C 8 0 5 1 F 3 3 6 / 7 / 8 / 9
8
Rev. 0.3
5.2. Target Board Power Options and Current Measurement
The C8051F338 target board supports three power options:
1.
12 VDC power using the AC to DC power adapter (P1)
2.
5 VDC USB VBUS power from PC via the USB Debug Adapter (J4)
3.
5 VDC USB VBUS power from PC via the CP2102 USB connector (P2)
All the three power sources are ORed together using reverse-biased diodes (D1, D2, D3), eliminating the need for
headers to choose between the sources. The target board will operate as long as any one of the power sources is
present. The ORed power is regulated to a 3.3V DC voltage using a LDO regulator (U2). The output of the
regulator powers the +3VD net on the target board, and is also connected to one end of the header J2. A shorting
block should be installed on J2 to power the VDD net, which powers the C8051F338 MCU. With the shorting block
removed, a multimeter can be used across J2 to measure the current consumption of the MCU.
5.3. System Clock Sources
5.3.1. Internal Oscillators
The C8051F338 device installed on the target board features a factory calibrated programmable high-frequency
internal oscillator (24.5 MHz base frequency, ±2%), which is enabled as the system clock source on reset. After
reset, the internal oscillator operates at a frequency of 3.0625 MHz (±2%) by default but may be configured by
software to operate at other frequencies. For low-frequency operation, the C8051F338 features a programmable
low-frequency internal oscillator (80 kHz base frequency, ±10%). Therefore, in many applications an external
oscillator is not required. However, if you wish to operate the C8051F338 device at a frequency not available with
the internal oscillator, an external crystal may be used. Refer to the C8051F336/7/8/9 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. Remove shorting blocks at headers
J9 and J10 and install the crystal at the pads marked Y1. Install a 10 M
resistor at R9 and install capacitors at
C13 and C14 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 C14 for capacitor mode, and populate R7 and C14 for RC mode. Refer to the C8051F336/7/8/9 data
sheet for more information on the use of external oscillators.
