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Programing an IC, how does it work?


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When you “PLUG” a chip into a programmer several things happen depending on the type of chip.

Good old Eproms have a glass window (Usually round) that you can see through.  That little square inside IS the actual ‘Chip’.  The reason for the window is not for vanity or decoration.  Actually it is there to allow UV light to strike the silicon.  UV light hitting the silicon causes all the electrons in thousands of tiny junctions to change to an ON state.

If you read a freshly ‘erased’ eprom every bit will be a ‘1’.  “Programming” an eprom is the process of clearing some of the ‘1’s and making them ‘0’s.  This happens usually because of a high voltage, sometimes as high as 24 volts.

An address is applied to the address pins (10110101 00110001 for example)
And the data is applied to the Data buss (1110 0010 lets say).
Then with the Vpp (Programming voltage) applied to one of the pins, usually set aside for this purpose, the ‘WRITE’ pin is asserted and any data line that has a ‘0’ on it will open up a bit in the chip at the address given.

Think of it as ‘Blowing the fuse’ When you read back that address later all the ‘BLOWN’ fuses will read zeros and the intact fuses will read ‘1’.

The true magic is that the UV light “Heals” the junctions so that you ca clear out the array and start again.  Writing to an EPROM takes a bright UV light source.  You can clear them by leaving the window exposed and setting them in the sun but the best way is to use the proper UV lamp.  The programmer not only needs to provide certain voltages but for each chip there can be different pin outs. Also different manufacturers and styles of chips require their own timing.  Too fast (And sometimes too slow) will cause programming problems)  The final problem is that the chips are slow to write.  Even if you built a PCB with a built in UV LED and power supply to enable programming on board, the WRITE speed is amazingly slow.

That’s where EEPROMS come in.  Notice the extra “e” ?  it stands for Electrically Erasable Programmable Read Only Memory.
These chips do not need a UV lamp.  BY applying the proper programming power these chips can ‘write over’ the existing data without having to clear the whole chip first. 
Some EEPROMS can even run on the standard 5 volt power supply.  The only drawbacks to the EEPROM are a lack of write speed, and the limited write cycles.

You can usually only write to an EEPROM about 10,000 to 1million times before they start to wear out.  Also the write speed is only slightly faster than the old EPROMS.

The limit to the number of write cycles and speed has made RAM a critical element.  Random Access Memory is very fast and it allows you to read and write very rapidly.  The down side is that when you loose power the memory goes away also.  There were ‘Bubble memories’ and ‘Static’ memories that tried to get around this by using a tiny amount of power to maintain their data when the main power was shut off but they never lived up to the speed of ram and the longevity of rom.

The newest version is called FLASH memory.  A similar technology by RAMTRON called FRAM memory works about the same.

These are very fast RAM style chips that can maintain their data for 10 or more years without any power.  They have a limited write cycle but it is in the ‘billions’ of cycles range and they can run on a single power supply.

There are 2 other classes of chips that are “Programmed” they are PGAs and Microcontrollers.

PGAs are (Programmable Gate Arrays)  These are chips that have a series of logic gates in them connected to a memory array.  Using special software you can design a complex logic gate that would need dozens of discrete parts to assemble, on one small part.
The software then creates a series of on/off signals to program into the memory much like data is stored in a Eprom.  The difference is that you can’t read and write the memory in these devices.  Instead the memory is used to set up the logic gates and ‘program’ the chip.

Microcontrollers are actually very small computers on a chip.  They have a processor, a ROM to store programs and a RAM for working data.  The program that the computer runs is stored in the ROM. Similar to the PGA, special software translates the program that is written into a series of binary data.  Once the program is compiled it is written to the EPROM on the microcontroller.  Again this data is usually not directly accessible under normal use.  The EPROM data on the microcontroller just holds the actual program for the chip.


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