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Equation 6 · Part 1 · When the Substrate Became the Product

Symbol C_fix

CfixC_{\mathrm{fix}}
CfixC_{\mathrm{fix}}

What this part means

CfC_fix is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.

Its job in the formula

CfC_fix is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.

The passage around this formula

where CfixC_{\mathrm{fix}} collects driver, receiver, bump and pad capacitance, cwc_w is capacitance per unit length of the conductor, ℓ\ell is the reach, and VswV_{\mathrm{sw}} is the signalling swing. Two assumptions are doing the work and both deserve stating. First, this treats the link as a lumped capacitance rather than a transmission line — reasonable for a few millimetres of on-package wire at moderate rates, wrong once the link is long or fast enough to need equalisation, retiming and serialisation, at which point the fixed circuit overhead dominates and the linear term stops being the whole story. Second, it ignores clocking and idle power, which for real links are not negligible.

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A subscript is a label attached below a symbol. It often selects a time step, component, category, or member of a sequence.

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