14_EEL4742-Notes#15 - EEL4742 Notes 15 Address Decoder and...

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EEL4742 Notes # 15 Address Decoder and Decoding Typical memory interface The address decoder determines when the memory IC’s are accessed . ce R/W Function 1 x Hold/Standby 0 1 Read 0 0 Write Consider 16 Bit address lines A 0 -A 15 64K memory map. and 8K by 16 Bit memory I.C. The question is where to put the 8K memory in the 64K memory map . Where you place the 8K block of memory determines when the ce must go low. Some important #s 10 Bits = 1K 20 Bits = 1 Meg 30 Bits = 1 G. Also 1000h = 4096 100h = 256 Typically the memory IC’s are smaller than the memory space
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Consider just A 15 . If A 15 = 0 and A 0 - A 14 are allowed to change between all zeros and all ones , we have the addresses: 0000h 7FFF. If A 15 = 1, then we have the address 8000h FFFFh. So, A 15 divides the memory space into 2 blocks 32K each . A 15 = 0, lower 32K (0000h 7FFFh) and A 15 =1, the upper 32K (8000h FFFFh). Now consider A 15 A 14 if A 15 A 14 = 00 and A 0 - A 13 are allowed to change between all zeros and all ones, then addresses are from 0000h 3FFFh (this becomes a 16 bit block) A 15 A 14 =
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