Digital Design: IMPLEMENTATION OF COMBINATIONAL LOGIC BY STANDARD ICs and PROGRAMMABLE ROM MEMORIES Part - II

Chapter 12

IMPLEMENTATION OF COMBINATIONAL LOGIC BY

STANDARD ICs and PROGRAMMABLE ROM

MEMORIES

Ch12L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 2

Lesson 2

Read Only Memory (ROM)


Outline

• ROM• Example —4k × 8 (4 kB)ROM • EEPROM, EPROM, OTP and Flash• Laboratory Programmer


ROM• Read only memory• Read means output data-bits corresponding to the

address bits set at the input. The bits are used by some processor or device or circuit for further operations.

• Data bits of different addresses store at ROM and all bits remain stored in the memory even after a power interruption or power switch OFF

• ROM is randomly accessed. An equal time interval is taken for a read of data-bits from each address. Address bits can be set randomly by an external circuit.


ROM• ROM is a previously programmed

‘Decoder-diode links’ array based logic device, programmed using appropriate masks at the manufacturing stage or by laboratory programming device

• ROM has an address decoder (n to 2n

decoder) within it• Each output line of decoder has 8

connections through a fusible diode. A diode n-end connects to a data bit output.


ROM• There are 8 data output lines D0-D7• Each output line Y of decoder has 8 connections

through the fusible link to diode. The diode n-ends connect to the data bit outputs and p-ends to Y . The n-ends also connect to ground line. Y is in tristate and only that Y is 1, which corresponds to address at input pins.

• A data bit is 1 when the corresponding diode link is not fused during masking, and 0 when fused. All diodes links are programmed by masking process such that only programmed diode links are fused and give 0s.


ROM Block DiagramA0

A1

.

.

.An–2

An–1

CS

n to 2n

Decoder

Y0

Yn–1

D0

.

.

D7


Pins• CS = Active 0 Chip-Select Pin to tristate

buffer enable input• A0-An = Address input bits to decoding

logic• D0-D7 = 8 data bits• RD = active 0 read-enable (output enable)

pin


Input Address and Output Data Bits• Each ROM has n inputs, called address bits (for

one of the 2n memory locations). This ROM on activating an input, called read input (actually a control gate input), generates for each set of address inputs, a distinct set of m outputs, called data bits (most often m = 8) from the addressed memory location. The data bits are as per programmed bits during manufacturing of ROM by silicon masking or any other process.


ROM Application• Preprogrammed toy circuit, • Preprogrammed robot circuit, • Standard look up table, • Arithmetic function table generator, • User defined code generator, • Character generator, • Printable or displayable fonts table,


ROM Application in a Computer• Set of data • Set of instructions


ROM special versions• Programmable logic devices (PLDs) called

PROMs (Programmable Read Only Memories),which use AND-OR arrays with fusible OR links

• EPROM • EEPROM • Flash• OTP ROM


Outline

• ROM• Example —4k ×××× 8 (4 kB)ROM • EEPROM, EPROM, OTP and Flash• Laboratory Programmer


4k × 8 ROM chipA0

A1

.

.

.A10

A11

CS

ROM

D0

D7

RD


Pins• CS = Active 0 Chip-Select Pin to tristate

buffer enable input• A0-A11 = 12 Address input bits to

decoding logic for 4096 addresses (called 4k addresses) between x000h to xFFFh

• D0-D7 = 8 data bits• RD = active 0 read-enable (output enable)

pin


Outline

• ROM• Example —4k × 8 (4 kB)ROM • EEPROM, EPROM, OTP and

Flash• Laboratory Programmer


EEPROM (E2PROM)• EEPROM means electrically erasable and

programmable read only memory, also called E2PROM

• An IC E2PROM can be erased as well as programmed

• For example, 2k x 8 chip 2816 for 2048 addresses and 8-bit data at each address


E2PROM Erasing• Erasable and programmable above 100

(above 10000 in latest E2PROMs) times by the electrical means.

