ECE 353
Introduction to
Microprocessor Systems
Week 9
Michael G. Morrow, P.E.
Topics
Memory technologies
Organization and operation of typical
SRAM, EPROM and flash memory devices
Memory subsystem design
Address decoder implementation
SRAM timing characteristics
Memory Terminology
How could we classify memory devices?
Read-Only Memory (ROM)

In common usage, ROM is memory that is
nonvolatile.
Random-Access Memory (RAM)


The time required to access any memory
location is the same – i.e. it does not need
to be accessed in a specific order.
In common usage, RAM is memory that can
be read or written with equal ease.
Memory Technologies
ROM (non-volatile)


Masked ROM
Field programmable
 EPROM
 OTP PROM (fuse or EPROM)

Electrically erasable
 EEPROM (or E2PROM)
 Flash memory
RAM (volatile)



SRAM
DRAM
Pseudo-SRAM
Emerging memory technologies
Memory Organization
Logical organization


Organization as seen looking at the device
from the outside
Linear array of registers (memory locations)
 Width – number of bits in each memory location
 Depth – number of memory locations
 Usually written as depth x width (i.e. 32k x 8)
Physical organization


Different physical organizations can be used
to implement the same logical organization
Physical organization affects performance
and cost
SRAM Interfaces
RAM with 3 control inputs




/CS, /OE, /WE
Read
Write
aka “Intel style”
/CS
/WE
/OE
internal w rite signal
internal read signal
RAM with 2 control inputs


E (CS), R/W (or /WE) E
R/W
aka “Motorola style”
internal w rite signal
internal read signal
SRAM Organization
Logical Organization

Typically 1, 4 , 8 or 16 bit widths
Physical Organization



Rectangular bit array
Two-level decoding (row and column)
Characteristic delays and timing requirements are
specified in memory devices datasheet (Example)
NV-SRAM


Uses an alternate power source to maintain SRAM
when system power is off
Requires logic to switch power sources and prevent
spurious writes during power-up/power-down
EPROM
Electrically programmable, non-volatile
Requires UV light to erase

Quartz window in package
Floating polysilicon gate avalanche injection
MOS transistor (FAMOS)

Operation
Programmer loads device out-of-circuit
OTP EPROMs eliminate quartz window
EEPROMs are electrically erasable


Byte-erasable / writeable
Low-density
JEDEC Packages
Flash Memory
Actually Flash EEPROM, commonly just
called flash memory
Characteristics



Technologies
Endurance
Blocking, programming and erasing
Applications



ROM replacement
GP NV-RAM
Solid-state disk (flash-disk) Example
Memory Subsystem Design
Memory banks



Increasing memory width
Increasing memory depth
Increasing memory width and depth
Address decoding


Exhaustive (full) vs. partial (reduced) decoding
Boundaries
n
 If address is a 2 boundary, then what is the result
of (address AND (2n-1))?
 We normally decode memory devices to be aligned
on boundaries at least as large as they are
Memory Architectures
Wide (n-byte) buses


Addressing effects
Byte transfer support
 Data lanes
 Control signals
Bus resizing



Static
Configurable
Dynamic
Memory Subsystems Review
What is the purpose of an address
decoder circuit, and where does its
output usually get connected?
What is exhaustive decoding, and what
effects does it have?
What is partial decoding, and what
effects does it have?
SRAM Timing Characteristics
An SRAM device has key timing parameters
specified for the read cycle.

tAA – address access time
 tRDHA – data valid after address changes

tACS – chip select access time
 tRHCS – data valid after chip select
 tCHZ – time until device floats bus

tOE – output enable access time
 tOHZ – time until device floats bus

tRC – read cycle time
The write cycle has a complementary set of
specifications.
7C1046
SRAM
27C512
EPROM
HM624100HC
SRAM
SRAM Timing Compatibility
In order to ensure that we will be able to
reliably read and write the memory
device, we need to ensure that the
processor system bus interface is
compatible with the memory device.
This is accomplished by analyzing the
timing for all relevant parameters of both
the processor and memory, and ensuring
that the operations can be completed
reliably.
Wrapping Up
Quiz #2 will be held on Thursday 3/29 at
7:15-8:30pm in 2255EH




Covers educational objectives for modules 3
and 4 (weeks 5 through 8)
Single 3x5 card with original handwritten
notes
No calculators
Instruction set references and any needed
datasheets will be provided
Reading for next week

Supplement #3, review chapter 9 in text
JEDEC
Standard
Packages
http://www.jedec.org/download/search/3_07_05R12.pdf
Flash Blocks
Flash Memory Application:
Disk-on-Key
Up to 64GB nonvolatile storage

And climbing
No battery or power supply
Specifications:
Data retention up to 10 years
Erase cycles: 1,000,000 times
Shock resistance: 1000 G (maximum)
RAM Read – 3 control signals
Ax
Dx
/CS
/WE
/OE
RAM Write – 3 control signals
Ax
Dx
/CS
/WE
/OE
Increasing Memory Depth
A15
A0
A14
D7
D7
D0
D0
A0
CE
A14
D7
D0
CE
A0
CE
Extending Depth
Increasing Memory Width
A15
A0
A15
D3
D7
D0
D0
A0
CE
A15
D3
D0
CE
A0
CE
Extending Width
Increasing Memory Depth & Width
A14
A15
D3
D0
D7
A0
D0
CE
A0
A14
D3
D0
A0
CE
A14
D3
D0
A0
CE
CE
A14
D3
D0
A0
CE
Extending Width and Depth