Product Specs

19 CLOCK — Clock control
Page
104
19.2.4 32.768 kHz crystal oscillator (LFXO)
For higher LFCLK accuracy (when better than +/- 250 ppm accuracy is required), the low frequency crystal
oscillator (LFXO) must be used.
The following external clock sources are supported:
• Low swing clock signal applied to the XL1 pin. The XL2 pin shall then be grounded.
• Rail-to-rail clock signal applied to the XL1 pin. The XL2 pin shall then be grounded or left unconnected.
The LFCLKSRC on page 108 register controls the clock source, and its allowed swing. The truth table for
various situations is as follows:
Table 25: LFCLKSRC configuration depending on clock source
SRC
EXTERNAL
BYPASS
Comment
0
0
0
Normal operation, RC is source
0
0
1
DO NOT USE
0
1
X
DO NOT USE
1
0
0
Normal XTAL operation
1
1
0
Apply external low swing signal to XL1, ground XL2
1
1
1
Apply external full swing signal to XL1, leave XL2 grounded or unconnected
1
0
1
DO NOT USE
2
0
0
Normal operation, synth is source
2
0
1
DO NOT USE
2
1
X
DO NOT USE
To achieve correct oscillation frequency, the load capacitance must match the specification in the crystal
data sheet. Figure 20: Circuit diagram of the 32.768 kHz crystal oscillator on page 104 shows the LFXO
circuitry.
XL1 XL2
C1 C2
32.768 kHz
crystal
Figure 20: Circuit diagram of the 32.768 kHz crystal oscillator
The load capacitance (CL) is the total capacitance seen by the crystal across its terminals and is given by:
C1 and C2 are ceramic SMD capacitors connected between each crystal terminal and ground. C
pcb1
and
C
pcb2
are stray capacitances on the PCB. C
pin
is the pin input capacitance on the XC1 and XC2 pins (see
32.768 kHz crystal oscillator (LFXO) on page 109). The load capacitors C1 and C2 should have the same
value.
For more information, see Reference circuitry on page 546.