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To reprogram the load capacitance, clear the XTALtune Rewrite-Once Enable bit (bit 7) of the Config
register (R0x0E) and then immediately set bits 7:4 of the register Xtune_R
desired value; Table 5 shows the available values.
The adjustment can set the frequency of the crystal oscillator to within +/- 4 ppm of the ideal value. As a
reference, a typical 32.768 kHz crystal changes its frequency 4 ppm for a 10°C change in temperature.
Since the temperature characteristics of crystals are well known and stable, the host microcontroller is
free to implement an algorithm for temperature compensation of the crystal oscillator using the adjustable
load capacitors, with the resulting accuracy of +/- 4 ppm over the entire temperature range.
4.1 Low-Frequency (LF) Oscillator Registers
NAME
0x0E
Config
SH3000UM version 0.95
Copyright ©2002 Semtech Corporation
Table 5: Load Capacitance Settings
Xtune
Padding
code
capacitance
(pF)
0000
26
0001
28
0010
30
0011
32
0100
34
0101
36
0110
38
0111
40
1000
10
1001
12
1010
14
1011
16
1100
18
1101
20
1110
22
1111
24
RESET
RESET
EVENT
VALUE
P
1
b7
XTALtune Rewrite-Once Enable (see notes for use)
P W B
0 b6 Register Page for R0x10 through R0x17. 0 =Page0, 1 =Page1.
P W B
0 b5 ForceDCOon. Forces HF Oscillator to run under all conditions.
P W B
0 b4 CLK
0x88
P W B
1 b3 CLK
P W B
0 b2 WdogClkSelect. 1 = CLK
P W B
0 b1 Interrupt Flag Clear. 1 = clear, 0 = no effect, always reads 0.
P W B
0 b0 Interrupt Enable. 1 = enable, 0 = disable.
2002-08
REF
Effective
Effective
crystal load
ppm
capacitance
adjustment
(pF)
13
-4
14
-10
15
-16
16
-21
17
-26
18
-31
19
-35
20
-37
5
118
6
91
7
70
8
52
9
36
10
23
11
15
12
4
DESCRIPTION
source. 1 = 32 kHz, 0 = HFCLK.
OUT
Enable. 1 = enable, 0 = disable.
OUT
, 0 = 32 kHz.
OUT
SH3000 User Manual
Preliminary
Adj (R0x12) with the
18
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