Oscillator fail detect

AN3060

12/23

Doc ID 16253 Rev 2

5

Oscillator fail detect

5.1

What is the OF bit? How does it work?

OF is the oscillator fail bit, and is set whenever the real-time clock detects approximately 4 to

6 consecutive missing pulses in the 4 KHz tap of the divider chain.

Figure 7.

Oscillator fail detect circuit

An RC circuit is used as an integrator. The 4 KHz pulses keep it discharged, and it slowly

charges between pulses. If sufficient pulses are missing, it charges high enough to trip the

comparator and set the OF bit. In the figure above, at low operating voltages, the time

constant will be shorter since the threshold voltage will be lower. Thus, fewer missing

pulses may be required to trigger the OF bit.

The basic mission of the OF function is to indicate when the oscillator has slowed or

stopped as might occur in capacitor backed applications or when the backup battery is

failing. When the backup voltage falls low enough, the part may stop keeping time, but the

registers will not necessarily become reset; the voltage may not drop low enough to

completely reset the part. Subsequently, after power-up, the oscillator will begin running

and, by merely reading the registers, it will not be clear that the timekeeping function had

become halted. The OF bit overcomes this. It provides a way for the system software to

monitor the timekeeping function and know when to advise the end user that the time may

need to be adjusted.

At first power-up, or anytime the oscillator has been turned off via the stop bit, the RC circuit

will be charged, and the OF bit will be set. The user should allow the oscillator to run for four

seconds prior to clearing the OF bit. This is done not only to discharge the RC circuit but to

ensure the oscillator has stabilized.

4KHz

SET

Q

OF

TIME CONSTANT

OSCILLATOR FAIL DETECT CIRCUIT

SET TO 4-6 PULSES

CRYSTAL

OSC

TFF

(÷ 2)

TFF

(÷ 2)

TFF

(÷ 2)

AM04725v1