Tiny url http://tinyurl.com/jdp3qqr

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Some games that run in RAM, particularly when in combination with OZ v4.6 and later - running in slot 1, fails to run on the 512K/512K Flash/RAM Card. This may be due to a timing error which is introduced by the decode chip CD74HCT139. This Although this is one of the fastest decode chips but , it is too slow for our purposes.

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.

The object of this exercise is to see if by redefining the design, a faster decode is achieved by using the NAND gates that are required.

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Choosing a fast NAND gate

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

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

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

Logic Diagram

Original

Cut Down Version using two inputs and two outputs

This ignores A1 GND line.

Logic Lab Test v Truth Table

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

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Extra components required