Use of ONE_HOT with Differential Input Cells

019_cell : Use of ONE_HOT with Differential Input Cells

Minimum Required Versions: AccuCell 2.6.0.R, SmartSpice 4.6.2.R

This example explains the characterization and modeling options for cells with differential inputs using the embedded SmartSpice SPICE engine to perform characterization.

Cells with differential inputs require that these these pin pairs be declared so that the vector generation process considers them as a single vector condition input that are always compilmentary in nature. The AccuCell .cfg command ONE_HOT can be used to define these pin pairings. Differential inputs are supported for all cell types including sequential cells.

It should be noted however, that timing measurement will NOT be done based on "true" differential signaling methods. Single-ended vs a fixed VSS measurement will still be done. ONLY the vector generation process is controlled so as to avoid the generation of illegal and excessive state conditions and thereby reduce the vector set for characterization.

Therefore caution should be used when attempting to characterize sensitive I/O cells based upon "true" differential signaling standards. For such cells it is often necessary that measurements not only be done on the basis of "true" differential measurement, but also with specific and different threshold definitions for rising/falling input/output edges and pins. While usable results have be achieve with the current method and restrictions, the user is cautioned to carefully validate that doing so is reasonable and appropriate for the end application and use.

The most common use of this feature is for control lines of passgate logic cells that utilize an external select control circuit to prevent out of sequence activation of multiple signal paths. Use on data lines is also permissable, but simultaneous Multiple Input Switching(MIS) effects will not be accounted for in such cases due to skewed arrive times between the individual signals of the differential pair. Therefore for critical cells such as long-line high-drive high-speed differential buffer cells this method may not be suitable without additional margining.

In the case of a differential buffer however, if a .tbl file is utilized then NO ONE_HOT statements need be specified in the .cfg file. The use of a .tbl table vector file defines the nature of the pin relationships and provides the vectors for characterization. The example in addition to employing differential inputs and outputs also acts as a level shifter. The most common use of such cells is to distribute clocks across chip with low noise pickup. In addition by driving the clock trunks at reduced voltage significant power savings can be achieved compared to other methods. AccuCell enables the use of these advanced design methods by allowing users to develop and characterize the custom cells required with minimal effort.