HD404889/HD404899/HD404878/HD404868 Series
161
Notes on PROM Programming
Principles of Programming/Erasure:
A memory cell in a ZTAT
microcomputer is the same as an
EPROM cell; it is programmed by applying a high voltage between its control gate and drain to inject hot
electrons into its floating gate. These electrons are stable, surrounded by an energy barrier formed by an
SiO
2
film. The change in threshold voltage of a memory cell with a charged floating gate makes the
corresponding bit appear as 0; a cell whose floating gate is not charged appears as a 1 bit (figure 98).
The charge in a memory cell may decrease with time. This decrease is usually due to one of the following
causes:
Ultraviolet light excites electrons, allowing them to escape. This effect is the basis of the erasure
principle.
Heat excites trapped electrons, allowing them to escape.
High voltages between the control gate and drain may erase electrons.
If the oxide film covering a floating gate is defective, the electron erasure rate will be greater. However,
electron erasure does not often occur because defective devices are detected and removed at the testing
stage.
Control gate
Floating gate
Drain
SiO
2
Source
N
N
+
+
Control gate
Floating gate
Drain
SiO
2
Source
N
N
+
+
Erasure (1)
Write (0)
Figure 98 Cross-Sections of a PROM Cell
PROM Programming:
PROM memory cells must be programmed under specific voltage and timing
conditions. The higher the programming voltage V
PP
and the longer the programming pulse t
PW
is applied,
the more electrons are injected into the floating gates. However, if V
PP
exceeds specifications, the pn
junctions may be permanently damaged. Pay particular attention to overshooting in the PROM
programmer. In addition, note that negative voltage noise will produce a parasitic transistor effect that may
reduce breakdown voltages.
The ZTAT
microcomputer is electrically connected to the PROM programmer by a socket adapter.
Therefore, note the following points:
Check that the socket adapter is firmly mounted on the PROM programmer.
Do not touch the socket adapter or the LSI during the programming. Touching them may affect the
quality of the contacts, which will cause programming errors.