How does a flash drive/pen drive store data?

  • Computer cannot understand analog signal, analog data refers to physical data like voice, radio signal, etc.
  • We humans understand analog data but computers only understand digital data.
  • To understand simple digital data, it has only two states as 0 or 1.
  • Basically it corresponds to either ON state ie. 1 or OFF state ie. 0.
  • Computers can’t be switched off entirely and hence OFF stage is represented by a low voltage (0) and ON stage is represented by a high voltage (1).
  • Now humans have devised various techniques to interpret the data into digital form, for example one of the various forms is ASCII (American Standard Code for Information Interchange).
  • In ASCII,
    • Capital "A" is stored as seven binary numbers: 1000001 while small “a” is stored as 1100001
    • Question mark “?” is stored as 0111111
    • Number “7” as 0110111
    • left bracket “[” as 01011011
  • ASCII is a code which is agreed upon by every computer round the globe. So if a computer sends tons of binary data to other computer, the other computer divides the data into 8 bit(only valid for 256 bit ASCII) pieces and the other PC can then convert the data on which is readable by humans.
  • Effectively, a computer memory is made up of billions and billions of microscopic switches (Transistors) which can either be in ON position or OFF position.
  • Every Transistor (microscopic switch) can only store one value called a BIT.
  • 8 Bits can be combined to form a byte. When you hear something like MBs megabytes, ie. millions of bytes.
Coming back to the specific topic of Flash Storage/Pen drives
  • Ordinary transistor switches can only be turned on or off by electricity.
  • As soon as the power is switched off, the transistor switch will revert to factory manufactured original state, losing all the stored data in process, this is exactly how RAM operates.
  • Flash drives use a modified version of transistor which can remember the state even after being turned OFF, called MOSFETs.
  • Not getting too technical here, A transistor has 3 connections source, drain and gate.
  • Think of transistor as a water pipe, one end of pipe is called source whichacts like a tap allowing the water(in this case electricity) to flow, drain is the other end of the pipe which lets water(in this case electricity) to flow out.
  • In between source and drain is a pipe blocking called gate.
  • When the gate is open, electricity flows and transistor stores one (1), when the gate is closed, electricity is blocked and transistor stores zero (0).
  • If the Power goes out, the transistors closes and information is lost.
  • To avoid this we add second gate above the first one called floating gate.
  • When the transistor is turned ON, some electricity leaks to the floating gate and stays there even when the transistor is turned OFF.
  • This way information is preserved even when the transistor is turned OFF.
  • The floating gate after number of write/erase cycles wears off and takes longer to operate, memory technologies are used to avoid writing/erasing same transistor again and again.
  • The estimated life time of a pen drive/flash storage is about 10,000 to 100,000 write/erase cycles, depending on the memory technology used.

Comments

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