Rev. 1.10
62
October 23, 2020
Rev. 1.10
63
October 23, 2020
BC66F5652
2.4GHz RF Transceiver A/D Flash MCU
BC66F5652
2.4GHz RF Transceiver A/D Flash MCU
Internal High Speed Internal RC Oscillator – HIRC
The internal RC oscillator is one of the high frequency oscillator choices, which is selected via a
software control bit, FHS. It is a fully integrated system oscillator requiring no external components.
The internal RC oscillator has three fixed frequencies of 8MHz, 12MHz and 16MHz, which is
selected using a configuration option. The HIRC1~HIRC0 bits in the HIRCC register must also
be setup to match the selected configuration option frequency. Setting up these bits is necessary to
ensure that the HIRC frequency accuracy specified in the A.C. Characterisitcs is achieved. Device
trimming during the manufacturing process and the inclusion of internal frequency compensation
circuits are used to ensure that the influence of the power supply voltage, temperature and process
variations on the oscillation frequency are minimised. Note that if this internal system clock is
selected, as it requires no external pins for its operation, I/O pins are free for use as normal I/O pins
or other pin-shared functional pins.
External 32.768kHz Crystal Oscillator – LXT
The External 32.768kHz Crystal System Oscillator is one of the low frequency oscillator choices,
which is selected via a software control bit, FSS. This clock source has a fixed frequency of
32.768kHz and requires a 32.768kHz crystal to be connected between pins XT1 and XT2. The
external resistor and capacitor components connected to the 32.768kHz crystal are necessary to
provide oscillation. For applications where precise frequencies are essential, these components may
be required to provide frequency compensation due to different crystal manufacturing tolerances.
After the LXT oscillator is enabled by setting the LXTEN bit to 1, there is a time delay associated
with the LXT oscillator waiting for it to start-up.
When the microcontroller enters the SLEEP or IDLE Mode, the system clock is switched off to stop
microcontroller activity and to conserve power. However, in many microcontroller applications
it may be necessary to keep the internal timers operational even when the microcontroller is in
the SLEEP or IDLE Mode. To do this, another clock, independent of the system clock, must be
provided.
However, for some crystals, to ensure oscillation and accurate frequency generation, it is necessary
to add two small value external capacitors, C1 and C2. The exact values of C1 and C2 should be
selected in consultation with the crystal or resonator manufacturer’s specification. The external
parallel feedback resistor, R
P
, is required.
The pin-shared software control bits determine if the XT1/XT2 pins are used for the LXT oscillator
or as I/O or other pin-shared functional pins.
• If the LXT oscillator is not used for any clock source, the XT1/XT2 pins can be used as normal I/
O or other pin-shared functional pins.
• If the LXT oscillator is used for any clock source, the 32.768kHz crystal should be connected to
the XT1/XT2 pins.
For oscillator stability and to minimise the effects of noise and crosstalk, it is important to ensure
that the crystal and any associated resistors and capacitors along with interconnecting lines are all
located as close to the MCU as possible.
