1. ECE 301 – Digital Electronics
Memory
(Lecture #22)

2. Random Access Memory

3. ECE 301 - Digital Electronics
Random Access Memory
Static Random Access Memory (SRAM)
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4. ECE 301 - Digital Electronics
Random Access Memory
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5. ECE 301 - Digital Electronics
Random Access Memory
Address Decoding
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6. ECE 301 - Digital Electronics
RAM: Address Decoding
k-bit address
Decoder requires 2k AND gates
Each AND gate has k inputs
For large k this becomes prohibitive.
Use 2-dimensional decoding
Two decoders
Each decoder requires 2(k/2) AND gates
Each AND gate has k/2 inputs
Far less combinational logic
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7. ECE 301 - Digital Electronics
RAM: Address Decoding
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8. ECE 301 - Digital Electronics
RAM: Address Decoding
The size of a chip package is often dictated by the number of input and output signals.
For large memories, the number of address lines often becomes prohibitive.
Use address multiplexing
The same address lines are used both for the row address and the column address
Use time multiplexing to first latch the row address and then latch the column address
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9. ECE 301 - Digital Electronics
RAM: Address Decoding
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10. Building a Bigger Memory System
Random Access Memory
Building a Bigger Memory System
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11. ECE 301 - Digital Electronics
RAM Systems
Often RAM chips are smaller than the required memory size.
What if you need a wider memory?
Larger word size
What if you need a deeper memory?
Greater number of memory locations
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12. Exercise: Design a 32K x 32 memory using RAM chips that are 32K x 8.
RAM Systems
Exercise:
Design a 32K x 32 memory using
RAM chips that are 32K x 8.
How many address bits are required?
How many data bits are required?
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13. ECE 301 - Digital Electronics
Exercise: 32K x 32 RAM
Addr 15
A14 - A0
D31 - D24 8
D23 - D16
D15 - D8
D7 - D0
Data
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14. Exercise: Design a 128K x 8 memory using RAM chips that are 32K x 8.
RAM Systems
Exercise:
Design a 128K x 8 memory using
RAM chips that are 32K x 8.
How many address bits are required?
How many address bits are connected to the RAM chips?
What are the remaining address bits connected to?
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15. ECE 301 - Digital Electronics
Example: 128K x 8 RAM
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16. ECE 301 - Digital Electronics
Read Only Memory
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17. ECE 301 - Digital Electronics
Read Only Memory
ROM store “permanent” binary information
One-time programmable memory
Multiple-time programmable memory
Address and Data
k address bits
n data bits
2k x n ROM includes
k-to-2k decoder
n 2k-input OR gates
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18. ECE 301 - Digital Electronics
Read Only Memory
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19. ECE 301 - Digital Electronics
Read Only Memory
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20. ECE 301 - Digital Electronics
Read Only Memory
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21. ECE 301 - Digital Electronics
Read Only Memory
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22. ECE 301 - Digital Electronics
Read Only Memory
EEPROM (E2PROM)
Electrically Erasable Programmable ROM
Flash ROM
Similar to E2PROM
Has additional circuitry to selectively erase and program the memory in-circuit
Does not require a special programmer
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23. Programmable Logic Devices
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24. Programmable Logic Devices
Programmable Logic Arrays (PLA)
Programmable Array Logic (PAL)
Simple Programmable Logic Device (SPLD)
Complex Programmable Logic Device (CPLD)
Field Programmable Gate Array (FPGA)
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25. ECE 301 - Digital Electronics
“That's All Folks!”
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26. Acknowledgments
The slides used in this lecture were taken, with permission, from those provided by Pearson Prentice Hall for
Digital Design (4th Edition).
They are the property of and are copyrighted by
Pearson Education.
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