Tiny url http://tinyurl.com/jdp3qqr

Introduction

The Z88 Flash/RAM card has been working for several years now. When OZ 4.6 was introduced, allowing applications to be run in RAM, some games failed to run.

This was found to be due to an intermittent timing error from the decode chip CD74HCT139 which switches between the RAM and Flash chip.

Although this is one of the fastest decode chips, taking 10nS, the software speed improvements pushed it outside these limits.

This documents highlights the steps taken to rectify this. The detail of the steps taken are now in subsections allowing the reader to go into detail if required.

Object

The object of this exercise is to see if by redefining the design on a small PCB, with the same footprint of the existing decode chip, a faster decode could be achieved by

Using high speed NAND gates, to see if then if that was successful
Then try using a decode chip from the same family.
Using the knowledge gained, add the circuit on the card to add the components to make a high speed 74139.

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

Adding 3 more of these decode chips allow full functionality using four 3mm wide high speed chips.

See http://www.neuroproductions.be/logic-lab/index.php?id=63191 for the ongoing logic in checking that the full chip will work.

NAND Gate Build

Building the circuit on a breadboard externally

5 NAND gates were used to test whether a faster time could be achieved with the Z88.

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.

DECODER Chip Build

http://tinyurl.com/j6zkfaj

Using 5 NAND Gates to prove that there is a hardware problem was very useful, but for a production run using a single 2-to-4 Line Decoder chip to replace them is the next step.
This circuit could now be bread-boarded and tested outside the Z88 card case.

The four signal and power lines can be seen connecting the card to the breadboard.

The Games play without crashing.

Printed Circuit Board

2-to-4 Line Decoder

Provisional Layout

Turbo Card to be produced

It has been proved that the decode chip in the Z88 Flash/RAM card is too slow. Existing users who wish to use OZ v4.6 (or later) will need their cards modified with the chip being replaced with this card.