Feedback

Clock sources

Low-frequency clock

For all Bluetooth Low Energy devices, the sleep phase is a crucial factor to consider during the design phase. It is even more important for battery-operated devices, as it has a direct influence on the device’s lifetime for each battery charge. When a low-current sleep phase is combined with longer connection/advertising intervals, some of the devices can run for many months on a single coin battery.

As opposed to the data exchange phase, the sleep phase is driven by a low-frequency clock.

The source of this clock can vary depending on the application. In the nRF54L Series, the low-frequency clock controller produces a 32.768 kHz clock signal that can be driven by the following sources (ordered by current consumption – least to most):

Crystal oscillator (LFXO)
RC oscillator (LFRC)
Synthesized from HFCLK (LFSYNT)

Accuracy

The accuracy of the low-frequency clock is represented by parts per million (ppm) and refers to how closely a clock’s output matches the ideal or intended frequency over time.

For device compatibility reasons, the Bluetooth Low Energy Standard specifies the low-frequency clock requirements, the Sleep Clock Accuracy (SCA), as follows:

Peripheral (Slave) ≤ 500 ppm
Central (Master) ≤ 50 ppm

Accuracy varies a lot depending on the source used to drive the low-frequency clock. The simplest option is to use an internal RC circuit integrated in the SoC. Despite its benefits, such as a short startup time (1 ms) and a smaller footprint, which reduce the device’s BOM cost, it comes with the cost of choosing this solution.

See the nRF54L Series datasheet 32.768 kHz clock source (LFCLK).

The LFRC frequency tolerance is at the 250 ppm level, and, moreover, it requires a calibration procedure to achieve this level of accuracy. Taking the SCA requirements into account, the accuracy does not meet the requirements for central devices, so this source is not an option for those cases.

To achieve better accuracy, you need to choose a different low-frequency clock source. The best option is using a crystal oscillator (LFXO). It requires an external crystal (with load capacitors, which are available on the SoC itself, to reduce the footprint).

See the crystal mounted on the nRF54L15 DK.

The BOM list includes the low-frequency crystal (20 ppm) document:

9HT11-32.768KDZC-T

The crystal data sheet specifies different values for frequency tolerance and load capacitance. When purchasing the crystal, you can specify which values the crystal should have. As specified in the BOM above, a 32 kHz crystal with +/-20 ppm and 9 pF for the nRF54L15 DK is used.

In addition, it can reduce the overall current consumption thanks to a lower run current (at least -0.6 µA @ 3V) and the absence of a calibration procedure.

The last possible option is to drive the low-frequency clock with a synthesized signal from a high-frequency clock. Although it can provide accuracy on LFXO level with no need for external components, the option is not recommended when battery life plays a role due to its huge current consumption.

Calibration procedure

A high-frequency clock can be used in the low-frequency clock controller not only for synthesizing the low-frequency clock. It also works as a solid reference for the calibration procedure. The internal RC circuit has a drift, causing inaccuracy in time measurement, so calibration is necessary. The nRF54L Series data sheet provides the procedure for calibration using a high-frequency crystal oscillator. Typically, calibration is performed every 4-8 s and enables a low-frequency clock with 250 ppm accuracy. The calibration procedure consumes a significant amount of energy and should be included in the power budget calculation.

Window widening

When the connection is established, the central and peripheral devices exchange data periodically according to the configured connection interval. Both use a low-frequency clock as a time source, which comes with a defined accuracy tolerance. This depends on the clock’s configuration and source. This inaccuracy can cause them to wake at slightly different times and miss transmissions. To prevent this, the Bluetooth Low Energy standard defines a mechanism called window widening (WW). Window widening allows the peripheral to increase the time it spends listening for packets from the central. The peripheral starts to listen before the expected transmission time (earlier by WW time ). It keeps listening to additional WW after that time (unless the transmission happens earlier). This time depends on the Sleep Clock Accuracy (SCA) of both devices. The peripheral knows its own SCA and receives the central’s SCA in the connection request. You can calculate the total window widening time using the following formula:

WindowWidening = (timeSinceLastAnchorPoint) × (SCA_central + SCA_peripheral) / 1,000,000 + 2* 16 µs

The timeSinceLastAnchorPoint is the last known point in time when both devices were communicating and is typically equal to the connection interval. During this time, the radio is enabled and configured to receive. The SoC current consumption in this mode needs to be included in the power budget calculation. The accuracy of the low-frequency clock significantly influences total power consumption. You can use the Online Power Profiler to see how different the window widening time will look when using an external crystal or an internal RC oscillator.

Current consumption

The previous sections covered the accuracy difference in low-frequency clock sources and the necessary actions when using an internal oscillator. Both the window-widening and calibration procedures increase the current consumption. Additionally, the run current of the oscillators varies. A device using the internal oscillator consumes approximately 0.6 μA more current than one with an external crystal during operation; this must be added to the total current consumption.

