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

A buffered-fanout ledger checks both the first-stage buffer inputs and each downstream output group, so total load is not enough if one group is oversized. The group pass/fail rows are exact only for the declared group sizes, input currents, drive limits, ideal buffers, and steady DC assumptions; real buffered logic adds leakage, input capacitance, output resistance, threshold and voltage-margin variation, switching noise, temperature drift, and measurement uncertainty.

Buffered fanout is checked by both the source-stage current and each downstream group current. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

A buffered fanout has source and group checks

The source first drives buffer inputs. Each buffer then drives a group of final inputs. A buffered handoff passes only when every stage stays inside the same current limit.

I_{\text{first}}\le I_{\text{limit}},\quad I_{\text{group}}\le I_{\text{limit}}

Three buffered rows show total load and split quality

The first two rows pass because each group is small enough. The last row has the same total load as the middle row, but an uneven split overloads one buffer output.

\begin{array}{c|c|c|c|c|c}N&\text{groups}&I_{\text{first}}&I_{\text{group}}&I_{\text{limit}}&\text{state}\\6&3+3&2\ \text{A}&3\ \text{A}&5\ \text{A}&\text{PASS}\\8&4+4&2\ \text{A}&4\ \text{A}&5\ \text{A}&\text{PASS}\\8&6+2&2\ \text{A}&6\ \text{A}&5\ \text{A}&\text{FAIL}\\\end{array}

The split matters as much as the total load

A buffer is not a magic capacity multiplier. It works only when the load distribution keeps each stage inside its own exact budget.

6\ \text{A}>5\ \text{A}\Rightarrow\text{FAIL}