endcase
end
// When CLEAR is pressed, the adder should go back to its initial
// state.
always @(negedge PIN_6)
begin
state <= 2'b00;
sum = 8'b0000000;
LED <= 0;
end
endmodule