OSCILLATORS — Oscillator control

The system oscillators are shared between the cores in the system and automatically controlled by the clock control system, see CLOCK — Clock control.

The system has the following crystal oscillators:

High-frequency 32 MHz crystal oscillator (HFXO)
Low-frequency 32.768 kHz crystal oscillator (LFXO)

The crystal oscillators can be configured to use either built-in or external capacitors.

High-frequency (32 MHz) crystal oscillator (HFXO)

The high-frequency crystal oscillator (HFXO) is controlled by a 32 MHz external crystal.

The crystal oscillator is designed for use with an AT-cut quartz crystal in parallel resonant mode, connected between pins XC1 and XC2. To achieve correct oscillation frequency, the load capacitance must match the specification in the crystal data sheet. The following figure shows how the 32 MHz crystal is connected to the high frequency crystal oscillator.

Figure 1. Circuit diagram of the high-frequency crystal oscillator

The load capacitance (CL) is the total capacitance seen by the crystal across its terminals and is calculated by the following equation.

C1 and C2 are ceramic SMD capacitors connected between each crystal terminal and ground. For more information, see Reference circuitry. C_pcb1 and C_pcb2 are stray capacitance on the PCB. C_pin is the pin input capacitance on pins XC1 and XC2. See table 32 MHz crystal oscillator (HFXO). The load capacitors C1 and C2 should have the same value.

For reliable operation, the crystal load capacitance, shunt capacitance, equivalent series resistance, and drive level must comply with the specifications in table 32 MHz crystal oscillator (HFXO). It is recommended to use a crystal with lower than maximum load capacitance and/or shunt capacitance. A low load capacitance will reduce both startup time and current consumption.

Using internal capacitors

Optional internal capacitors ranging from 7.0 pF to 20.0 pF in 0.5 pF steps, are provided on pins XC1 and XC2.

Enabling internal capacitors eliminates the need for external capacitors for the 32 MHz crystal. The configuration of the internal capacitors must take place before starting the high-frequency crystal oscillator (HFXO).

The internal capacitors are used instead of the external capacitors C1 and C2, and the total capacitance seen by the crystal across its terminals is calculated by the load capacitance equation in High-frequency (32 MHz) crystal oscillator (HFXO).

To enable the internal capacitors, find the correct XOSC32MCAPS.CAPVALUE field using the following equation.

CAPVALUE = (((FICR->XOSC32MTRIM.SLOPE+56)*(CAPACITANCE*2-14))
           +((FICR->XOSC32MTRIM.OFFSET-8)<<4)+32)>>6;

The equation has the following variables

CAPACITANCE is the desired capacitor value in pF, holding any value between 7.0 pF and 20.0 pF in 0.5 pF steps.
FICR->XOSC32MTRIM are factory trim values which usually are different from device to device.

Finally, set XOSC32MCAPS.ENABLE to Enabled, to activate the capacitors.

After this, when HFXO is started, it will use the internal capacitor values together with the external crystal.

Note: It is possible to avoid using floating point numbers by pre-calculating the (CAPACITANCE*2-14) field of the above equation.

Low-frequency (32.768 kHz) crystal oscillator (LFXO)

For higher LFCLK accuracy (when greater than ± 250 ppm accuracy is required), the 32.768 kHz crystal oscillator (LFXO) must be used.

To use the LFXO, a 32.768 kHz crystal must be connected between the XL1 and XL2 pins, as illustrated in the following figure.

Figure 3. Circuit diagram of the low-frequency crystal oscillator

To enable oscillator functionality on XL1 and XL2 pins, use value Peripheral for the MCUSEL bitfield of the register PIN_CNF[n] (n=0..31) (Retained).

To achieve correct oscillation frequency, the load capacitance (CL) must match the specification in the crystal data sheet. The load capacitance (CL) is the total capacitance seen by the crystal across its terminals. It is calculated by the following equation.

C1 and C2 are ceramic SMD capacitors connected between each crystal terminal and ground. C_pcb1 and C_pcb2 are stray capacitance on the PCB. C_pin is the pin input capacitance on the XL1 and XL2 pins (see 32.768 kHz crystal oscillator (LFXO)). The load capacitors C1 and C2 should have the same value.

For more information, see Reference circuitry.

Using internal capacitors

Optional internal capacitors of 6 pF, 7 pF, and 9 pF are provided between pins XL1 and XL2.

Enabling the internal capacitor between XL1 and XL2 pins eliminates the need for external capacitors for the 32 kHz crystal, as shown in the following figure.

