Programming Bipolar PROM

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Programming Bipolar PROM

ROM stands for Programmable Read Only Memory. Unlike ordinary ROMs, which are programmed at the manufacturer’s site, PROMs are programmed by the users.So, user can load a program into the PROM according to their own specifications. This is an advantage of PROM over ROM. They are also known as Write Once Memory (WOM). PROM is made by connecting the address lines to the data lines by a decoder with connections between decoder o/p lines and data lines made by bipolar transistors with fuses at their emitters. To program the PROM appropriate fuse links are flown by sending a high current pulse through the fuses. This process is illustrated below using 2 address lines and 4 data line with a decoder connecting them.

Initially the PROM from the manufacturer contains bipolar transistor fuse links between decoder lines are all intact as shown in the following schematic diagram.



In such a case, the output of all the data lines is logic 1 irrespective of the address on the address line. At the user site user can selectively flow the fuses by sending a high current pulse through them leaving logic 0s at selected data lines for specific addresses.  For an example, for the following diagram shown with flown fuses the data read for all the possible addresses are given in the table below.


ADDRESS
DATA
A0
A1
D3
D2
D1
D0
0
0
0
1
0
0
0
1
0
1
0
1
1
0
1
0
0
1
1
1
0
0
1
0

The programming is done through a programmer called PROM burner. Three technologies exist for making fusible links at the emitters of bipolar transistors. They are nichrome metal links, poly silicon links and pn junction links.