Low Power UART

The Low Power UART sample demonstrates the capabilities of the Low power UART driver module.

Requirements

The sample supports the following development kits:

Hardware platforms

PCA

Board name

Board target

nRF9160 DK

PCA10090

nrf9160dk

nrf9160dk/nrf9160/ns

nRF54L15 DK

PCA10156

nrf54l15dk

nrf54l15dk/nrf54l15/cpuapp

nRF5340 DK

PCA10095

nrf5340dk

nrf5340dk/nrf5340/cpuapp

nRF52 DK

PCA10040

nrf52dk

nrf52dk/nrf52832

nRF52840 DK

PCA10056

nrf52840dk

nrf52840dk/nrf52840

nRF52833 DK

PCA10100

nrf52833dk

nrf52833dk/nrf52833

nRF21540 DK

PCA10112

nrf21540dk

nrf21540dk/nrf52840

When built for a board target with the */ns variant, the sample is configured to compile and run as a non-secure application with Cortex-M Security Extensions enabled. Therefore, it automatically includes Trusted Firmware-M that prepares the required peripherals and secure services to be available for the application.

The sample also requires the following pins to be shorted:

Pin connections.

Development Kit

nRF52 DK

nRF52833 DK

nRF52840 DK

nRF21540 DK

nRF5340 DK pins

nRF54L15 DK pins

nRF9160 DK pins

Request-Response Pins

P0.24-P0.25

P0.22-P0.23

P1.14-P1.15

P1.07-P1.08

P1.14-P1.15

P1.08-P1.09

P0.12-P0.13

UART RX-TX Pins

P0.22-P0.23

P0.20-P0.21

P1.12-P1.13

P1.05-P1.06

P1.12-P1.13

P1.10-P1.11

P0.10-P0.11

Additionally, it requires a logic analyzer.

Overview

The sample implements a simple loopback using a single UART instance. By default, the console and logging are disabled to demonstrate low power consumption when UART is active.

Configuration

See Configuring and building for information about how to permanently or temporarily change the configuration.

FEM support

You can add support for the nRF21540 front-end module to this sample by using one of the following options, depending on your hardware:

  • Build the sample for one board that contains the nRF21540 FEM, such as nrf21540dk/nrf52840.

  • Manually create a devicetree overlay file that describes how the nRF21540 FEM is connected to the SoC. See Configuring devicetree for different ways of adding the overlay file.

  • Provide nRF21540 FEM capabilities by using a shield, for example the nRF21540 EK shield that is available in the nRF Connect SDK. In this case, build the project for a board connected to the shield you are using with an appropriate variable included in the build command, for example -DSHIELD=nrf21540ek. This variable instructs the build system to append the appropriate devicetree overlay file.

    To build the sample in the nRF Connect for VS Code extension for an nRF52840 DK with the nRF21540 EK attached, add the shield variable in the build configuration’s Extra CMake arguments and rebuild the build configuration. For example: -DSHIELD=nrf21540ek.

    See How to work with build configurations in the nRF Connect for VS Code extension documentation for more information.

    See Programming nRF21540 EK for information about how to program when you are using a board with a network core, for example the nRF5340 DK.

Each of these options adds the description of the nRF21540 FEM to the devicetree. See Developing with Front-End Modules for more information about FEM in the nRF Connect SDK.

To add support for other front-end modules, add the respective devicetree file entries to the board devicetree file or the devicetree overlay file.

Building and running

This sample can be found under samples/peripheral/lpuart in the nRF Connect SDK folder structure.

When built as firmware image for a board target with the */ns variant, the sample has Cortex-M Security Extensions (CMSE) enabled and separates the firmware between Non-Secure Processing Environment (NSPE) and Secure Processing Environment (SPE). Because of this, it automatically includes the Trusted Firmware-M (TF-M). To read more about CMSE, see Processing environments.

To build the sample, follow the instructions in Building an application for your preferred building environment. See also Programming an application for programming steps and Testing and optimization for general information about testing and debugging in the nRF Connect SDK.

Note

When building repository applications in the SDK repositories, building with sysbuild is enabled by default. If you work with out-of-tree freestanding applications, you need to manually pass the --sysbuild parameter to every build command or configure west to always use it.

Testing

After programming the sample to your development kit, test it by performing the following steps:

  1. Connect the logic analyzer to the shorted pins, to confirm UART activity.

  2. Measure the current to confirm that the power consumption indicates that high-frequency clock is disabled during the idle stage.

During the idle stage, the UART receiver is ready to start reception, as the request pin wakes it up.

Dependencies

This sample uses the following nRF Connect SDK driver:

In addition, it uses the following Zephyr libraries:

The sample also uses the following secure firmware component: