Page Comparison

Choosing a Fast NAND gate

Looking through the data sheets for NAND gates found the following timings:-

Part No	Chip	Manufacture	Speed				Package	Price
			Min	Typ	Max
1085297	74HCT00D	NXP		10nS			SOIC	0.221
381755	74HCT00N	NXP		10nS			DIP	0.667
9590889	74HC00	Ti		8nS			DIP	0.31
1105915	SN74AHCIG00	Ti		5,2		8 mA	SOT-23-5	1.78
1287697	SN74LVCIG007 SN74LVC1G00DBVR	Ti		4.0		32mA	SOT-23-5	1.98
	SN74LVC1G00-EP	Ti	1.0	4.0
	SN74LVC1G00DBVR	Ti	1.0		4.0		SOT-23-5

The Single 2-input Positive NAND gates display the fastest times.

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Full Data Sheet for SN74LVC1G00 Single 2-Input Positive-NAND Gate & Timing

View file

name	sn74lvc1g00.pdf
height	150

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Circuit Description

There are 2 inputs,

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Truth Table of half a 74139 using two outputs

Inputs Enable Select			Outputs
/E /CE	A1 GND	A0 A19	/Y1 /CE1 FLASH	/Y0 /CE0 RAM
0	0	0	1	0
0	0	1	0	1
1	X	X	1	1

Logic Diagram

Original

Cut Down Version using two inputs and two outputs

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The Z88 only uses one half of this dual decode chip. This circuit may be made with

3 inverters
2 NAND gates

The inverters are slower than the NAND gates, so it was decided to use 5 NAND gates using 3 of the gates as inverters.

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The logic was checked with thanks to http://www.neuroproductions.be/logic-lab/ for the simulator.

Inputs Enable Select			Outputs
	/E /CE	A0 A19	/Y1 /CE1 FLASH	/Y0 /CE0 RAM
00	0	0	1	0
01	0	1	0	1
02	1	0	1	1
03	1	1	1	1

00 Image Modified	01 Image Modified
02 Image Modified	03 Image Modified

They both agree.

Circuit Diagram

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Building the circuit on a breadboard externally

Components required

Image Modified

10 sot to IC adaptors were obtained in addition to the 5 NAND gates.
The circuit could now be bread-boarded and tested outside the Z88 card case.

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