Figure 5. Internal capacitor for the 32 kHz crystal

To enable this capacitor, choose the correct C_INT capacitance in register XOSC32KI.INTCAP. The value of the internal capacitor C_INT must match the specification in the crystal data sheet. C_INT is the capacitance seen by the crystal across its terminals, including pin capacitance but excluding PCB stray capacitance.

Low-frequency (32.768 kHz) external source

The 32.768 kHz crystal oscillator (LFXO) is designed to work with external sources.

The following external sources are supported:

A low swing clock. The signal should be applied to the XL1 pin with the XL2 pin grounded. Set OSCILLATORS.XOSC32KI.BYPASS=Disabled.
A rail-to-rail clock. The signal should be applied to the XL1 pin with the XL2 pin left unconnected. Set OSCILLATORS.XOSC32KI.BYPASS=Enabled.

In order to use an external source, configure LFCLKSRC.SRC=LFXO.

Registers

Instances


Instance	Domain	Base address	TrustZone			Split access	Description
Instance	Domain	Base address	Map	Att	DMA	Split access	Description
OSCILLATORS : S OSCILLATORS : NS	APPLICATION	0x50004000 0x40004000	US	S	NA	No	Oscillator configuration

Register overview


Register	Offset	Description
XOSC32MCAPS	0x5C4	Programmable capacitance of XC1 and XC2 This register is retained.
XOSC32KI.BYPASS	0x6C0	Enable or disable bypass of LFCLK crystal oscillator with external clock source This register is retained.
XOSC32KI.INTCAP	0x6D0	Control usage of internal load capacitors This register is retained.

XOSC32MCAPS (Retained)

Address offset: 0x5C4

Programmable capacitance of XC1 and XC2

This register is retained.


Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID																											B				A	A	A	A	A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	RW	CAPVALUE										Value representing capacitance, calculated using provided equation
B	RW	ENABLE										Enable on-chip capacitors on XC1 and XC2
			Disabled	0								Capacitor disabled (use external caps)
			Enabled	1								Capacitor enabled

XOSC32KI.BYPASS (Retained)

Address offset: 0x6C0

Enable or disable bypass of LFCLK crystal oscillator with external clock source

This register is retained.


Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID																																			A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	RW	BYPASS										Enable or disable bypass of LFCLK crystal oscillator with external clock source
			Disabled	0								Disable (use with crystal or low-swing external source)
			Enabled	1								Enable (use with rail-to-rail external source)

XOSC32KI.INTCAP (Retained)

Address offset: 0x6D0

Control usage of internal load capacitors

This register is retained.


Bit number				31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ID																																		A	A
Reset 0x00000000				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
ID	R/W	Field	Value ID	Value								Description
A	RW	INTCAP										Control usage of internal load capacitors
			External	0								Use external load capacitors
			C6PF	1								6 pF internal load capacitance
			C7PF	2								7 pF internal load capacitance
			C9PF	3								9 pF internal load capacitance

