Digital Systems: Principles and Design

439RAM, Address and Data Buses, Memory Decoding, Semiconductor Memories

Step G

4

: Find the decoder address select pin inputs. The decoder activates one of the Y

output as per the combination of inputs at these pins. Write in column 6, the conditions

for connecting the address select pins of the decoder. Use here only common unused

address lines for decoding. Consider DEC-1. It decodes the memories: PROM, RAM-

1 and RAM-2. RAM-2 and PROM conditions are specified for A

14

, A

15

as well as A

13

.

But RAM-1 condition for A

13

is not specified in this column. This is because RAM-1

connects A

13

directly to address bus. Consider DEC-2. It decodes the P

1

and P