Tiny url http://tinyurl.com/jdp3qqr

The object of this exercise is to see if by redefining the design, a faster decode is could be achieved by

using the NAND gates

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, then if that was successful, try
a decode chip from the same family.

Choosing a

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Fast NAND gate

Looking through the datasheets for NAND gates found the following timings:-

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Here is the chip that has been chosen:-

Circuit Description

There are 2 inputs,

A19 - Selects either the top half of the 1M memory space for the flash chip or the bottom half for the RAM.
/CE - Selects the 1M card.

Truth Table of the 74139

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

This ignores A1 GND line.

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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

01

02

03

They both agree.

Circuit Diagram

Circuit updated 15:04 6/9/16

Can a Full 74139 chip be built?

The Z88 only uses one half of the dual functionality of the full chip.

Using just NAND gates requires too many of them. See http://www.neuroproductions.be/logic-lab/index.php?id=63191 for the unfinished work in getting a full chip to work.

Building a prototype board internally

A prototype board was built inside the Flash card..

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It was very difficult to solder onto the legs of the inverted small chips. As a result, this prototype failed to work.

Building the circuit on a breadboard externally

Components required

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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Note

title	Signals not the same abbreviations as the circuit

The names of the signals on the scope are not all the same as shown in the circuit.

Signal	Scope	Circuit
A19	A19	A19
/CE1	_CE1	/CE1
/CE0	_CE	/CE0

Printed Circuit Board

http://tinyurl.com/h8dxsct

A PCB has been laid out (just in case Tony's prototype works).

Main points

A letterbox slot has been made in the centre of the HD1, HD2, (which are connected both sides,) so that either direct soldering or wire links may be used to connect the signals from the 512K/512K card to the PCB.
The pads of the footprint of the SOT-23-5 have been made longer, to enable easier soldering of the small parts.
The bottom Left Hand corner of the PCB matches the shape of the 512K/512K Card for easy alignment.
C2 on the 512K/512K, needs to be removed and put onto the PCB, in the same space. This is to make the PCB a bit larger for stability.

The 7 layers

Here are the layers printed on A4 sheets. The board is small, zooming in is generally helpful.

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View file

name	Circuits.pdf
height	250

Results

The latest software was tried again in a standard 512K/512K Flash/RAM Card and Vic could not make the games software go wrong. The prototype board was packed and sent to Mr T who had a failing Z88 and 512K/512K Flash/RAM Card. He confirmed that it was the decoder chip that was causing the fault and suggested that another decoder could be found.

Choosing a fast DECODER chip

Looking through the datasheets for a Texas Instruments VC1 chip found the following timings:-

Part No	Chip	Manufacture	Speed nS			Package	Price
			Min	Typ	Max
1741279	SN74LVC1G139DCTR	Ti			<2.5	SM8	$ 0.69
	SN74LVC1G139DCUT	Ti / Farnell			<2.5	VSSOP	£0.404

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PART NUMBER	PACKAGE	BODY SIZE (NOM)
SN74LVC1G139DCT	SM8 (8)	2.95 mm × 2.80 mm
SN74LVC1G139DCU	VSSOP (8)	2.30 mm × 2.00 mm