Silicon Laboratories C8051F983-GM, User Manual

C 8 0 5 1 F 9 9 x

14

Rev. 0.2

5.3.  System Clock Sources

5.3.1. Internal Oscillators

The C8051F996 device installed on the target board features a factory calibrated programmable high-frequency
internal oscillator (24.5 MHz base frequency, ±2%) and a low power internal oscillator (20 MHz ±10%). After each
reset, the low power oscillator divided by 8 results in a default system clock frequency of 2.5 MHz (±10%). The
selected system clock and the system clock divider may be configured by software for operation at other
frequencies. For low-frequency operation, the C8051F996 features a smaRTClock real time clock. A 32.768 kHz
Watch crystal (Y2) is included on the target board. If you wish to operate the C8051F996 device at a frequency not
available with the internal oscillators, an external crystal may be used. Refer to the C8051F99x/98x data sheet for
more information on configuring the system clock source.

5.3.2. External Oscillator Options

The target board is designed to facilitate the installation of an external crystal (Y1). Install a 10 M



 resistor at R17

and install capacitors at C20 and C21 using values appropriate for the crystal you select. If you wish to operate the
external oscillator in capacitor or RC mode, options to install a capacitor or an RC network are also available on the
target board. Populate C21 for capacitor mode, and populate R16 and C21 for RC mode. Refer to the C8051F99x/
98x data sheet for more information on the use of external oscillators.

5.4.  Port I/O Headers (J2, J3, J18, J6)

Access to all Port I/O on the C8051F996 is provided through the headers J2, J3, and J18. The header J6 provides
access to the ground plane for easy clipping of oscilloscope probes.