Gray counter

Task: Design a 3-bit Gray counter.

Inputs:

• button BTN0 is a clock ( signal name - clk_i ),
• button BTN3 is an asynchronous reset ( signal name - rst_i ).

Outputs:

• LED LD0 - bit 0 (LSB) of the Gray counter ( signal name - led_o(0) ),
• LED LD1 - bit 1 of the Gray counter ( signal name - led_o(1) ),
• LED LD2 - bit 2 (MSB) of the Gray counter ( signal name - led_o(2) ).

Fig. 1. Photograph of the Digilent Spartan 3 board.

The clock is generated by pressing the BTN0 button and therefore the contact bouncing may occur, that is, the generation of more than one clock slope after a single press of a button.

Perform a functional simulation and verify the circuit practically by programming the prototype board.

UCF file for the exercise, Digilent Spartan-3 board, Spartan-3 3S200 FT256-4:

# Push-buttons:
NET "rst_i" LOC = "L14" ; # pressed high BTN3
NET "clk_i" LOC = "M13" ; # pressed high BTN0
NET "clk_i" CLOCK_DEDICATED_ROUTE = FALSE;
# LEDs:
NET "led_o<0>" LOC = "K12" ; # high on
NET "led_o<1>" LOC = "P14" ; # high on
NET "led_o<2>" LOC = "L12" ; # high on
#