UPD17708 datasheet(331/356 Pages) NEC

331 / 356 page

331

µPD17704, 17705, 17707, 17708, 17709

Data memory destruction start voltage

5 V

0 V

VDD

Values of data memory addresses not used for RAM judgment may be destroyed.

Data memory for RAM judgment (normal)

Data memory

21.7.4 Cautions on power failure detection by RAM judgment method

Because the values of the data memory on application of supply voltage VDD are basically “undefined”, 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 judgment method is “n bits”, the

probability that the value of the data memory happens to coincide the value to be compared on application

of VDD is (1/2)n.

In other words, a power failure detected by the RAM judgment method may be judged as backup at a probability

of (1/2)n.

To minimize this probability, compare as many bits as possible.

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 that the data to be compared consist of a combination of “0”s and “1”s, such

as “1010B” and “0110B”.

(2) Cautions on program

If VDD rises from a level at which the contents of the data memory are destroyed as shown in Figure 21-17,

even if the value of the data memory to be compared is normal, the other parts of the data memory may be

destroyed.

If a power failure detection is performed by the RAM judgment method at this time, it is judged to be a backup.

Therefore, the program must be designed so that a hang-up does not occur even if the contents of the data

memory are destroyed.

Figure 21-17. VDD and Destruction of Data Memory Contents

(3) Cautions on using RESET pin

Caution If the device is reset by the RESET pin during program execution, the data in the data memory

may be corrupted.

Therefore, be careful when resetting with the RESET pin.

UPD17708 Datasheet(PDF) 331 Page - NEC