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:-
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 |
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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
Inputs Enable Select | Outputs | |||
---|---|---|---|---|
/E /CE | A1 | 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.
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
Component | Manufacturer Part No | Order Code | Price | Min Qty |
---|---|---|---|---|
N95BT | Maplin | 4.09 | 1 | |
YA29G | Maplin | 1.79 | 1 | |
MFR3 2K2 FC | 1565301 | £0.085 | 10 |