Rev. 1.00
44
September 11, 2018
Rev. 1.00
45
September 11, 2018
HT45F4050
A/D NFC Flash MCU
HT45F4050
A/D NFC Flash MCU
Crystal Oscillator C1 and C2 Values
Crystal Frequency
C1
C2
16MHz
0pF
0pF
12MHz
0pF
0pF
8MHz
0pF
0pF
4MHz
0pF
0pF
1MHz
100pF
100pF
Note: C1 and C2 values are for guidance only.
Crystal Recommended Capacitor Values
Internal RC Oscillator – HIRC
The internal RC oscillator is a fully integrated system oscillator requiring no external components.
The internal RC oscillator has three fixed frequencies of
4
MHz, 8MHz, and 12MHz. 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 option
is selected, it requires no external pins for its operation.
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 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 logic 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.