PC87570-ICCSLASHVUL datasheet(65/168 Pages) NSC

65 / 168 page

Real-Time Clock (RTC) and Advanced Power Control (APC)

www.national.com

6.2.6

Internal Oscillator

The internal oscillator employs an external crystal connect-

ed to the on-chip amplifier. The on-chip amplifier is accessi-

ble on the 32KX1 input pin and 32KX2 output pin. See

Figure 6-1 for the recommended external circuit, and Table

6-1 for a listing of the circuit components. The oscillator may

be disabled in certain conditions. See Section 6.2.15 on

page 68 for more details.

Figure 6-1. Recommended Oscillator External Circuitry

Table 6-1. Crystal Oscillator Circuit Components

External Elements

Choose C1 and C2 capacitors (see Figure 6-1) to match

the crystal’s load capacitance. The load capacitance CL

“seen” by crystal Y is comprised of C1 in series with C2 and

in parallel with the parasitic capacitance of the circuit. The

parasitic capacitance is caused by the chip package, board

layout and socket (if any), and can vary from 0 to 8 pF. The

rule of thumb in choosing these capacitors is:

CL = (C1 * C2) / (C1 + C2) + CPARASITIC

Oscillator Start-up

The oscillator starts to generate 32.768 KHz pulses to the

RTC and on-chip clock multiplier after about 100 ms from

when VBAT is higher than VBATMIN (2.4 V) or VCC is higher

than VCCMIN (3.0 V). The oscillation amplitude on the

32KX2 pin stabilizes to its final value (approximately 0.4 V

peak-to-peak around 0.7 V DC) in about 1 s.

C1 can be trimmed to achieve precisely 32.768 KHz. To

achieve a high time accuracy, use crystal and capacitors

with low tolerance and temperature coefficients.

6.2.7

External Oscillator

32.768 KHz can be applied from an external clock source,

as shown in Figure 6-2.

Figure 6-2. External Oscillator Connections

Connections

Connect the clock to the 32KX1 pin, leaving the oscillator

output, 32KX2, unconnected.

Signal Parameters

The signal levels should conform to the voltage level re-

quirements for 32KX1, stated in "DC ELECTRICAL CHAR-

ACTERISTICS" on page 134. The signal should have a

duty cycle of approximately 50%. It should be sourced from

a battery-backed source in order to oscillate during power-

down. This will assure that the RTC delivers updated

time/calendar information.

6.2.8

Timing Generation

The timing generation function divides the 32.768 KHz

clock by 215 to derive a 1 Hz signal, which serves as the in-

put for the seconds counter. This is performed by a divider

chain composed of 15 divide-by-two latches, as shown in

Figure 6-3.

Component

Parameters

Values

Toler-

ance

Crystal

Resonance

Frequency

32.768 KHz

Parallel Mode

User-

deﬁned

Type

N-Cut or XY-

bar

Serial Resistance 40 K

Ω

Max

Quality Factor, Q

35000

Min

Shunt Capacitance 2 pF

Max

Load

Capacitance, CL

9-13 pF

Temperature

Coefficient

User Choice

Resistor R1 Resistance

20 M

Ω

Resistor R2 Resistance

120 K

Ω

Capacitor C1 Capacitance

10 pF

Capacitor C2 Capacitance

33 pF

VBAT

Internal

External

32KX1

32KX2

To other

modules

Battery

0.1uF

32.768 KHz

Clock Generator

Internal

External

CLKIN

32KX2

To other

modules

Battery

VBAT

OUT

POWER

CF=0.1µF

(32KX1)

PC87570-ICC/VUL 数据表(PDF) 65 Page - National Semiconductor (TI)

PC87570-ICC/VUL 数据表(HTML) 65 Page - National Semiconductor (TI)

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