• Erase of a byte is by writing by ‘1111 1111’at D0 to D7 inputs from some internal processor circuit.

• Before programming, the 0s must be erased by replacing them by 1s.


EEPROM Programming• Programming of a byte is by writing appropriate

bits (for example ‘1000 1111’ for writing 1000 1111) by the successive D0 to D7 inputs and the corresponding address inputs one by one from some external circuit (processor or laboratory programmer).

• Erase is byte by byte. In some chips, several bytes (4 or 8 or 16) can be erase simultaneously by single instruction.


EEPROM• A byte is set at D0-D7 lines for writing FFh on

application of a programming pulse or instruction. FFh at an address means, the data bits at that address are in erased state.

• After erasing, a byte as per program can be written at an address on application of a programming pulse or instruction. [D0-D7 lines are set for writing any set of data bits for the address.]


Flash• Flash means an EEPROM in which erase of whole

sector(s) is done in very short interval. A sector may be of 4k or 16k.

• An IC flash can be erased by an instruction All addresses of a sector or a set of sectors get erased concurrently.

• For example, 2M × 8 flash chip for 2 × 220

addresses and 8-bit data at each address and a sector of 216 bytes can erase by single instruction


Flash Erasing• Erase of a byte is by writing by ‘1111 1111’

at D0 to D7 inputs • Before programming a flash for the 0s, an

instruction must erase by replacing them by 1s.

• Flash can be erased and rewritten over 10000 times.


Flash Programming• Programming of a byte is by writing

appropriate bits (for example ‘1000 1111’for writing 1000 1111) by the successive D0 to D7 inputs and the corresponding address inputs one by one by a processor internal instruction or from some external circuit (called laboratory programmer).


OTP ROM• OTP means an E2PROM one time

programmability in read only memory• An IC E2PROM can be programmed only

once • An OTP is like an electronic paper onto

which writing is done by permanent ink


EPROM• EPROM means UV erasable and

programmable read only memory• An IC EPROM all bytes at all addresses get

erased on UV exposure for 10-20 minutes• For example, 2k x 8 chip 2716 is EPROM

for 2048 addresses and 8-bit data at each address


EPROM• Erase of a byte is by writing by ‘1111 1111’

at D0 to D7 on electrical charges internal accumulation at MOSFETs on UV exposure

• Before programming, all 0s must erase by replacing them by 1s.


EPROM Programming• Programming of a byte is by writing

appropriate bits (for example ‘1000 1111’for writing 1000 1111) by the successive D0 to D7 inputs and the corresponding address inputs one by one from some external circuit (called laboratory programmer).

• Erase is once for all addressable bytes.


EPROM• In erased EPROM, a byte as per program

can be written at an address on application of a programming pulse by laboratory programmer [D0-D7 lines are set for writing any set of data bits after setting the address bits.


Outline

• ROM• Example —4k × 8 (4 kB)ROM • EEPROM, EPROM, OTP and Flash• Laboratory Programmer


Laboratory programmer unit• Performs the following steps in a sequence:• 1. Applies gives the n bits (address bits) to

the decoder of the array of cells and applies as inputs the D bits, which are meant for the outputs later on during the read operation corresponding to the selected address.


Laboratory programmer steps2. Applies an high Voltage to make programming feasible and applies a very short duration (as per EPLD specification) to cause fusing (snapping) of the desired links in the array due to the high Voltage or program pulse


Laboratory programmer steps• 3. Repeat from the step 1 by applying the

next higher address than the previous one, • 4. Repetition is till all addressed are

programmed. 5.Verify the programmed bits


Summary


• ROM is a read only memory to store data bytes at the number of addresses

• ROM is preprogrammed at masking process

• ROM versions — Masked ROM, EPROM, EEPROM, Flash, OTP and PROM

• Laboratory programmer


• ROM has 2n address in case of n input line • At each address, stores FFh after erasing • At each address, stores bytes according a

program• ROM has applications in Computer and in

large number of devices


End of Lesson 2

Read Only Memory (ROM)


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