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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.

Standard VersionModified Turbo Version

This was thought to be due to an intermittent timing error from the decode chip CD74HCT139 which switches between the RAM and Flash chip but after the Z88 Hardware Investigation, it has  been discovered to be a misunderstanding by both the hardware and software engineers on how to write faultlessly to the flash chip.

Object

Although the best solution would be a software upgrade to OZ 4.7 allowing users to update their cards, it has been decided to see if by changing the hardware the same result may be achieved. See Writing to Flash Chips for a full explanation of what needs to be fixed.

The object of this exercise is to see if by redefining the address map and by hardware prevent the flash chip from writing to the data bus when it is running OZ 4.7.

It has already been proved that if OZ 4.7 is run in a 256K EPROM card, the application runs faultlessly. This is because the code that writes to the Flash chip is ignored by the EPROM. There are several ways of achieving the same effect by changing the address mapping in the 1M space available on the card.


EPROMFLASHRAM




OZ 4.7FILESPROTECTEDFILES
CEA19A18ROE
Replace Flash with EPROM 1256K


768KXXX
Replace Flash with EPROM 2256K256K

512KXX

Protect Flash

256K256K512KXXXX

Decoder Connections

The decode chip used, is repeated twice in the same device. The first part does the 512K/512K Decode (as before) whilst the unused part is used for the changes.

In all cases, the input pins of the unused part (Pins 13, 14 & 15), need to be isolated as they have been connected to GND.

Replace Flash with EPROM 1

256K EPROM 768 RAM


CEA19A18
256K EPROM011
768K RAM000

Connection List


I/O
PIN
No







I/O
PIN
No







CEIE101000
CEFLSIE1501000

A19IA020x101
A19IA0140x101

0vIA130x011
A18IA1130x011

CERAO

Y0

411110
CERAO

Y0

1211110CE TO RAM
CEFLSO

Y1

511101
CEFLO

Y1

1111101CE TO EPROM

Replace Flash with EPROM 2

512K EPROM 512 RAM

NO CHANGE TO DECODE CHIP



PIN
No





CEE101000
A19A020x101
0vA130x011
CERA

Y0

411110
CEFL

Y1

511101

Protect Flash

256K FLASH PROTECTED, 256K FLASH & 512 RAM


CEA19A18ROES
256K FLASH PROTECTEDX111
256K FLASH110ROE
512K RAM00

Connection List


I/O
PIN
No







I/O
PIN
No







CEIE101000
ROEIE1501000

A19IA020x101
A19IA0140x101

0vIA130x011
A18IA1130x011

CERAO

Y0

411110

O

Y0

1211110

CEFLO

Y1

511101

O

Y1

1111101ROE_S TO FLASH

Decoder Tests

The decode circuits were built on a breadboard externally using DIL instead of using the SMD used on the card for easier testing.

16 pin DIL16 pin SO 16


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