PD17012_15 Datasheet, PDF (291/320 Pages) Renesas Technology Corp – 4-BIT SINGLE-CHIP MICROCONTROLLERS WITH DIGITAL TUNING SYSTEM HARDWARE

µPD17012, 17P012
22.6.4 Notes on detecting power failure by RAM judgement method
The value of the data memory on application of supply voltage VDD is basically undefined, and therefore, the
following points (1) and (2) must be noted.
(1) Data to be compared
Where the number of bits of the data memory to be compared by the RAM judgement method is “n bits”,
the probability at which the value of the data memory matches the value to be compared on application
of VDD is (1/2)ⁿ.
This means that backup is judged at a probability of (1/2)ⁿ when a power failure is detected by the RAM
judgement method.
To lower this probability, as many bits as possible must be compared.
Because the contents of the data memory on application of VDD are likely to be the same value such as
“0000B” and “1111B”, it is recommended to mix “0” and “1” as data to be compared, such a “1010B” and
“0110B” to reduce the possibility of a wrong judgment.
(2) Notes on program
If VDD rises from the level at which the data memory contents may be destroyed as shown in Figure 22-
12, and even if the value of the data memory area to be compared is normal, the values of the other data
memory areas may be destroyed.
This is judged as backup if a power failure is detected by the RAM judgement method. Therefore,
consideration must be given so that the program does not hang up even if the contents of the data memory
are destroyed.
Figure 22-12. VDD and Destruction of Data Memory Contents
5V
VDD
0V
Data memory destruction start voltage
Data memory
Data memory for RAM judgement (normal)
Values of data memory areas not used for RAM judgement may be destroyed.
Data Sheet U10101EJ4V0DS
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