PC87570 Datasheet, PDF (65/168 Pages) National Semiconductor (TI) – PC87570 Keyboard and Power Management Controller

Real-Time Clock (RTC) and Advanced Power Control (APC)
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.
V_BAT
32KX1
C_F
0.1uF
To other
modules
Internal
32KX2 ^External
R_{1 R2}
B₁
C₁ Y
C₂
Battery
Figure 6-1. Recommended Oscillator External Circuitry
Table 6-1. Crystal Oscillator Circuit Components
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 V_BAT is higher than V_BATMIN (2.4 V) or V_CC is higher
than V_CCMIN (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.
V_BAT
C_F
CLKIN
(32KX1)
To other
modules
Internal
External
32KX2
NC
OUT
32.768 KHz
POWER Clock Generator
Component Parameters
Values
Toler-
ance
Crystal
Resonance
Frequency
32.768 KHz User-
Parallel Mode defined
Type
N-Cut or XY-
bar
Serial Resistance 40 KΩ
Max
Quality Factor, Q 35000
Min
Shunt Capacitance 2 pF
Max
Load
9-13 pF
Capacitance, C_L
Temperature
Coefficient
User Choice
Resistor R₁ Resistance
20 MΩ
5%
Resistor R₂ Resistance
120 KΩ
5%
Capacitor C₁ Capacitance
10 pF
5%
Capacitor C₂ Capacitance
33 pF
5%
B₁
Battery
C_F C_F=0.1µF
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.
External Elements
Choose C1 and C2 capacitors (see Figure 6-1) to match
the crystal’s load capacitance. The load capacitance C_L
“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:
C_L = (C₁ * C₂) / (C₁ + C₂) + C_PARASITIC
www.national.com
65