Rev. 1.0
49
Si4010-C2
16. Crystal Oscillator (XO)
The crystal oscillator produces an accurate clock reference for applications demanding a high-accuracy
transmit carrier frequency. It uses a 1-pin crystal oscillator circuit (Colpitt's oscillator) and the output is con-
nected to the frequency counter. When crystal is used, the accuracy of the radio center frequency is deter-
mined by the parameters of the crystal (such as load capacitance, crystal accuracy, etc.) and the parasitic
of the PCB associated with the crystal circuit. To reduce the impact of these crystal parameters and to
guarantee safe startup of the oscillator, the user has to do the following:
Check the crystal data sheet for the “Cload” capacitor value that should be placed across the crystal’s
terminals to oscillate at the correct frequency
If Cload > 14 pF, XO_LOWCAP bit of the bXO_CTRL register have to be set to 0. In this case, the input
capacitance of the XTAL pin of the Si4010 is approximately 5.5 pF, so a (Cload – 5.5)pF capacitor
should be placed externally across the crystal terminals.
If Cload < 14 pF XO_LOWCAP bit of the bXO_CTRL register have to be set to 1. In this case, the input
capacitance of the XTAL pin of the Si4010 is approximately 3 pF, so the external capacitor placed
across the crystal has to be (Cload – 3)pF.
16.1. Register Description
XREG Address = 0x4016
XREG Definition 16.1. bXO_CTRL
Bit
7
6
5
4
3
2
1
0
Name
Reserved Reserved Reserved Reserved Reserved Reserved XO_LOWCAP XO_ENA
Type
R/W
R/W
Reset
0
0
0
0
0
0
0
0
Bit
Name
Function
7:2
Reserved
Reserved.
1
XO_
LOWCAP
XO Low Capacitance.
Bit should be set for crystals that require less than 14 pF of total capacitance.
0: Crystal requires 14 pF or more of total capacitance.
1: Crystal requires less than 14 pF of total capacitance.
0
XO_ENA
Enable XO.
Note that operation of the XO requires that the bandgap be enabled with the System
Module Function API. The input XO_CKGOOD status bit is in the SFR SYSTEM reg-
ister.
0: Crystal oscillator disabled.
1: Crystal oscillator enabled.
