The Typical Embedded System Memory

1. Introduction to
EMBEDDED SYSTEM
(2nd Edition)
SHIBU K V
Dr Moe Moe Myint
Department of Computer Engineering & Information Technology
Mandalay Technological University
www.slideshare.net/MoeMoeMyint
moemoemyint@moemyanmar.ml
moe2myint.mdy@gmail.com
drmoemoemyint.blogspot.com

2. Agenda
2.1 Core of the Embedded System 17
2.2 Memory 28
2.3 Sensors and Actuators 35
2.4 Communication Interface 45
2.5 Embedded Firmware 59
2.6 Other System Components 60
2.7 PCB and Passive Components 64
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Department of Computer Engineering and Information Technology

3. Learning Objectives
 Learn the building blocks of a typical Embedded System
 Learn about General Purpose Processors (GPPs), Application
Specific Instruction Set Processors (ASIPs), Microprocessors,
Microcontrollers, Digital Signal Processors, RISC & CISC
processors, Harvad and Von-Neumann Processor Architecture, Big-
endian v/s Little endian processors, Load Store operation and
Instruction pipelining
 Learn about different PLDs like Complex Programmable Logic
Devices (CPLDs), Field Programmable Gate Arrays (FPGAs), etc.
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4. Cont’d
 Learn about the different memory technologies and memory types used in
embedded system development
 Learn about Masked ROM (MROM), PROM, OTP, EPROM, EEPROM, and
FLASH memory for embedded firmware storage
 Learn about Serial Access Memory (SAM), Static Random Access Memory
(SRAM), Dynamic Random Access Memory (DRAM) and Nonvolatile
SRAM (NVRAM)
 Understand the different factors to be considered in the selection of memory
for embedded systems
 Understand the role of sensors, actuators and their interfacing with the I/O
subsystems of an embedded system
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5. Cont’d
 Learn about the interfacing of LEDs, 7-segment LED Displays, Piezo Buzzer,
Stepper Motor, Relays, Optocouplers, Matrix keyboard, Push button switches,
Programmable Peripheral Interface Device (e.g. 8255 PPI), etc. with the I/O
subsystem of the embedded system
 Learn about the different communication interfaces of an embedded system
 Understand the various chip level communication interfaces like I2C, SPI,
UART, 1-wire, parallel bus, etc
 Understand the different wired and wireless external communication interfaces
like RS-232C, RS-485, Parallel Port, USB, IEEE1394, Infrared (IrDA),
Bluetooth, Wifi, ZigBee, GPRS, etc.
 Know what embedded firmware is and its role in embedded systems
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6. Cont’d
 Understand the different system components like Reset Circuit,
Brown-out protection circuit, Oscillator Unit, Real-Time Clock
(RTC) and Watchdog Timer unit
 Understand the role of PCB in embedded systems
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7. 2.2 Memory
 Memory is an important part of a processor/controller based embedded
systems.
 Some of the processors/controllers contain built in memory and this
memory is referred as on-chip memory.
 Others do not contain any memory inside the chip and requires external
memory to be connected with the controller/processor to store the
control algorithm. It is called off-chip memory.
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8. Cont’d
 There are different types of memory used in embedded system applications:
i. Program Storage Memory (ROM)
 Masked ROM (MROM)
 Programmable Read Only Memory (PROM)/ (OTP)
 Erasable Programmable Read Only Memory (EPROM)
 Electrically Erasable Programmable Read Only Memory (EEPROM)
 FLASH
ii. Read-Write Memory/Random Access Memory (RAM)
 Static RAM (SRAM)
 Dynamic RAM (DRAM)
 NVRAM
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9. Program Storage Memory (ROM)
 The program memory or code storage
memory of an embedded system stores
the program instructions and it can be
classified into different types as per the
block diagram representation given in
Figure.
 The code memory retains its contents
even after the power to it is turned
off. It is generally known as non-
volatile storage memory.
 Depending on the fabrication, erasing
and programming techniques they are
classified into the following types.
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Figure. Classification of Program Memory (ROM)
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Non Volatile
Memory
FLASH

10. Classification of ROM
 Mask ROM : Masked ROM is a static ROM which comes programmed into an integrated
circuit by its manufacturer. Masked ROM makes use of the hardwired technology for
storing data. It is a good candidate for storing the embedded firmware for low cost
embedded devices. The primary advantage of this is low cost for high volume production.
The limitation with MROM based firmware storage is the inability to modify the device
firmware against firmware upgrades. They are used in network operating systems, server
operating systems, storing of fonts for laser printers, sound data in electronic musical
instruments.
 PROM/ OTP : Unlike MROM, One Time Programmable Memory (OTP) or PROM is not
pre-programmed by the manufacturer. The end user is responsible for programming
these devices. They have several different applications, including cell phones, video game
consoles, RFID tags, medical devices, and other electronics.
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11. Cont’d
 EPROM : EPROM gives the flexibility to re-program the same chip. EPROM stores the bit
information by charging the floating gate of an FET and contains a quartz crystal window for
erasing the stored information. Even though the EPROM chip is flexible in terms of re-
programmability, it needs to be taken out of the circuit board and put in a UV eraser device for 20 to
30 minutes. So it is a tedious and time-consuming process.
 EEPROM : The information contained in the EEPROM memory can be altered by using electrical
signal at the register/Byte level. They can be erased and reprogrammed in-circuit. These chips
include a chip erase mode and in this mode they can be erased in a few milliseconds. It provides
greater flexibility for system design. The only limitation is their capacity is limited when
compared with the standard ROM (a few kilobytes). It is used for storing the computer system
BIOS.
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12. How MOS-FET Transistor works?12
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13.  FLASH : It is an enhanced version of EEPROM. It combines the re-programmability of
EEPROM and the high capacity of standard ROMs. FLASH memory is organized as sectors
(blocks) or pages. FLASH memory stores information in an array of floating gate MOS-FET
transistors. The erasing of memory can be done at sector level or page level without affecting
the other sectors or pages. Each sector/ page should be erased before re-programming. The
typical erasable capacity of FLASH is 1000 cycles. Many modern PCs have their BIOS
stored on a flash memory chip, called as flash BIOS and they are also used in memory
cards, USB flash drives, modems as well.
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Cont’d

