pc87591l-n05 National Semiconductor Corporation, pc87591l-n05 Datasheet - Page 319

• Manufacturer Part Number: pc87591l-n05
• Description: Lpc Mobile Embedded Controller
• Manufacturer: National Semiconductor Corporation

Revision 1.2
Host-Controlled Modules and Host Interface
External Elements
Choose C1 and C2 capacitors (see Figure 105) to match the crystal’s load capacitance. The load capacitance C
crystal Y is comprised of C
pacitance is caused by the chip package, board layout and socket (if any), and can vary from 0 to 8 pF. The rule of thumb
for choosing these capacitors is:
To provide accurate 32.768 KHz frequency, the value of C
Table 55), according to the specific board implementation. See the PC87591x RTC Oscillator Circuit Design and Layout
Guidelines Application Note for details on how to select the capacitors value and how to lay out the PCB.
Oscillator Start-Up
The oscillator starts to generate 32.768 KHz pulses to the RTC after approximately t
V
C
tolerance and temperature coefficients.
6.2.4
32.768 KHz can be applied from an external clock source, as shown in Figure 106.
BATMIN
1
can be trimmed to achieve precisely 32.768 KHz. To achieve a high time accuracy, use crystal and capacitors with low
C
L
= (C
External Oscillator
(2.4 V) or from when V
1
* C
Crystal
Resistor R
Resistor R
Capacitor C
Capacitor C
Component
2
) / (C
1
+ C
1
2
1
2
1
2
Resonance Frequency
Type
Serial Resistance
Quality Factor, Q
Shunt Capacitance
Load Capacitance, C
Temperature Coefficient
Resistance
Resistance
Capacitance
Capacitance
) + C
in series with C
CC
Battery
PARASITIC
B1
Parameters
is higher than V
Table 55. Crystal Oscillator Circuit Components
Figure 106. External Oscillator Connections
V
BAT
CF
2
and in parallel with the parasitic capacitance of the circuit. The parasitic ca-
CF
L
OUT
POWER
CCMIN
32KCLKIN /
32KX1
32.768 KHz Parallel Mode
(3.0V).
319
N-Cut or XY-bar
1
User defined
Clock Generator
and C
9-13 pF
121 K
5-20 pF
2-15 pF
Values
(Continued)
20 M
40 K
35000
32.768 KHz
2 pF
NC
2
32KX2
capacitors may vary (within the range defined in
To other
modules
CF = 0.1 F
External
Internal
32KW
User defined
Tolerance
from when V
10%
10%
Max
Max
Min
5%
5%
BAT
is higher than
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“seen” by