1. EET 3350 Digital Systems Design
Textbook: John Wakerly
Chapter 9: 9.1
Memory
Read-Only Memory: ROM, PROM, EPROM
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2. Memory
• Sequential circuits all depend upon the presence of
memory
– A flip-flop can store one bit of information
– A register can store a single “word”
• typically 32 or 64 bits
– Memory stores a large number of words
• Memory stores this large amounts of data using two
primary device types
– Read Only Memory (ROM, PROM, EPROM, EEPROM)
– Random Access Memory (RAM)
• Static RAM (SRAM)
• Dynamic RAM (DRAM)
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3. Memory
Address Data
• You can think of memory as being 00000000 0110101100111101
00000001 1011111100100100
one big array (list) of data 00000002 1001110011110111
– The address serves as an array .
index .
– Each address refers to one word .
.
of data (e.g., 8-bits, 16-bits, etc.)
.
.
• You can read (or modify) the data .
.
at any given memory address, .
just like you can read (or modify) .
the contents of an array at any FFFFFFFD 0000101100001111
given index FFFFFFFE 1100101000110001
FFFFFFFF 0110101111010000
word
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4. Memory
Memory signals fall into three groups:
• Address bus - selects one of many memory locations
• Data bus -
– Read (ROM/RAM): the selected location’s stored data is
put on the data bus
– Write (RAM): The data on the data bus is stored into the
selected location
• Control signals - specifies what the memory is to do
– Control signals are usually active low
– Most common signals are:
• CS: Chip Select; must be active to do anything
• OE: Output Enable; active to read data
• WR: Write; active to write data
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5. Memory
• Memory is not a single chip (device)
– Made up of many identical or similar devices
– A specific device (part of memory) is selected by control
signals and the address lines (bus)
– All devices are connected to the same bus, and see the
signals at the same time
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6. Memory
• Memory Connection to CPU
– RAM and ROM chips are connected to a CPU through
the data and address buses
– The low-order lines in the address bus select the byte
within the chips and other lines in the address bus select
a particular chip through its chip select inputs
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7. Memory
• Location - the smallest selectable unit in memory
– Has 1 or more data bits per location
– All bits in location are read/written together
– Cannot manipulate single bits in a location
• For k address signals, there are 2k locations in a
memory device
• Each location contains an n-bit word
• Memory size is specified as
– #loc x bits per location
• 224 x 16 RAM - 224 = 16M words, each 16 bits long
• 24 address lines, 16 data lines
– #bits
• The total storage capacity is 224 x 16 = 228 bits
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8. Memory
• Memory sizes are usually specified in numbers of
bytes (1 byte= 8 bits)
• The 228-bit memory on the previous page translates
into:
228 bits / 8 bits per byte = 225 bytes
• With the abbreviations below, this is equivalent to 32
megabytes
Prefix Base 2 Base 10
K Kilo 210 = 1,024 103 = 1,000
M Mega 220 = 1,048,576 106 = 1,000,000
G Giga 230 = 1,073,741,824 109 = 1,000,000,000
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9. Memory
• Non-volatile
– If un-powered, its content is retained
• Read-only
– normal operation cannot change
contents 2k x n
ROM
• k-bit ADRS specifies the address or k
Δ
n
location to read from ADRS
CS
• A Chip Select, CS, enables or
Data
OE
Out
disables the RAM/ROM
• An Output Enable, OE, turns on or off
tri-state output buffers
• Data Out will be the n-bit value stored
at ADRS
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10. Memory
• Content loading (programming) done many
ways depending on device type
– ROM: mask programmed, loaded at the factory
• hardwired - can’t be changed
• embedded mass-produced systems
– PROM: OTP (One Time Programmable),
programmed by user, using an external
programming device
– EPROM: reusable, erased by UV light,
programmed by user, using an external
programming device
– EEPROM: electrically erasable, clears entire
blocks with single operation, programmed in-
place (no need to remove from circuit board)
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11. Read-Only Memories
• Definition
– ROM consists of an array of semiconductor devices
interconnected to store an array of memory data.
– Data can only be read, it cannot be changed under
normal operating conditions.
• Types of ROM
– Mask programmable ROM (at the factory)
– Field-Programmable ROM (PROM)
– UV-Erasable and re-Programmable ROM (EPROM)
– Electrically-Erasable and re-Programmable ROM
(EEPROM)
– Flash
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12. Read-Only Memories
• The logic symbol below is used in circuit diagrams
– Focus is on the basic structure of a ROM
– A combinational logic circuit
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13. Logic-in-ROM Example
• As we discussed previously, a ROM is simply a
combinational circuit, basically a truth-table lookup
– Can perform any combinational logic function
– Address inputs = function inputs
– Data outputs = function outputs
(address) (data)
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14. Logic-in-ROM Example
• Two alternative implementations for the 3-input, 4-output
logic function
– 2-to-4 decoder with output polarity control
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15. 4x4 Multiplier Example
• ROM implementation of a 4x4 unsigned binary
multiplier
– Multiplier and multiplicand form the address
– Product is pre-programmed into the storage location
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16. 4x4 Multiplier Example
• ROM contents for the 4x4 unsigned binary multiplier
x0 x1 x2 x3 x4 x5 x6 x7 x8 x9 xA xB xC xD xE xF
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17. Internal ROM Structure
• Typical implementation of “primitive” ROM
Diode means a
“1” is stored at
this location
data output
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active low
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20. ROM Control and I/O Signals
• n Address lines
– An-1 … A0
• b Data lines
– Db-1 … D0
• Chip Select
– One or more
– Active low
• Output Enable
– Active low
• Tri-state output
buffers
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21. ROM Timing
• tAA (access time from address): propagation delay from stable
address inputs to valid data output
• tACS ( access time from chip select): propagation delay from time
CS is asserted until the valid data output
• tOE (output-enable time): propagation delay from time OE and CS
are asserted until the tri-state drivers have left Hi-Z state
• tOZ (output-disable time): propagation delay from time OE and
CS are negated until the tri-state drivers have entered Hi-Z
state
• tOH (output-hold time): the length of time the outputs remain valid
after a change in the address inputs, or after OE and CS are
negated
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22. ROM Timing
• tAA access time from address
• tACS access time from chip select
• tOE/tOZ output-enable/disable time
• tOH output-hold time
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23. ROM -Advantages and Disadvantages
• Ease of speed and design
• For moderately complex function a ROM-based circuit is
usually faster than a circuit using multiple SSI/MSI devices and
PLDs
• The program that generates the ROM contents can easily be
structured to handle unusual or undefined cases that would
require additional hardware in any other design. e.g. the adder
program easily handles out-of-range sums.
• A ROM’s function is easily modified just by changing the stored
pattern, usually without changing any external connections
• The prices of ROMs are dropping and densities increasing
making them more economical and expanding the scope with a
single chip
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24. ROM -Advantages and Disadvantages
• May consume more power
• For functions with more inputs a ROM based circuit is
impractical because of the limit on ROM sizes that are available
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