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4. Low-Frequency (LF) Oscillator
The Low Frequency (LF) Oscillator System provides the 32 kHz clock to all internal circuits and to the
dedicated output pin, CLK32. The 32 kHz clock can also be output to the CLK
the Config register (R0x0E).
If enabled, the CLK32 output continues normal operations when V
available; in this case, the output voltage swings between zero and V
When power is first applied to the SH3000, the RC oscillator takes over. It supplies the 32 kHz clock for
start-up and initialization. However, if the CLK
Once the crystal has started and stabilized, the internal 32 kHz clock switches to the very accurate crystal
frequency (and the xtal oscillator stable bit, bit 7 in the Status register R0x1A, is set). See Figure 6.
X
IN
5
32768 Hz
Watch Crystal
12.5 pF Load
X
OUT
Capacitance
6
V
REG
CLK
SEL
X-
TAL
7
RC
R
REF
9
External
V
SS
Lock / Unlock
Reference
8
Resistor
V
SS
1M 1% or
variable
1
The default calibration values for the RC oscillator are loaded on power-up from the factory-programmed
OTP non-volatile memory. They can be reprogrammed at any time, or they can be permanently protected
from any changes by setting the Lock flag or an OTP write-protect flag. Factory calibration brings the
frequency of the RC oscillator within +/- 3 % of the 32768 Hz for the internal reference resistor, and 2% for
the external 1M 1 % resistor, over the entire temperature and supply voltage range.
The frequency of the RC oscillator can be tuned or modulated by varying the external reference
resistor,which should be located as close as possible to R
The crystal oscillator has the unique feature of adjustable load capacitors. It permits tuning of the circuit
for initial tolerances of the crystal (often +/- 20 ppm) as well as an adjustment for the required load
capacitance (with possible variations from the PCB layout). While the oscillator was designed for a crystal
with a nominal load capacitance of 12.5 pF, the circuit accommodates any value from 7–22 pF (depending
on parasitics of the layout). All of these corrections can be performed when the part is already installed on
the PCB in the actual circuit.
The default value for load capacitance (12.5 pF) loaded on power-up from the factory-programmed OTP
non-volatile memory can be re-programmed at any time, or it can be completely protected from any
changes by a permanent OTP write-protect flag.
SH3000UM version 0.95
Copyright ©2002 Semtech Corporation
SEL
I
Logic
4-bit Value
6-bit Value
Figure 6: Low-Frequency (LF) Oscillator
2002-08
is absent and backup power is
DD
.
BATT
pin is set high, then the crystal oscillator is enabled.
X-tal stable?
NTERNAL
R
O
REF
N
RC
Oscillator
Internal R
REF
4-bit Value
Lock Logic
, pin 9, to minimize noise pickup.
REF
SH3000 User Manual
Preliminary
pin by setting bit 4 in
OUT
CLK32 O
N
CLK32
13
Internal
32 kHz
Clock
From / To
Serial I/O
17
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