You can use the Online Power Profiler to see how selecting different low-frequency clock sources affects the device current consumption, including all the mentioned factors:

Calibration procedure
Window widening
Run current

Change the LF clock option in the Chip settings pane.

Footprint

Selecting a low-frequency clock significantly impacts the device’s footprint. When the accuracy and current consumption are the key factors, the crystal oscillator is the preferred source.

It requires an external crystal for the operation, which leads to increased footprint and cost of the device. The external crystal needs capacitors for operation.

The nRF54L Series devices not only support external capacitors for the crystal but also feature programmable internal capacitors ranging from 3 pF to 18 pF in 0.65 pF steps. This helps minimize the footprint of the device when selecting an external crystal as a low-frequency clock source.

When an external crystal is not an option, the internal RC circuit (LFRC) can still be used without any additional components. Worse clock accuracy and increased current consumption are expected in this configuration.

Summary

	External oscillator (LFXO)	Internal RC (LFRC)
Accuracy	High	Low
Startup time	Very long (0.43 s)	Short (1 ms)
Current consumption	Low	Medium/high
Calibration needed	No	Every 4 s
Windows Widening (longer peripheral RX time)	Short	Long
Footprint	High/medium (capacitors in the SoC)	Small (no external components needed)

High-frequency clock

The SoC needs a high-frequency clock for operation. It is distributed to the CPU, peripherals, and other system components. As with the low-frequency clock, there are more than one possible source for it. The high-frequency clock can run from:

Phase-locked loop (PLL) with internal oscillator (HFINT)
External crystal oscillator (HFXO)

Both solutions have strengths and weaknesses that affect the performance and current consumption. The nRF54L Series features dynamic control of these sources to deliver better performance when needed and reduce current consumption during idle or non-critical tasks.

In addition, when the device is powered up, it starts with an internal oscillator due to its short startup time of approximately 6 μs. For the external crystal oscillator, it also depends on the crystal size, affecting the footprint. It takes 200-300 μs to start the oscillator and 415-700 μs for a high-frequency clock to be calibrated, which is sufficient for many cases. The HFXO calibration is necessary for the radio operation and calibrating the low-frequency oscillator.

The external crystal has higher accuracy than other crystals. The following peripherals need the HFXO for operation and request the clock automatically when needed:

Radio
NFCT
UARTE

There is an additional group of components that can run using the internal clock HFINT, but enabling the external crystal improves their accuracy:

SAADC (reducing clock jitter)
Serial interfaces (accuracy and bitrate):
- SPIM
- SPIS
- TWIM
- TWIS

Using the external crystal comes with the additional factors that affect the footprint and power budget. Looking at the oscillator electrical specifications, you can notice that using a smaller crystal comes with increased current consumption and longer startup time. To help reduce the footprint, the nRF54L Series devices are equipped with internal configurable crystal capacitors. The external crystal oscillator, due to its higher accuracy, has a higher startup time and consumes much more current than an internal oscillator.

	Internal oscillator (HFINT)	External oscillator (HFXO) – crystal 2.0×1.6 mm crystal	External oscillator (HFXO) – crystal 1.2×1.0 mm
Footprint	Small	Moderate/High (internal capacitors)	Moderate (internal capacitors)
Startup time	Short (6 μs)	Moderate (200 μs)	Long (400 μs)
Startup & calibration time	–	Moderate (300 μs)	Long (700 μs)
Current consumption	Moderate	High	High
Accuracy	Low	High	High

Nordic Developer Academy Terms of Service

1. Introduction

‍These terms and conditions (“Terms of Use”) apply to the use of the Nordic Developer Academy, provided by Nordic Semiconductor ASA, org. nr. 966 011 726, a public limited liability company registered in Norway (“Nordic Semiconductor”). ‍

Nordic Developer Academy allows the user to take technical courses related to Nordic Semiconductor products, software and services, and obtain a certificate certifying completion of these courses. By completing the registration process for the Nordic Developer Academy, you are agreeing to be bound by these Terms of Use.

These Terms of Use are applicable as long as you have a user account giving you access to Nordic Developer Academy.‍

‍2. Access to and use of Nordic Developer Academy

‍‍Upon acceptance of these Terms of Use you are granted a non-exclusive right of access to, and use of Nordic Developer Academy, as it is provided to you at any time. Nordic Semiconductor provides Nordic Developer Academy to you free of charge, subject to the provisions of these Terms of Use and the Nordic Developer Academy Privacy Policy.

To access select features of Nordic Developer Academy, you need to create a user account. You are solely responsible for the security associated with your user account, including always keeping your login details safe.

You will able to receive an electronic certificate from Nordic Developer Academy upon completion of courses. By issuing you such a certificate, Nordic Semiconductor certifies that you have completed the applicable course, but does not provide any further warrants or endorsements for any particular skills or professional qualifications.

