Compute the next value from the old value

Learning goals

• Identify the old state used by the expression
• Read the priority between reset and increment

Calculate the next state separately

In count <= count + 2'b01, the right-hand count is the pre-update value. If it is 01, the computed next value is 10. Do not treat the expression as sequential edits to the same written value.

The nonblocking assignment <= schedules its evaluated value for the nonblocking update phase. Separating old and next values is also useful when several registers update on the same edge.

always_ff @(posedge clk)
    if (rst) count <= 2'b00;
    else count <= count + 2'b01;

One branch is taken at an edge

The if and else select one branch. With rst=1, the register stores 00 and does not take the increment branch. It does not reset to 00 and then increment to 01 on that same edge.

For a hand trace, write current count, current rst, selected branch and next count. Compare these columns with the code to check reset priority and wraparound.