E202075_HD404889 Datasheet, PDF (163/202 Pages) Renesas Technology Corp – Low-Voltage AS Microcomputers with On-Chip LCD Circuit

HD404889/HD404899/HD404878/HD404868 Series
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
SiO₂ 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.
SiO₂
Source
N+
Control gate
Floating gate
Drain
N+
SiO₂
Source
N+
Control gate
Floating gate
Drain
N+
Write (0)
Erasure (1)
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.
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