PCF2123TS datasheet(11/55 Pages) NXP

11 / 55 page

PCF2123_2

Product data sheet

Rev. 02 — 4 December 2009

11 of 55

NXP Semiconductors

PCF2123

SPI Real time clock/calendar

The oscillator may be stopped, for example, by grounding one of the oscillator pins, OSCI

or OSCO. The oscillator is also considered to be stopped during the time between

power-on and stable crystal resonance. This time may be in the range of 200 ms to 2 s

depending on crystal type, temperature and supply voltage. At power-on the OS ﬂag is

always set.

9.5 Reset

A reset is automatically generated at power-on. A reset can also be initiated with the

software reset command. It is generally recommended to make a software reset after

power-on.

A software reset can be initiated by setting the bits 6, 4 and 3 in register Control_1 logic 1

and all other bits logic 0 by sending the bit sequence 01011000 (58h), see Figure 10. If

this bit sequence is not correct, the software reset instruction will be ignored to protect the

device from accidently being reset. When sending the software instruction, the other bits

are not written.

Fig 8.

OS set by failing VDD

Fig 9.

OS ﬂag

001aai561

VDD

VOSC(MIN)

battery operation

main supply

001aai553

OS = 1 and flag can not be cleared

OS = 1 and flag can be cleared

OS flag cleared

by software

OS flag set when

oscillation stops

oscillation now stable

VDD

oscillation

OS flag

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