ATSAMB11XR datasheet(18/87 Pages) MICROCHIP

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6.3

32.768 kHz RTC Crystal Oscillator (RTC XO)

6.3.1

General Information

The ATSAMB11-XR2100A and ATSAMB11-ZR210CA contain a 32.768 kHz RTC oscillator that is

preferably used for BLE activities involving connection events. To be compliant with the BLE

specifications for connection events, the frequency accuracy of this clock has to be within ±500 ppm.

Because of the high accuracy of the 32.768 kHz crystal oscillator clock, the power consumption can be

minimized by leaving radio circuits in low-power sleep mode for as long as possible, until they need to

wake-up for the next connection timed event.

The block diagram in Figure 6-2 below shows how the internal low-frequency Crystal Oscillator (XO) is

connected to the external crystal.

The RTC XO has a programmable internal capacitance with a maximum of 15 pF on each terminal,

RTC_CLK_P, and RTC_CLK_N. When bypassing the crystal oscillator with an external signal, the user

can program down the internal capacitance to its minimum value (~1 pF) for easier driving capability. The

driving signal can be applied to the RTC_CLK_P terminal, as illustrated in Figure 6-2 below.

The need for external bypass capacitors depends on the chosen crystal characteristics. Typically, the

crystal should be chosen to have a load capacitance of 7 pF to minimize the oscillator current. Refer to

the datasheet of the preferred crystal and take into account the on-chip capacitance.

Alternatively, if an external 32.768 kHz clock is available, it can be used to drive the RTC_CLK_P pin

instead of using a crystal. The XO has 6 pF internal capacitance on the RTC_CLK_P pin. To bypass the

crystal oscillator, an external signal capable of driving 6 pF can be applied to the RTC_CLK_P terminal,

as illustrated in Figure 6-2. RTC_CLK_N must be left unconnected, when driving an external source into

RTC_CLK_P. Refer to the Table 6-1 for the specification of the external clock to be supplied at

RTC_CLK_P.

Figure 6-2. Connections to RTC XO

ATSAMB11XR/ZR

Datasheet Preliminary

DS70005342A-page 18

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