14. Read-Write Memory/Random Access Memory (RAM)
 The Random Access Memory
(RAM) is the data memory or
working memory of the
controller/processor.
 Controller/processor can read from
it and write to it.
 RAM is volatile, meaning when the
power is turned off, all the contents
are destroyed.
 RAM generally falls into three
categories: Static RAM (SRAM),
dynamic RAM (DRAM) and non-
volatile RAM (NVRAM).
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Figure. Classification of Working Memory (RAM)
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15. Static RAM (SRAM)
 SRAM : SRAM stores data in the
form of voltage. They are made up
of flip-flops. A flip-flop for a
memory cell takes four or six
transistors (or 6 MOSFETs) along
with some wiring, four of the
transistors are used for building the
latch (flip-flop) part of the memory
cell and two for controlling the
access. SRAM is fast in operation
due to its resistive networking and
switching capabilities. In its simplest
representation an SRAM cell can be
visualized as shown in Figure.
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Figure. SRAM Cell Implementation
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16. Cont’d
 This implementation in its simpler form can be visualized as two-cross coupled inverters
with read/write control through transistors. The four transistors in the middle form the
cross-coupled inverters. This can be visualized as shown in Figure.
 The major limitations of SRAM are low capacity and high cost.
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Figure. Visualization of SRAM cell

17. Dynamic RAM (DRAM)
 DRAM : DRAM stores data in the form of charge. They are made up
of MOS transistor gates.
 The advantages of DRAM are its high density and low cost compared
to SRAM.
 The disadvantage is that since the information is stored as charge it
gets leaked off with time and to prevent this they need to be
refreshed periodically.
 Special circuits called DRAM controllers are used for the refreshing
operation.
 The refresh operation is done periodically in milliseconds interval.
 The MOSFET acts as the gate for the incoming and outgoing data
whereas the capacitor acts as the bit storage unit.
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Figure. DRAM Cell Implementation
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18. Summary of the relative merits and demerits of
SRAM and DRAM technology
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SRAM Cell DRAM Cell
Made up of 6 CMOS transistors (MOSFET) Made up of a MOSFET and a capacitor
Doesn’t require refreshing Requires refreshing
Low capacity (Less dense) High capacity (Highly dense)
More expensive Less expensive
Fast in operation.
Typical access time is 10 ns.
Slow in operation due to refresh
requirements.
Typical access time is 60 ns. Write operation
is faster than read operation.
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Department of Computer Engineering and Information Technology

19. NVRAM
 NVRAM: Non-volatile RAM is a random access memory with battery backup.
 It contains static RAM based memory and a minute battery for providing supply to the
memory in the absence of external power supply.
 The memory and battery are packed together in a single package.
 NVRAM is used for the non-volatile storage of results of operations.
 The life span of NVRAM is expected to be around 10 years.
 DS1744 from Maxim/Dallas is an example for 32 KB NVRAM.
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20. Understanding Test Questions III
1. Which of the following is one-time programmable memory?
(a) SRAM (b) PROM (c) FLASH (d) NVRAM
2. Which of the following memory type is best suited for development purpose?
(a) EEPROM (b) FLASH (c) UVEPROM (d) OTP
(e) (a) or (b)
3. EEPROM memory is alterable at byte level. State True or False
(a) True (b) False
4. Non-volatile RAM is a Random Access Memory with battery backup. State True or
False
(a) True (b) False
5. Execution of program from ROM is faster than the execution from RAM. Sate True
or False
(a) True (b) False
6. Dynamic RAM stores data in the form of voltage. State True or False
(a) True (b) False
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21. Reviewed Questions III
1. What is the difference between RAM and ROM?
2. What are the different types of RAM used for Embedded System design?
3. What are the different types of memories used for Program storage in
Embedded System Design?
4. What are the advantages of FLASH over other program storage in
Embedded System Design?
5. Explain the operation of Static RAM (SRAM) cell.
6. Explain the merits and limitations of SRAM and DRAM as Random
Access Memory.
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22. Assignment III
Write a “C” program to find the endianness of the
processor in which the program is running. If the
processor is big endian, print “The processor architecture
is Big endian”, else print “The processor architecture is
Little endian” on the console window.
Remark: for all groups
Deadline : 2. 1. 18 (Coming Tuesday)
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23.  Only Original Owner has full rights reserved for copied images.
 This PPT is only for fair academic use.
 Coming soon for chapter 2 (2nd portion)
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