Nordic Semiconductor will continuously develop Nordic Developer Academy with new features and functionality, but reserves the right to remove or alter any existing functions without notice.

‍3. Acceptable use

You undertake that you will use Nordic Developer Academy in accordance with applicable law and regulations, and in accordance with these Terms of Use.‍ You must not modify, adapt, or hack Nordic Developer Academy or modify another website so as to falsely imply that it is associated with Nordic Developer Academy, Nordic Semiconductor, or any other Nordic Semiconductor product, software or service.

You agree not to reproduce, duplicate, copy, sell, resell or in any other way exploit any portion of Nordic Developer Academy, use of Nordic Developer Academy, or access to Nordic Developer Academy without the express written permission by Nordic Semiconductor. You must not upload, post, host, or transmit unsolicited email, SMS, or \”spam\” messages.

You are responsible for ensuring that the information you post and the content you share does not;

contain false, misleading or otherwise erroneous information
infringe someone else’s copyrights or other intellectual property rights
contain sensitive personal data or
contain information that might be received as offensive or insulting.
Such information may be removed without prior notice.

‍Nordic Semiconductor reserves the right to at any time determine whether a use of Nordic Developer Academy is in violation of its requirements for acceptable use.

Violation of the at any time applicable requirements for acceptable use may result in termination of your account. We will take reasonable steps to notify you and state the reason for termination in such cases.

‍4. Routines for planned maintenance

‍Certain types of maintenance may imply a stop or reduction in availability of Nordic Developer Academy. Nordic Semiconductor does not warrant any level of service availability but will provide its best effort to limit the impact of any planned maintenance on the availability of Nordic Developer Academy.

5. Intellectual property rights

‍Nordic Semiconductor retains all rights to all elements of Nordic Developer Academy. This includes, but is not limited to, the concept, design, trademarks, know-how, trade secrets, copyrights and all other intellectual property rights.

Nordic Semiconductor receives all rights to all content uploaded or created in Nordic Developer Academy. You do not receive any license or usage rights to Nordic Developer Academy beyond what is explicitly stated in this Agreement.

‍6. Liability and damages

‍Nothing within these Terms of Use is intended to limit your statutory data privacy rights as a data subject, as described in the Nordic Developer Academy Privacy Policy. ‍You acknowledge that errors might occur from time to time and waive any right to claim for compensation as a result of errors in Nordic Developer Academy. When an error occurs, you shall notify Nordic Semiconductor of the error and provide a description of the error situation.

You agree to indemnify Nordic Semiconductor for any loss, including indirect loss, arising out of or in connection with your use of Nordic Developer Academy or violations of these Terms of Use. ‍Nordic Semiconductor shall not be held liable for, and does not warrant that (i) Nordic Developer Academy will meet your specific requirements, (ii) Nordic Developer Academy will be uninterrupted, timely, secure, or error-free, (iii) the results that may be obtained from the use of Nordic Developer Academy will be accurate or reliable, (iv) the quality of any products, services, information, or other material purchased or obtained by you through Nordic Developer Academy will meet your expectations, or that (v) any errors in Nordic Developer Academy will be corrected.

You accept that this is a service provided to you without any payment and hence you accept that Nordic Semiconductor will not be held responsible, or liable, for any breaches of these Terms of Use or any loss connected to your use of Nordic Developer Academy. Unless otherwise follows from mandatory law, Nordic Semiconductor will not accept any such responsibility or liability.

‍7. Change of terms

‍Nordic Semiconductor may update and change the Terms of Use from time to time. Nordic Semiconductor will seek to notify you about significant changes before such changes come into force and give you a possibility to evaluate the effects of proposed changes. Continued use of Nordic Developer Academy after any such changes shall constitute your acceptance of such changes. You can review the current version of the Terms of Use at any time at https://academy.nordicsemi.com/terms-of-service/

‍8. Transfer of rights

‍Nordic Semiconductor is entitled to transfer its rights and obligation pursuant to these Terms of Use to a third party as part of a merger or acquisition process, or as a result of other organizational changes.

‍9. Third Party Services

‍‍To the extent Nordic Developer Academy facilitates access to services provided by a third party, you agree to comply with the terms governing such third party services. Nordic Semiconductor shall not be held liable for any errors, omissions, inaccuracies, etc. related to such third party services.

‍10. Dispute resolution

‍‍The Terms of Use and any other legally binding agreement between yourself and Nordic Semiconductor shall be subject to Norwegian law and Norwegian courts’ exclusive jurisdiction.

Designing Low-Power Bluetooth LE Products

Clock sources

Low-frequency clock

Accuracy

High-frequency clock

Switch language?

Change summary

What's new in the latest version

General updates

Bluetooth LE updates

MCUboot & Partition Manager