12 / 24 · Concept

Compute the next value from the old value

Read nonblocking assignments and reset priority in the code.

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.

RTL excerpt
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.

Try it yourself

At a rising edge with count=11 and rst=1, is the result 00 or 01?

Read the explanation

00. The reset branch is selected, so the increment branch is not taken on that edge.