AN2156

2 - Clock management

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2

Clock management

2.1

Crystal oscillator pins XTAL1, XTAL2

All current STR73x devices have an on-chip oscillator that allows the driving of external crystals

or resonators with a fundamental frequency of 4-8 MHz. The recommended circuitry for a

crystal is shown below. C1, C2 and R1 values depend greatly on the crystal type and

manufacturer. It is suggested that you ask your crystal supplier for the best values for these

components.

Figure 4.

Recommended circuitry for crystal oscillator pins XTAL1 and XTAL2

oscillator of STR73x devices has automatic gain control, there is no need to add a resistance in

series.

The values of the load capacitors C1 and C2 are also heavily dependent on the crystal type and

frequency. For best oscillation stability they normally have the same value. Typical values are in

the range from below 10pF up to 30pF. The parasitic capacitance of the board layout also

needs to be considered and typically adds a few pF to the component values.

In the PCB layout all connections should be as short as possible. Any additional signals,

especially those that could interfere with the oscillator, should be locally separated from the

PCB area around the oscillation circuit using suitable shielding.

2.2

On-chip RC oscillator and VBias pin

All devices of the STR73x family have an on-chip RC oscillator in addition to the main oscillator.

This on-chip RC oscillator is capable of running at either 2MHz or 32kHz.

●

The default 2MHz mode requires no external components.

●

GND.

Note:

The oscillator frequencies can be adjusted through software after reset, where the reset

frequencies are around 2.34Mhz / 29Khz.

XTAL2

XTAL1

R1

C2

C1