05 Larger Combinational Systems (Part B)

05 Larger Combinational Systems (Part B) - Programmable...

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39 Programmable Logic Device
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40 Read-Only Memory (ROM) ± A device in which permanent binary information is stored ( non-volatile memories ) ± Even when power is turned off, the information is still there ± k -bit address inputs and n -bit data outputs ± Inputs: address for the memory ± Outputs: the word ( n data bits) stored in ROM selected by the k -bit input address ± k address input can specify 2 k words k -bit address inputs n -bit data outputs 2 k x n ROM k n
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41 Truth Table for ROM ± e.g. 2 3 x 4 ROM ( k = 3, n = 4) 0 0 0 1 0 1 1 0 F 2 1 1 1 0 1 1 0 1 F 3 0 1 1 1 0 1 0 0 0 1 0 1 1 0 1 0 0 0 1 1 1 0 1 0 A 0 Outputs (Word) Inputs (Address) 1 0 0 0 F 1 1 1 1 0 F 0 1 1 1 0 0 0 0 0 A 1 A 2 Example data stored in ROM (2 3 words of 4 bits each) ROMs work as a lookup table. In this example, When you input (0, 0, 0), it will output (1, 0, 0, 0). When you input (0, 0, 1), it will output (0, 1, 0, 1), …etc
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42 Pros and Cons of ROM ± Advantage ± Simpler and lesser connections ± Cheap for mass production ± Disadvantage ± Doubling the size for each additional input ± Expensive for development
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43 Other ROMs ± ROM usually works as a lookup table ± No data inputs as ROM does not have a write operation (only read operation) ± ROMs that can be write / rewrite ± PROM ± EPROM ± EEPROM ± FLASH Memory
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44 Implementation Technologies ± Implementation technologies introduced so far are not programmable ± Fixed integrated circuits ± Programmable Logic Device (PLD) ± Integrated circuit with internal logic gates connected together ± These internal connections can be reconfigured (erased / programmed) to form a specific logic circuit ± Three types of basic PLDs ± ROM, PLA, PAL
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45 PROM ± P rogrammable R ead-O nly M emory ± Perform same functions as ROM ± Oldest programmable logic device ± Fuse or anti-fuse programmed ± One-time writing process has been deferred to the end-user ± Advantage: easy programming and inexpensive ± Disadvantage: non-reprogrammable
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46 Fuse and Anti-fuse ± Fuses which are initially CLOSED (1) are selectively “blown out” by a higher-than-normal voltage to established OPEN (0) connections ± The pattern of OPEN and CLOSED fuses establishes the connections defining the logic ± Connections are permanent,
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05 Larger Combinational Systems (Part B) - Programmable...

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