Non-RF design considerations

AN3206

10/14

Doc ID 17406 Rev 3

3

Non-RF design considerations

To operate properly, each STM32W Reference design contains additional, non-RF

components. These include a 24-MHz crystal, appropriate decoupling capacitors and RF

test point as well as an InSight Port connector.

3.1

High-frequency (24-MHz) crystal reference (X1, C5, and C6)

The STM32W requires a single, accurate 24-MHz crystal source in order to develop the

proper STM32W clock distribution and IEEE 802.15.4 timing (see Figure 6).

Figure 6.

24MHz high-frequency crystal

ST recommends that the shunt loading capacitors, C5 and C6, be located fairly close

together and share a common ground via. The shunt capacitors and the crystal form a

resonant circuit with a reasonably high Q, so the circulating currents are high. As a result,

despite only consuming 1mA (for example), the circulating current through the shunt

capacitors may be several mA. If the grounds of the capacitors are widely separated, this

several mA loop current has to flow through the ground plane to get between the two

capacitors. The high current causes voltage to be developed due to the via inductance, as

well as potentially inducing a voltage in the ground plane. Bringing the capacitors closer

together means the majority of the loop current flows in surface tracks-only and an

imbalance current flows to ground, which should be small if the signal across the crystal is

symmetric and sinusoidal. It is difficult to say if this really makes a difference or not, and

capacitors are commonly taken to ground separately. However, the crystal is very near the

RF and the VCO in particular, and 24 MHz tones of the VCO are very detrimental, so the

problem is best avoided.

Due to the timing requirements of the IEEE 802.15.4-2003 Standard, this 24-MHz crystal

must meet an accuracy of ±40 ppm over tolerance, temperature, and aging. In addition, the

crystal must have a maximum ESR of 60

Ω to control the integrated oscillator signal

strengths.

A manufacturing token within the STM32W device stores the crystal bias register setting.

This setting is automatically determined during PCB manufacturing testing when ST's

NodeTest or MFG Library software is used, and it is dependent upon the selected crystal.

For more information on this token, refer to application note AN3188: Preparing custom

devices for the STM32W108 platform. Loading capacitors C5 and C6 should be chosen

according to the crystal manufacturer's requirements.