Direct Fanout Load Three-Row Scan - Electronic Circuits IV & XII: Digital Logic and Timing Closure Step by Step

A direct-fanout ledger multiplies the number of driven inputs by the current each input draws, then compares that total with the output drive limit. The pass/fail rows are exact only for the declared load count, input current, drive limit, ideal logic levels, and steady DC load model; real logic adds leakage, input capacitance, threshold spread, output resistance, voltage-margin loss, simultaneous switching noise, temperature drift, and current measurement uncertainty.

Direct fanout is a three-row boundary scan over exact load count, total current, and source limit. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Direct fanout is one multiplication

Each extra input adds the same training current. The formula has two variables, but this scan holds input current fixed and moves the load count through three rows.

I_{\text{total}}=N\cdot I_{\text{in}}

Below, equal, and over-limit rows expose the boundary

The load count column is the only changing input. The total current column follows by exact multiplication, then the state column follows by exact comparison.

\begin{array}{c|c|c|c|c}N&I_{\text{in}}&I_{\text{total}}&I_{\text{limit}}&\text{state}\\3&1\ \text{A}&3\ \text{A}&5\ \text{A}&\text{PASS}\\5&1\ \text{A}&5\ \text{A}&5\ \text{A}&\text{PASS}\\6&1\ \text{A}&6\ \text{A}&5\ \text{A}&\text{FAIL}\\\end{array}

The failed row is one load beyond the limit

The edge row and fail row use the same source limit. That makes the transition easy to see: equality is allowed, but one more load crosses the boundary.

5\ \text{A}=5\ \text{A},\quad 6\ \text{A}>5\ \text{A}