Click HERE to download as PDF
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.