Logic Design for Array-Based Circuits

by Donnamaie E. White

External Set-Up and Hold Times

Last Edit July 22, 2001

Case 1: When The Timing Specifications Are Nominal

For the case where the vendor has specified timing delays as nominal, with adjustment factors or worst-case multipliers, the following procedures can be applied. The worst-case multiplier (WCM) may be the product of adjustment factors for temperature, voltage and process.

Set-Up Time - Generic Equation

When computing the set-up time, it is desirable to assume that the data propagation path delay is the worst-case maximum and that the clock path propagation delay is a worst-case minimum for the operating conditions.

The generic equation is:

t_{su_external} = WCM_max * (t_D + T_{su_macro}) - WCM_min * t_C

Hold Time - Generic Equation

When computing the hold time, it is desirable to assume that the data propagation path delay is the worst-case minimum and that the clock path propagation delay is a worst-case maximum for the operating conditions.

The generic equation is:

t_{h_external} = WCM_max * (t_C + T_{h_macro}) - WCM_min * t_D

Terminology Definitions

t_D: Nominal data path propagation delay from the circuit input and up to the flip/flop or latch data input pin; including the interface macro intrinsic delay and the extrinsic net delay for the net driven by the interface macro, computed using Front-Annotation methodology before layout, Back-Annotation after layout.

t_C: Nominal clock path propagation delay from the circuit input and up to the flip/flop or latch clock input pin; including the interface macro intrinsic delay and the extrinsic net delay for the net driven by the interface macro, computed using Front-Annotation methodology before layout, Back-Annotation after layout.
T_{su_macro}: Macro intrinsic set-up time as specified in the macro library documentation. (For this case, it is assumed to be specified as typical.)
T_{h_macro}: Macro intrinsic hold time as specified in the macro library documentation. (For this case, it is assumed to be specified as typical)

Variations in Specifications

Even when the propagation delays through a macro are specified as typical or nominal, the set-up and hold times for flip/flops and latches may be specified as worst-case maximum. In that case, the macro set-up (T_{su_macro}) and hold (T_{h_macro}) times will not be multiplied by a worst-case multiplier or adjustment factor