UPD6P9 datasheet(20/73 Pages) RENESAS

20 / 73 page

µPD67, 67A, 68, 68A, 69

Data Sheet U14935EJ2V1DS

3.3.1 RAM retention flag (bit 3 of P3)

The RAM retention flag indicates whether the supply voltage has fallen below the level at which the contents

of the RAM are lost while the battery is being exchanged or when the battery voltage has dropped.

This flag is at bit 3 of control register 0 (P3).

It is cleared to 0 if the supply voltage drops below the RAM retention detection voltage (approx. 1.4 V TYP.).

If this flag is 0, it can be judged that the RAM contents have been lost or that power has just been applied. This

flag can be used to initialize the RAM via software. After initializing the RAM and writing the necessary data to it,

set this RAM retention flag to 1 by software. At this time, 1 means that data has been set to the RAM.

Figure 3-2. Supply Voltage Transition and Detection Voltage

(1) If the supply voltage rises after the battery has been set, and exceeds VPOC (POC detection voltage),

reset is cleared. Because the supply voltage rises from 0 V, which is lower than VID (RAM retention

detection voltage), the RAM retention flag remains in the initial status 0.

(2) The supply voltage has now risen to the level at which the device can operate. Write the necessary data

to the RAM and set the RAM retention flag to 1.

(3) The device is reset if the supply voltage drops below VPOC. At point (A) in the above figure, the RAM

retention flag remains 1 because the supply voltage is higher than VID at this point.

(4) If the RAM retention flag is checked by software after reset has been cleared, it is 1. This means that

the contents of the RAM have not been lost. It is therefore not necessary to initialize the RAM by software.

(5) The device is reset if the supply voltage drops below VPOC. At point (B) in the figure, the voltage is lower

than VID. Consequently, the RAM retention flag is cleared to 0.

(6) If the RAM retention flag is checked by software after reset has been cleared, it is 0. This means that

the contents of the RAM may have been lost. If this case, initialize the RAM by software.

VDD

VPOC

VID

0 V

RAM retention flag

(A)

(B)

(6)

(5)

(4)

(3)

(2)

Set to 1

Flag contents

are read

Flag contents

are read

(1)

POC detection voltage

(Refer to 7. POC CIRCUIT)

VPOC = 1.85 V (TYP.)

RAM retention detection voltage

VID = 1.4 V (TYP.)

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