Logic Design for Array-Based Circuits

by Donnamaie E. White

Sizing the Design - Selecting the Array

Array Sizing

Cell Structure

Each cell in an array consists of a number of uncommitted transistors, resistors and other discrete components and is designed around the performance criteria for the intended macro library. The cells will vary between array series, regardless of the vendor.

Equivalent gates

The number of equivalent gates has been a design measure dating from the days of discrete designs first converting into SSI-level ICs. Integrated circuits were classed as SSI, MSI and LSI based on their equivalent gate counts. Circuits were "sized" based on the number of equivalent gates it would take to create them. CMOS arrays carried on with the equivalent gate count and it was reasonable because the internal cell in a CMOS array can be sized as 1, 2 or 3 gates.

Bipolar arrays carry equivalent gate counts on their data sheets as a sizing measure but it serves only to show relative sizing between arrays in the same series. Bipolar array cell complexities render equivalent gates a rough measure at best. BiCMOS cells are more complex than CMOS and equivalent gate estimates are not recom mended for them either.

To complicate the problem, vendors use many different methods for computing equiva lent gates. The designer would need the algorithms before a rational comparison based on equivalent gates can be made between and two array series, even from the same vendor.

Example - Method 1

One approach to array sizing is to count the number of transistors in the internal core cells, assume that 2.5 transistors is equivalent to a gate (Digital Equipment's defini tion), and compute the number of equivalent gates per cell. The product of the number of cells times the number of gates per cell provides the equivalent gates per array.

equivalent gates = ( number of transistors in core / 2.5 )

Example - Method 2

Another method is to use the D flip/flop. Sizing the D flip/flop as 11 gates, the Q20000 Series D flip/flop uses 2 internal cells.

equivalent gates = ( number of internal cells / 2 ) * 11

Example - Method 3

The usual AMCC method is to size a 3:1 MUX-D flip/flop macro as 11 gates. The Q20000 Series 3:1 MUX-D flip/flop uses 3 internal cells.

equivalent gates = (number of internal cells / 3) * 11

Example - Method 4

The last method discussed here is to size a full adder at 16 gates. For the Q20000 Series, a 1-bit full adder takes 3 internal cells.

equivalent gates = (number of internal cells / 3) * 16

or:

equivalent gates = [(number of internal cells / number of cells required for measuring function) * number of gates in function]

AMCC ASIC Product Selection Guide with Equivalent Gates Listed (1996) (repeated)

AMCC ASIC PRODUCT SELECTION GUIDE (1990's)
Part Number	Technology	Equivalent Gates (Full Adder Method)	Number of I/O	Structured Array Blocks
Q20004	1 Micron Bipolar	671	28	None
Q20010	1 Micron Bipolar	1469	66	None
Q20025	1 Micron Bipolar	4032	100	None
Q20045	1 Micron Bipolar	6782	128	None
Q20080	1 Micron Bipolar	11242	162	None
Q20120	1 Micron Bipolar	18777	198	None
Q20P010	1 Micron Bipolar	928	34	1 GHz PLL
Q20P025	1 Micron Bipolar	3120	51	1 GHz PLL
Q20M100	1 Micron Bipolar	13475	195	RAM

I/O cell contributions

None of these methods for estimating equivalent gates take the logic capability of the interface cells into account. Some vendors do count them in their published equivalent gate counts and others do not.

Example - AMCC cell design

AMCC cell design is optimized for MUX, latch and flip/flop implementations. Each cell is designed to support high-speed requirements so that there are no placement re strictions on the high-speed option macros due to cell limitations. No power is used by a cell in its base configuration.

For the AMCC BiCMOS arrays, a cell is roughly 3 gates. For the bipolar arrays, a logic cell is a more complex structure and varies with the series.