C8051F970-A-GM Datasheet, PDF (258/454 Pages) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

C8051F97x
25.2.4. Programmable Load Capacitance
The programmable load capacitance has 16 values to support crystal oscillators with a wide range of
recommended load capacitance. If Automatic Load Capacitance Stepping is enabled, the crystal load capacitors
start at the smallest setting to allow a fast startup time, then slowly increase the capacitance until the final
programmed value is reached. The final programmed loading capacitor value is specified using the LOADCAP bits
in the RTC0XCF register. The LOADCAP setting specifies the amount of on-chip load capacitance and does not
include any stray PCB capacitance. Once the final programmed loading capacitor value is reached, the LOADRDY
flag will be set by hardware to logic 1.
When using the SmaRTClock oscillator in Self-Oscillate mode, the programmable load capacitance can be used to
fine tune the oscillation frequency. In most cases, increasing the load capacitor value will result in a decrease in
oscillation frequency.Table 25.2 shows the crystal load capacitance for various settings of LOADCAP.
Table 25.2. SmaRTClock Load Capacitance Settings
LOADCAP
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
Crystal Load Capacitance
4.0 pF
4.5 pF
5.0 pF
5.5 pF
6.0 pF
6.5 pF
7.0 pF
7.5 pF
8.0 pF
8.5 pF
9.0 pF
9.5 pF
10.5 pF
11.5 pF
12.5 pF
13.5 pF
Equivalent Capacitance seen on
XTAL3 and XTAL4
8.0 pF
9.0 pF
10.0 pF
11.0 pF
12.0 pF
13.0 pF
14.0 pF
15.0 pF
16.0 pF
17.0 pF
18.0 pF
19.0 pF
21.0 pF
23.0 pF
25.0 pF
27.0 pF
Rev 1.0
259