Semtech 2006
www.semtech.com
7-6
XE8805/05A
7.5.2
Xtal oscillator
7.5.2.1
Xtal configuration
The Xtal operates with an external crystal of 32’768 Hz.
During Xtal oscillator start-up, the first 32768 cycles are masked. The two bits EnableXtal and ColdXtal in register
RegSysClock control the oscillator.
At power-on reset or during sleep mode, EnableXtal is reset and ColdXtal is set (Xtal oscillator is not selected at
start-up). The user can start Xtal oscillator by setting EnableXtal. When the Xtal oscillator starts, bit ColdXtal is
reset after 32768 cycles. Before ColdXtal is reset by the system, the Xtal frequency precision is not guaranteed.
The Xtal oscillator can be stopped by the user by resetting bit EnableXtal.
When the user enters into sleep mode, the Xtal is stopped.
When an external clock is detected (ExtClk = 1) or the EnExtClock is set 1, the EnableXtal bit can not be set to 1.
7.5.2.2
Xtal oscillator specifications
The crystal oscillator has been designed for a crystal with the specifications given in Table 7-8. The oscillator
precision can only be guaranteed for this crystal.
Symbol
Description
Min
Typ
Max
Unit
Comments
Fs
Resonance frequency
32768
Hz
CL
for
nominal
frequency
8.2
15
pF
Rm
Motional resistance
40
100
k
Cm
Motional capacitance
1.8
2.5
3.2
fF
C0
Shunt capacitance
0.7
1.1
2.0
pF
Rmp
Motional resistance of
6
th overtone (parasitic)
4
8
k
Q
Quality factor
30k
50k
400k
-
Table 7-8. Crystal specifications.
For safe operation, low power consumption and to meet the specified precision, careful board layout is required:
Keep lines OSCIN and OSCOUT short and insert a VSS line in between them.
Connect the crystal package to VSS.
No noisy or digital lines near OSCIN or OSCOUT.
Insert guards where needed.
Respect the board specifications of Table 7-9.
Symbol
Description
Min
Typ
Max
Unit
Comments
Rh_oscin
Resistance OSCIN-VSS
10
M
Rh_oscout
Resistance OSCOUT-VSS
10
M
Rh_oscin_oscout
Resistance OSCIN-OSCOUT
50
M
Cp_oscin
Capacitance OSCIN-VSS
0.5
3.0
pF
Cp_oscout
Capacitance OSCOUT-VSS
0.5
3.0
pF
Cp_oscin_oscout
Capacitance OSCIN-OSCOUT
0.2
1.0
pF
Table 7-9. Board layout specifications.
The oscillator characteristics are given in Table 7-10. The characteristics are valid only if the crystal and board
layout meet the specifications above.
Not
Recommended
for
New
Designs