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
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.

Figure 2. Load capacitance equation
Load capacitance equation

C1 and C2 are ceramic SMD capacitors connected between each crystal terminal and ground. For more information, see Reference circuitry. Cpcb1 and Cpcb2 are stray capacitance on the PCB. Cpin is the pin input capacitance on pins XC1 and XC2. See table High-frequency (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 High-frequency (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
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.

Figure 4. Load capacitance equation
Load capacitance equation

C1 and C2 are ceramic SMD capacitors connected between each crystal terminal and ground. Cpcb1 and Cpcb2 are stray capacitance on the PCB. Cpin is the pin input capacitance on the XL1 and XL2 pins (see Low-frequency (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
Using internal capacitor instead of external capacitor

To enable this capacitor, choose the correct CINT capacitance in register XOSC32KI.INTCAP. The value of the internal capacitor CINT must match the specification in the crystal data sheet. CINT 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

Table 1. Instances
Base address Domain Peripheral Instance Secure mapping DMA security Description Configuration

0x50004000
0x40004000

APPLICATION OSCILLATORS

OSCILLATORS : S
OSCILLATORS : NS

US

NA

Oscillator configuration

   
Table 2. Register overview
Register Offset Security Description
XOSC32MCAPS 0x5C4  

Programmable capacitance of XC1 and XC2

Retained

XOSC32KI.BYPASS 0x6C0  

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

Retained

XOSC32KI.INTCAP 0x6D0  

Control usage of internal load capacitors

Retained

XOSC32MCAPS (Retained)

Address offset: 0x5C4

This register is a retained register

Programmable capacitance of XC1 and XC2

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

This register is a retained register

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

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

This register is a retained register

Control usage of internal load capacitors

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
fHFXO

External crystal frequency

32 MHz
fTOL_HFXO

Frequency tolerance requirement for 2.4 GHz proprietary radio applications

±60 ppm
fTOL_HFXO_BLE

Frequency tolerance requirement, Bluetooth Low Energy applications, packet length ≤ 200 bytes

±40 ppm
fTOL_HFXO_BLE_LP

Frequency tolerance requirement, Bluetooth Low Energy applications, packet length > 200 bytes

±30 ppm
CL_HFXO

Load capacitance

6 8 12 pF
C0_HFXO

Shunt capacitance

7 pF
RS_HFXO_7PF

Equivalent series resistance 3 pF < C0 <= 7 pF

60 Ω
RS_HFXO_3PF

Equivalent series resistance C0 <= 3 pF

100 Ω
PD_HFXO

Drive level

100 µW
CPIN_HFXO

Input capacitance XC1 and XC2 with internal capacitors disabled

5.5 pF
CPIN_HFXO_INT

Input capacitance XC1 and XC2 with internal capacitors enabled

2.5 pF
CHFXO_INT_MIN

Input capacitance XC1 and XC2, internal capacitor at minimum value, excluding CPIN_HFXO_INT

7 pF
CHFXO_INT_MAX

Input capacitance XC1 and XC2, internal capacitor at maximum value, excluding CPIN_HFXO_INT

20 pF
ISTBY_X32M

Core standby current for various crystals

 
ISTBY_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
ISTBY_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
ISTBY_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
ISTBY_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
ISTART_X32M

Average startup current for various crystals, first 1 ms

 
ISTART_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
ISTART_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
ISTART_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
ISTART_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
tPOWERUP_X32M

Power-up time for various crystals

 
tPOWERUP_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
tPOWERUP_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
tPOWERUP_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
tPOWERUP_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
fLFXO

External crystal frequency

32.768 kHz
fTOL_LFXO_BLE

Frequency tolerance requirement, Bluetooth Low Energy applications

±500 ppm
fTOL_LFXO_ANT

Frequency tolerance requirement for ANT applications

±50 ppm
CL_LFXO

Load capacitance

7 9 pF
C0_LFXO

Shunt capacitance

1 2 pF
RS_LFXO

Equivalent series resistance

60 90
PD_LFXO

Drive level

0.5 μW
Cpin

Input capacitance on XL1 and XL2 pads when internal capacitor is disabled

4 pF
Cpin

Total capacitance between XL1 and XL2 pads when internal capacitor enabled

6 7 9 pF
VAMP,IN,XO,LOW

Peak-to-peak amplitude for external low swing clock. Input signal must not swing outside supply rails.

200 1000 mV

This document was last updated on
2021-09-27.
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