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WM8321

PD, February 2012, Rev 4.0

13 CLOCKING AND OSCILLATOR CONTROL

13.1 GENERAL DESCRIPTION

The WM8321 incorporates a 32.768kHz crystal oscillator in order to maintain the Real Time Clock

(RTC). An external crystal is normally required. Alternatively, a 32.768kHz signal may be input directly

on the XTI pin. The crystal oscillator and RTC are enabled at all times, including the OFF and

BACKUP power states. It is possible to disable the crystal oscillator in BACKUP for power-saving

RTC ‘unclocked’ mode if desired. The WM8321 clock functions are illustrated in Figure 16.

Figure 16 Clocking Configuration

The 32.768kHz crystal oscillator is enabled using the XTAL_ENA register. The crystal oscillator is

enabled in the OFF, ON and SLEEP power states when XTAL_ENA = 1. The status of the crystal

oscillator in BACKUP is selected using the XTAL_BKUPENA register.

Note that the XTAL_ENA field is set via OTP/ICE settings only; it cannot be changed by writing to the

control register. If the crystal is omitted, and an external 32.768kHz signal is connected as an input to

the XTI pin, it is still required to set XTAL_ENA = 1 for normal operation.

The crystal oscillator can be disabled in the BACKUP state by setting the XTAL_BKUPENA register

bit to 0. This feature may be used to minimise the device power consumption in the BACKUP state,

as described in Section 20.5. The crystal oscillator is maintained in the BACKUP state if both

XTAL_ENA and XTAL_BKUPENA are set to 1.

The CLKOUT signal, derived from the 32.768kHz oscillator, can be enabled or disabled directly by

writing to the CLKOUT_ENA register in the ON or SLEEP power states. The CLKOUT can also be

controlled as part of the power state transitions using the CLKOUT_SLOT and CLKOUT_SLP_SLOT

The CLKOUT pin may be configured as a CMOS output or as an Open-Drain output. The CLKOUT

signal is referenced to the DBVDD power domain.

The status of the crystal oscillator is indicated by the XTAL_OK register bit. If the crystal oscillator

fails to start, or if it stops for any reason, then the XTAL_OK register will be set to 0.

An internal RC oscillator is available in order to provide CLKOUT functionality during start-up of the

crystal oscillator. This function is selectable using the XTAL_INH register bit, as described below.

If XTAL_INH = 0, then the internal RC oscillator provides the CLKOUT signal in the event that the

crystal oscillator has not fully started up prior to an ‘ON’ state transition event. A glitch-free transition

between the clock sources is implemented after the crystal oscillator is ready.

If XTAL_INH = 1, then an ‘ON’ state transition is delayed until the crystal oscillator has fully started

up. This may be desirable if the CLKOUT signal is used as a clock for another circuit, to ensure that

CLKOUT signal has been verified before the ‘ON’ state transition occurs. Note that the CLKOUT

output is always disabled in the OFF power state; it is typically enabled as part of the ‘ON’ state

transition sequence. Setting XTAL_INH = 1 ensures that the CLKOUT output cannot be enabled until

the crystal frequency has been verified.

If XTAL_INH = 1, and the crystal oscillator fails to start, then a System Reset will be scheduled after a

timeout period of approximately 32 seconds. See Section 24 for details of System Resets.

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