C
C
O
L
ESR
R
=
1+
m
2
32KX
OC0
OC1
019-019
SWCS037G
– MAY 2008 – REVISED APRIL 2011
Table 11-4. Crystal Electrical Characteristics (continued)
Parameter
Min
Typ
Max
Unit
Crystal ESR(2)
75
k
Crystal shunt capacitance, CO
1
pF
Crystal tolerance at room temperature, 25
°C
–30
30
ppm
Crystal tolerance versus temperature range (
–40°C to 85°C)
–200
200
ppm
Maximum drive power
1
μW
Operating drive level
0.5
μW
(2)
The crystal motional resistance Rm relates to the equivalent series resistance (ESR) by the following formula:
Measured with the load capacitance specified by the crystal manufacturer. If CXIN = CXOUT = 10 pF, CL = 5 pF. Parasitic capacitance
from the package and board must also be considered.
When selecting a crystal, the system design must consider the temperature and aging characteristics of a
crystal versus the user environment and expected lifetime of the system.
Table 11-5 and
Table 11-6 list the switching characteristics of the oscillator and the timing requirements of
the 32.768-kHz input clock.
Figure 11-6 shows the crystal oscillator output in normal mode.
Table 11-5. Base Oscillator Switching Characteristics
Name
Parameter Description
Min
Typ
Max
Unit
fP
Oscillation frequency
32.768
kHz
tSX
Start-up time
0.5
s
LOJIT
<1:0> = 00
1.8
IDDA
Active current consumption
μA
LOJIT
<1:0> = 11
8
Low battery mode (1.2 V)
1
IDDQ
Current consumption
μA
Startup
8
Table 11-6. 32-kHz Crystal Input Clock Timing Requirements
Name
Parameter Description
Min
Typ
Max
Unit
OC0
1/tC(32KHZ)
Frequency, 32 kHz
32.768
kHz
OC1
tW(32KHZ)
Pulse duration, 32 kHz low or high
0.40*tC(32KHZ)
0.60*tC(32KHZ)
μs
Figure 11-6. 32-kHz Crystal Input
11.2.3.2 External Clock Description
When an external 32K clock is used instead of a crystal, three configuration can be used:
A square- or sine-wave input can be applied to the 32KXIN pin with amplitude of 1.85 or 1.8 V. The
32KXOUT pin can be driven to a dc value of the square- or sine-wave amplitude divided by 2. This
configuration, shown in
Figure 11-7, is recommended if a large load is applied on the 32KXOUT pin.
A square- or sine-wave input can be applied to the 32KXIN pin with amplitude of 1.85 or 1.8 V. The
32KXOUT pin can be left floating. This configuration, showed in
Figure 11-8, is used if no charge is
applied on the 32KXOUT pin.
The oscillator is in bypass mode and a square-wave input can be applied to the 32KXIN pin with
amplitude of 1.8 V. The 32KXOUT pin can be left floating. This configuration, shown in
Figure 11-9, is
88
Clock Specifications
Copyright
2008–2011, Texas Instruments Incorporated