Memory INTERFACING

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Memory INTERFACING

• NOTE
• Your text book covers memory interfacing in
  Chapter 10.
• That chapter is the core of these slides,
• However, these slides and the lectures contain
  some extra useful stuff.
• So, BE FULLY alert in the class.

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What, Why and How? ?

• What is memory?
• You now it very well !!
• Why memory?
• Whether simple or complex, every
  microprocessor-based system has a memory system.
  Basically, two main types are required ROM that
  contains system software and permanent system
  data, and RAM that contains temporary data and
  application software.
• How We will talk a lot about HOW !! ?

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Link to the previous material

• The link is straightforward
• All what we were doing is to Prepare the
  processor to be able generate the address, data
  and control Busses.
• Now, how to connect these to the memory,
• Looks simple, isnt it? !!

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Prior to interfacing

• Before attempting to interface memory, it is
  essential to understand the operation of memory
  components. And memory types
• ROM
• PROM
• EEPROM
• SRAM
• DRAM

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What differentiate between memory types

• Memory characteristics
• Access time.
• Volatile or Nonvolatile.
• Modifiable.
• space (density).
• current drain (power consumption).

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Memory types1- ROM

• Mask- programmable ROM
• n address lines, 2n memory locations, m data
  lines.
• programmed by placing diodes in the proper
  places.
• programmed once at the factory according to the
  user truth table.
• Should be mass production.

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ROM

• Mask- programmable ROM
• n address lines, 2n memory locations, m data
  lines.
• programmed by placing diodes in the proper
  places.
• programmed once at the factory according to the
  user truth table.
• Should be mass production.
• Example in the next slide

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Field-Programmable ROMs

• One time programmable (OTP) ROM
• UV- light Erasable PROM (EPROM)
• Electrically Erased PROM E2PROM
• Flash Memory

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One time programmable (OTP) ROM

• It is very similar to the previous one.
• Fusible link in series with diode in every bit.
• Selected fuses can be melted (blown) by the user
  giving logic 1.

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UV-light Erasable PROM (UV-EPROM)

• Can be erased-reprogrammed many times.
• Should be erased entirely before reprogramming.
• Quartz window is placed on the top of the ceramic
  package containing the EPROM( why).
• Erased by subjecting it for 15 min. to UV light
  of 2537 angstroms wavelength produced by
  commercial erasers.
• These erasers can erase many EPROMS in the same
  time.
• According to Intel, fluorescent (three years) or
  direct sunlight of one weak could erase it.

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Programming (writing to) the EPROM

• Programmers send the address for location to be
  programmed, its data, and pulse the PGM pin low
  for 100 ms.

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Comparison

• EPROM
• Should be removed from circuit board to be erased
  
• long erasing time.
• Byte eraser is not possible.
• The quartz window is expensive.
• E2PROM
• Can be erased in its place.
• Both byte and bulk eraser modes are possible.
• It has a lifetime 10,000 to 1000,000 read/write
  cycles.

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Flash Memory

• as an EEPROM, it can be programmed and erased in
  its place
• Simple cell compared to the traditional EEPROM.
  (then?)
• - Lowering cost
• - Improving reliability
• -Very dense memory part (then?)
• - small size
• Where is it Used ?
• Network cards, Laser printers , BIOS,

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Flash disk example

• Erase Cycles1,000,000 times
• Data retention time 10 years
• Read transfergt 750KB/second
• Write transfergt 450KB/second

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RAM

• read write volatile memory
• Workspace for the Microprocessor
• Temporary storage keeps the operating system
  and the running applications, data and programs,
  while the power is on.
• When a file is loaded into memory, a copy of the
  file is loaded from the hard disk.

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Basic RAM Types

• DRAM Dynamic RAM
• INEXPENSIVE
• MUST BE CONSTANTLY REFRESHED (recharged)
• SLOW
• Dense (high capacity) (hundreds of Mbytes)
• SRAM Static RAM
• CACHE
• FAST
• EXPENSIVE
• Capacity is much less than DRAM (few Kbytes)

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SRAM vs. DRAM Cells (for you information)
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Memory pins

• These are the basic pins.
• There are some extra pins like
• clk related pins (for synchronous).
• RAS (Raw Address Select) and CAS (Column Address
  Select) in DRAM.
• PGM for programming programmable chips.

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Examples
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Memory size and its relation to address and data
pins.

• Address pins can tell the number of memory
  locations and visa versa.
• If the address pins are A0, A1,..An which makes
  the number of memory locations 2n1
• You should be able to relate the size, 1st
  address and last address wile written in HEX.

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Continue
Section 10.1 in the text book.
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Dram Size is an exception

• number of the locations
• 2x ? 2y
• x and y are the number of row address bits
  and column address bits respectively.

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Still about memory size

• What about DATA pins? Do they affect the memory
  size?
• Certainly, because the number of data pins
  reflects how many bit in that location.
• One small note here some memories has one output
  pin and another input pin for the same data bit.
  Other memories has one I/O pin for each bit.

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Still on the memory size

• A 4K x 8 memory contains 4,096 (4K) memory
  locations that each contain 8-bits.
• It is 32 Kbits or 4 KB
• A 16M x 4 memory has 16M memory locations that
  are each 4-bits wide.
• It is 64 Mbits or 8 MB