Electrical specification

32 MHz crystal oscillator (HFXO)


Symbol	Description	Min.	Typ.	Max.	Units
f_HFXO	External crystal frequency		32		MHz
f_{TOL_HFXO}	Frequency tolerance requirement for 2.4 GHz proprietary radio applications			±60	ppm
f_{TOL_HFXO_BLE}	Frequency tolerance requirement, Bluetooth Low Energy applications, packet length ≤ 200 bytes			±40	ppm
f_{TOL_HFXO_BLE_LP}	Frequency tolerance requirement, Bluetooth Low Energy applications, packet length > 200 bytes			±30	ppm
C_{L_HFXO}	Load capacitance	6	8	12	pF
C_{0_HFXO}	Shunt capacitance			7	pF
R_{S_HFXO_7PF}	Equivalent series resistance 3 pF < C0 <= 7 pF			60	Ω
R_{S_HFXO_3PF}	Equivalent series resistance C0 <= 3 pF			100	Ω
P_{D_HFXO}	Drive level			100	µW
C_{PIN_HFXO}	Input capacitance XC1 and XC2 with internal capacitors disabled		5.5		pF
C_{PIN_HFXO_INT}	Input capacitance XC1 and XC2 with internal capacitors enabled		2.5		pF
C_{HFXO_INT_MIN}	Input capacitance XC1 and XC2, internal capacitor at minimum value, excluding C_{PIN_HFXO_INT}		7		pF
C_{HFXO_INT_MAX}	Input capacitance XC1 and XC2, internal capacitor at maximum value, excluding C_{PIN_HFXO_INT}		20		pF
I_{STBY_X32M}	Core standby current for various crystals
I_{STBY_X32M_X0}	Typical parameters for a given 2.5mm x 2.0mm crystal: CL_HFXO = 8 pF, C0_HFXO = 1 pF, LM_HFXO = 7 mH, RS_HFXO = 20 Ω		65		µA
I_{STBY_X32M_X1}	Typical parameters for a given 1.6mm x 1.2mm crystal: CL_HFXO = 8 pF, C0_HFXO = 0.4 pF, LM_HFXO = 20 mH, RS_HFXO = 40 Ω		187		µA
I_{STBY_X32M_X2}	Typical parameters for a given 2.0mm x 1.6mm crystal: CL_HFXO = 8 pF, C0_HFXO = 0.73 pF, LM_HFXO = 9.47 mH, RS_HFXO = 16.32 Ω		135		µA
I_{STBY_X32M_X3}	Typical parameters for a given 1.2mm x 1.0mm crystal: CL_HFXO = 8 pF, C0_HFXO = 0.42 pF, LM_HFXO = 22.7 mH, RS_HFXO = 100 Ω		181		µA
I_{START_X32M}	Average startup current for various crystals, first 1 ms
I_{START_X32M_X0}	Typical parameters for a given 2.5mm x 2.0mm crystal: CL_HFXO = 8 pF, C0_HFXO = 1 pF, LM_HFXO = 7 mH, RS_HFXO = 20 Ω		363		µA
I_{START_X32M_X1}	Typical parameters for a given 1.6mm x 1.2mm crystal: CL_HFXO = 8 pF, C0_HFXO = 0.4 pF, LM_HFXO = 20 mH, RS_HFXO = 40 Ω		790		µA
I_{START_X32M_X2}	Typical parameters for a given 2.0mm x 1.6mm crystal: CL_HFXO = 8 pF, C0_HFXO = 0.73 pF, LM_HFXO = 9.47 mH, RS_HFXO = 16.32 Ω		467		µA
I_{START_X32M_X3}	Typical parameters for a given 1.2mm x 1.0mm crystal: CL_HFXO = 8 pF, C0_HFXO = 0.42 pF, LM_HFXO = 22.7 mH, RS_HFXO = 100 Ω		863		µA
t_{POWERUP_X32M}	Power-up time for various crystals
t_{POWERUP_X32M_X0}	Typical parameters for a given 2.5mm x 2.0mm crystal: CL_HFXO = 8 pF, C0_HFXO = 1 pF, LM_HFXO = 7 mH, RS_HFXO = 20 Ω		60		µs
t_{POWERUP_X32M_X1}	Typical parameters for a given 1.6mm x 1.2mm crystal: CL_HFXO = 8 pF, C0_HFXO = 0.4 pF, LM_HFXO = 20 mH, RS_HFXO = 40 Ω		187		µs
t_{POWERUP_X32M_X2}	Typical parameters for a given 2.0mm x 1.6mm crystal: CL_HFXO = 8 pF, C0_HFXO = 0.73 pF, LM_HFXO = 9.47 mH, RS_HFXO = 16.32 Ω		60		µs
t_{POWERUP_X32M_X3}	Typical parameters for a given 1.2mm x 1.0mm crystal: CL_HFXO = 8 pF, C0_HFXO = 0.42 pF, LM_HFXO = 22.7 mH, RS_HFXO = 100 Ω		211		µs

32.768 kHz crystal oscillator (LFXO)


Symbol	Description	Min.	Typ.	Max.	Units
f_LFXO	External crystal frequency		32.768		kHz
f_{TOL_LFXO_BLE}	Frequency tolerance requirement, Bluetooth Low Energy applications			±500	ppm
f_{TOL_LFXO_ANT}	Frequency tolerance requirement for ANT applications			±50	ppm
C_{L_LFXO}	Load capacitance		7	9	pF
C_{0_LFXO}	Shunt capacitance		1	2	pF
R_{S_LFXO}	Equivalent series resistance		60	90	kΩ
P_{D_LFXO}	Drive level			0.5	μW
C_pin	Input capacitance on XL1 and XL2 pads when internal capacitor is disabled		4		pF
C_pin	Total capacitance between XL1 and XL2 pads when internal capacitor enabled	6	7	9	pF
V_{AMP,IN,XO,LOW}	Peak-to-peak amplitude for external low swing clock. Input signal must not swing outside supply rails.	200		1000	mV