7.6 A Read/Write RAM Interface Example

Some special decoder IC's have been produced which help simplify the design of address decoding circuitry. One such decoder in common use is the 8205 (alias 74LS138). Pertinent data for this IC is listed below.

PIN CONFIGURATION

LOGIC SYMBOL

PIN NAMES A0-A2 Address Inputs E1-E3 Enable Inputs O0-O7 Decoded Outputs ADDRESS ENABLE OUTPUTS A0 A1 A2 E1 E2 E3 0 1 2 3 4 5 6 7 L L L H L L L H L H H L L L H H L H L H H H H H X X X X X X X X X X X X X X X X X X X X X L L H L L H L L H L L H L L H L L H L L H L L H L L L H L L L H L H H L H L H L H H H H H L H H H H H H H H L H H H H H H H H L H H H H H H H H L H H H H H H H H L H H H H H H H H L H H H H H H H H L H H H H H H H H L H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H

A commonly-used static RAM IC, the 2114, is illustrated below. Containing 4K bits of storage, it is organized as 1024 four-bit memory locations. Note the different speed/power varieties of this memory IC which are available.

	2114-2	2114-3	2114	2114L3	2114L
Max. Access Time (ns)	200	300	450	300	450
Max. Power Dissipation (mw)	710	710	710	370	370

PIN CONFIGURATION	LOGIC SYMBOL	PIN NAMES A0-A9 Address Inputs VCC Power (+5V) WE Write Enable GND Ground CS Chip Select I/O1-I/O4 Data Input/Output
BLOCK DIAGRAM

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