Case Study: Sizing A Design
Last Edit July 22, 2001
The clock input will use IE34H, a differential high-speed input with
a Y and YN output. For CML-compatible input, use IE31H. The clock will
have two loads. It uses two I/O cells and two pads. The clock is in the
Other options that could be considered include the use of the driver
version of the differential input, IE32D. The driver handles 32 loads
and has k-factors with less skew than those of the H-option IE34H. If
the IE34H proves to be too slow or the inter-macro delays too long, the
IE32D would be the choice for a speed upgrade. The driver is shown in
Figure A-5 Differential Input Macro
ECL outputs - first pass
All outputs in the initial version of the circuit were the OE42S, a cut-off
(ECL output with an enable) macro used with the enable tied low (always
on) except for the two controlled outputs. (This macro was the only 50
ohm non-Darlington termination in the initial release of the library.)
The (111) version of the library added OE11S, a NOR-input 50 ohm termination,
rated for 350MHz. The other option is to have a custom 50 ohm macro created,
not worth the effort for the case study but something that should be reviewed
in a real circuit where power and cell space are at a premium.
The OE42S enable is tied low by way of the GT87D static driver, a macro
that supplies steady HIGH and LOW signals when unused macro pins cannot
be "clipped" low or allowed to float.
There will be 64 data output for the pipeline, six outputs for the pass-through
signals, two MUX outputs and an output for the parametric gate tree for
a total of 73 outputs. Each OE42S uses one I/O cell and one pad.
The fan-out load limit for the GT87D is 50 loads so two will be required
to supply the OE42S enable pins in this first version of the design. The
basic module is shown in Figure A-6.
Note that the OE11S is easier to use and uses less power - reasons to
consider challenging the initial solution.
Figure A-6 Macro Design Restrictions