Memory Interface

Memory Interface CEN433 King Saud University Dr. Mohammed Amer Arafah

1

Address Decoding n

n

When interfaced to a microprocessor with 20 address signals there is a mismatch.

n

The extra 9 address pins (A11-A19) are decoded using a decoder such that they select the memory device for a unique position in the memory map of the processor.

n

n

BA0-19

19

11

10

0

Memory chip A10-A0

For example, the 2716 is 2 K x 8 memory device has 11 (= 1 + 10) address inputs (A0-A10).

n

LS (11 bits)

MS (9 bits)

Memory devices interfaced are usually of smaller storage capacity than the full address space of the processor

11 Address bits

9 Selector bits

9 to 1 Decoder

FFFFFH 511 MS (9 bits)

LS (11 bits)

.. .

00FFFH

Decoding A11-A19 and using them for selecting the memory chip fixes the position of the memory locations in the mP address map

High Memory (ROM)

510

000000001 xxxxxxxxxxx

Here the mP address space is 29 times the size of the memory chip

CS/

00800H

211 = 1 2048

00000H 0

29 = 512 times

Low Memory (RAM)

mP Memory Map CEN433 - King Saud University

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Mohammed Amer Arafah

Example 1: Exhaustive Address Decoding (Simple NAND Gate Decoder)

Memory locations:FF800H-FFFFFH CEN433 - King Saud University

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Mohammed Amer Arafah

Example 2: Exhaustive Address Decoding

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Example 3: Exhaustive Address Decoding (Simple NAND Gate Decoder) n

32 K x 8 memory device: 15 bit address: 215 locations

U2A BA19

n

Selector address: 20 – 15 = 5 bits

n

If we want the memory locations to start at 10000H, What is the selector address to decode?:

n

n

n

n

Start Address 00010000000000000000 = 10000H End address 00010111111111111111 = 17FFFH Selector 5 bits: fixed)

2

74LS04 U2B BA18

3

1 2 3 4

U1

8 4

CS/

5 6 11 12 74LS30

74LS04 U2C BA17

5

6

74LS04 BA16 U2C BA15

00010 (Remain

1

5

6

74LS04

Check: These are 7FFF+1 = 8000H = 215 locations

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Mohammed Amer Arafah

Exhaustive Address Decoding (74LS138 – 74LS139)

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Example 4: Exhaustive Address Decoding (74LS138) Module 0: 1111 000X XXXX XXXX XXXX 1111 0000 0000 0000 0000 to 1111 0001 1111 1111 1111 . . .

Module 7: 1111 111X XXXX XXXX XXXX 1111 1110 0000 0000 0000 to 1111 1111 1111 1111 1111

Memory locations:F0000H-FFFFFH CEN433 - King Saud University

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Mohammed Amer Arafah

Example 5: Exhaustive Address Decoding (74LS138) 64 KB EPROM Starting at F0000H Answer: à F0000 – FFFFFH. à BA0-BA15: Location Addressing. à BA16-BA19: Space Addressing.

BA16 BA17

1 U1A 2

BA18 BA19

4 5

6

CS/

74LS20

Assume that 64 KB EPROM is not found! We can replace it with 8 of 8KB EPROM. Starting address is F0000H. Answer: à BA16-BA19: Space Addressing. à 8KB EPROM

U2

BA16 BA17

1 U1A 2

BA18 BA19

4 5

BA13 BA14 BA15 6

1 2 3 6 4 5

A B C G1 G2A G2B

15 Y0 14 Y1 13 Y2 12 Y3 11 Y4 10 Y5 9 Y6 7 Y7

CS0/ CS1/ CS2/ CS3/ CS4/ CS5/ CS6/ CS7/

74LS20

à

à

BA0-BA12: Location Addressing

74LS138

BA13-BA15: 8KB Module Addressing

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Mohammed Amer Arafah

Example 6: Exhaustive Address Decoding (74LS139) 64 KB EPROM Starting at F0000H Answer: à F0000 – FFFFFH. à BA0-BA15: Location Addressing. à BA16-BA19: Space Addressing.

à

1 U1A 2

BA18 BA19

4 5

6

CS/

74LS20

Assume that 64 KB EPROM is not found! We can replace it with 4 of 16KB EPROM. Starting address is F0000H. Answer: à BA16-BA19: Space Addressing. à 16KB EPROM à

BA16 BA17

U2A BA16 BA17

1 U1A 2

BA18 BA19

4 5

BA14 BA15 6

2 3 1

A B G

4 Y0 5 Y1 6 Y2 7 Y3

CS0/ CS1/ CS2/ CS3/

74LS139 74LS20

BA0-BA13: Location Addressing

BA14-BA15: 8KB Module Addressing

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Mohammed Amer Arafah

Exhaustive Address Decoding (PLD Decoders ) n

Many modern systems use programmable logic decoders in place of integrated decoders

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They give total freedom in decoding different addresses for individual memory devices

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Programmable logic devices have may be called: ¨ ¨ ¨ ¨ ¨ ¨

PLDs: Programmable logic devices PLAs: Programmable logic array PALs: Programmable array logic GALs: Gate Array Logic SPLDs: Simple programmable logic devices CPLDs: Complex programmable logic devices

n

They are all programmable logic devices. Nowadays they can be programmed using VHDL (Verilog Hardware Definition Language)

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Some types are programmed only once (fused links), similar to PROMs Some types are erasable like EPROMs

n n

The next slide shows one of the most common low cost devices: the PAL16L8

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PAL16LR

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PAL16LR

xx

yy

zz

xyz xyz x x x x x

F = (xyz)+(xyz)

x

y

z

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Mohammed Amer Arafah

Example 7: Exhaustive Decoding

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Example 7: Exhaustive Decoding library ieee; use ieee.std_logic_1164.all; entity DECODER_10_17 is port ( BA19, BA18, BA17: in STD_LOGIC; Input declaration ROMCS/, RAMCS/ : out STD_LOGIC; output declaration ); end; architecture V1 of DECODER_10_17 is begin ROMCS/