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A17 / A18 wiring
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Author:  guitarzombie [ Mon May 26, 2014 11:56 am ]
Post subject:  A17 / A18 wiring

Quick question about these pins for EPROMS. Do you only need to wire pin 30/31 ONLY if your EPROM size is 256k or higher?

Author:  tokumaru [ Mon May 26, 2014 12:31 pm ]
Post subject:  Re: A17 / A18 wiring

Each address line doubles the amount of memory that can be accessed. With one address line (bit), you can only address 2 positions, 0 and 1. Add a second address line and you double the amount of positions you can address, because you can now access both positions you could before with the new address line with values of 0 and 1. This continues all the way up to the higher address lines:

1 address line: 2 positions;
2 address lines: 4 positions;
3 address lines: 8 positions;
4 address lines: 16 positions;
5 address lines: 32 positions;
6 address lines: 64 positions;
7 address lines: 128 positions;
8 address lines: 256 positions;
9 address lines: 512 positions;
10 address lines: 1024 (1K) positions;
11 address lines: 2048 (2K) positions;
12 address lines: 4096 (4K) positions;
13 address lines: 8192 (8K) positions;
14 address lines: 16384 (16K)positions;
15 address lines: 32768 (32K) positions;
16 address lines: 65536 (64K) positions;
17 address lines: 131072 (128K) positions;
18 address lines: 262144 (256K) positions;
19 address lines: 524288 (512K) positions;
X address lines: 2 to the Xth power positions;

If you're using 128KB EPROMs, you only need 17 address lines, and since address lines are numbered from 0 up, you'll be using A0-A16. You shouldn't connect any higher address the board may have to anything.

In the opposite situation though (chip has more address lines than the board), you'll want to connect the unused address lines to GND, otherwise those unconnected lines might cause the chip to access random locations.

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