Tutorial 2 Sequential Logic

Registers A register is basically a D Flip-Flop A D Flip Flop has 3 basic ports. D, Q, and Clock

Registers Registers are edge sensitive to the clock On a rising edge of the clock, the output of the flip flop (Q) takes on the value of the input (D). This is known as ‘clocking in’. When it’s NOT the rising edge of the clock, Q doesn’t change even if D does A register is often considered the most basic block of memory, because the value of D is stored in the register until the next clock cycle

Sequential Circuits A sequential circuit is any digital design that has registers in it

Processes In VHDL, sequential logic is described in a process process(clk) begin if rising_edge(clk) q <= d; end if; End process; The rising_edge (or falling_edge) statement is a key word. Anything assignments in this “if” block is registered.

Processes In a process, inside an ‘if rising_edge’ block, all assignments are registered This means that on an assignment statement, everything to the LEFT of the <= is the output of a DFF and everything to the RIGHT is the input to a DFF

Processes The previous example described a DFF. Let’s describe something else. If rising_edge(clk) then a <= b; c <= a; end if; What does the hardware look like?

Processes

Let’s mix it up a bit If rising_edge(clk) then a <= b; c <= b; end if What does the hardware look like?

Processes

Let’s put some combinational logic between those registers! If rising_edge(clk) then a <= b and c; d <= a and b; end if; What does the logic look like?

Processes

Registers can also feed back on themselves Process (clk) if rising_edge(clk) then a <= b or c; d <= a xor d; end if; end process; What does this hardware look like?

Processes