AN3206

Non-RF design considerations

Doc ID 17406 Rev 3

11/14

3.2

Optional Low-frequency crystal reference (X2, C27 and C26)

The STM32W contains an integrated sleep timer that allows for software configuration of

sleep/wake cycles of the wireless node. There are two options for driving the sleep timer: a

32.768 kHz crystal oscillator or an internal RC oscillator. The crystal oscillator is more

accurate but consumes more current than the RC oscillator. The RC oscillator is sufficient

for operating ST's IEEE 802.15.4 software libraries and stacks. Therefore, the application's

sleep timing accuracy will decide if a 32-kHz crystal is required.

Figure 7 illustrates the crystal oscillator with its loading capacitors. To reduce the effects of

overdriving the oscillator circuit within the STM32W, ST recommends an asymmetrical

loading of the crystal.

Figure 7.

32-kHz low-frequency crystal

If the application does not need the 32-kHz crystal, then the pins OSC32A (GPIO22) and

OSC32B (GPIO23) can be used as a general-purpose IO.

Table 2.

High frequency crystal requirements

Parameter

Test Conditions

Min.

Typ.

Max.

Unit

Frequency

24

MHz

Duty cycle

40

60

%

Phase noise from 1kHz to

100kHz

- 120

dBc/Hz

Accuracy

Initial, temperature, and

aging

- 40

+ 40

ppm

Crystal ESR

Load capacitance of 10pF

100

Crystal ESR

Load capacitance of 18pF

60

Start-up time to stable clock

(max. bias)

1ms

Start-up time to stable clock

(optimum bias)

2ms