PCA146X datasheet(11/24 Pages) PHILIPS

11 / 24 page

1998 Apr 21

Philips Semiconductors

Product speciﬁcation

32 kHz watch circuits with adaptive motor

pulse

PCA146x series

Time calibration

Taking a normal quartz crystal with frequency 32768kHz,

frequency deviation (

∆f/f) of ±15 × 10−6 and CL = 8.2 pF;

the oscillator frequency is offset (by using non-symmetrical

internal oscillator input and output capacitances of 10 pF

and 15 pF) such that the frequency deviation is

positive-only. This positive deviation can then be

compensated for to maintain time-keeping accuracy.

Once the positive frequency deviation is measured, a

corresponding number ‘n’ (see Table 2) can be

programmed into the device’s EEPROM. This causes n

pulses of frequency 8192 Hz to be inhibited every minute

of operation, which achieves the required calibration.

The programming circuit is shown in Fig.9. The required

number n is programmed into EEPROM by varying VDD

according to the steps shown in Fig.10, which are

explained below:

1. The positive quartz frequency deviation (

∆f/f) is

measured, and the corresponding values of n are

found according to Table 2.

2. VDD is increased to 5.1 V allowing the contents of the

EEPROM to be checked from the motor pulse period

tT3 at nominal frequency.

3. VDD is decreased to 2.5 V during a motor pulse to

initialize a storing sequence.

4. The first VDD pulse to 5.1 V erases the contents of

EEPROM.

5. When the EEPROM is erased a logic 1 is at the TEST

pin.

6. VDD is increased to 5.1 V to read the data by pulsing

VDD n times to 4.5 V. After the n edge, VDD is

decreased to 2.5 V.

7. VDD is increased to 5.1 V to store n bits in the

EEPROM.

8. VDD is decreased to 2.5 V to terminate the storing

sequence and to return to operating mode.

9. VDD is increased to 5.1 V to check writing from the

motor pulse period tT3.

10. VDD is decreased to the operation voltage between

two motor pulses to return to operating mode.

(Decreasing VDD during the motor pulse would restart

the programming mode).

The time calibration can be reprogrammed up to 100

times.

Table 2

Quartz crystal frequency deviation, n and tT3

Notes

1. Increments of 2.03

× 10−6/step.

2. Increments of 122

µs/step.

FREQUENCY

DEVIATION

∆f/f

(

× 10−6)

NUMBER OF

PULSES

(n)

tT3

(ms)

0(1)

31.250(2)

+2.03

31.372

+4.06

31.494

+127.89

38.936

Fig.9 Circuit for programming the time calibration.

MSA940

VDD

RESET

VSS

TEST

OSC IN

OSC OUT

PCA146x

SERIES

32 kHz

SIGNAL GENERATOR

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