Getting started with LPBAM

This article explains the LPBAM feature, when and how to use it, it also provides code examples.

1. Introduction to LPBAM

The LPBAM stands for low-power background autonomous mode. It is an operating mode available in STM32U5 products series that allows peripherals to be functional and autonomous independently from the device power modes, down to Stop 2 mode, without any software running. The LPBAM subsystem can chain different operations thanks to DMA linked-list transfers.
File:LPBAM in U5.png
The DMA operations can be related to:

• Peripheral data transfer
• Peripheral reconfiguration

Using LPBAM optimizes automatically the consumption:
The device can be in a low power mode down to Stop 2, without any need to wake up for managing peripheral operation, thus saving energy loss during device wakeup time and run operation.
The bus clock and kernel clocks of peripherals are distributed only when requested by the autonomous peripherals.

1.1. Smart Run Domain (SRD)

The STM32U5 is split into two domains: CPU domain (CD) and SmartRun domain (SRD) File:Smart Run Domain.png

The SRD architecture relies on a DMA allowing autonomous operation during low-power modes down to Stop 2. This architecture also features a 32-bit AHB bus matrix that interconnects:

• two masters:

– the main AHB bus matrix
– LPDMA1 (low-power DMA featuring one master port)

• two slaves:

– AHB3 peripherals including AHB to APB bridge connected to APB3
– internal SRAM4

Information

The SRAM4 is the only SRAM that can be accessed by the LPDMA1.

1.2. Peripherals supporting LPBAM

There are 2 types of peripherals :

• Autonomous peripherals : which have clock request capability and support DMA transfer in STOP mode.
• Passive peripherals : can only be reconfigured or used as trigger, they do not support DMA or clock request.

LPBAM is an operating mode that allows peripherals to function autonomously, independently of power modes, and without running any software. So autonomous peripherals can request their clock (kernel or bus clocks), and can then stay functional in Stop mode where the clocks are disabled by default to reduce consumption (except for LSE and LSI low-power low-speed oscillators).
The table below lists all peripherals that support LPBAM

Low-power mode	Peripherals
Stop 0 and Stop 1	ADC4, ADF1, DAC1, GPDMA1, LPDMA1, LPTIMx (x = 1 to 4), LPUART1, MDF1, I2Cx (x = 1 to 4), SPIx (x = 1 to 3), USARTx (x = 1 to 5)
Stop 2	ADC4, ADF1, DAC1, LPDMA1, LPTIM1, LPTIM3, LPUART1, I2C3, SPI3

1.3. LPBAM main use cases

Here below examples of use cases that can be done using LPBAM while remaining in Stop mode

1.3.1. I2C slave/Master transfer - SPI or UART RX/TX

This case is applicable when interfacing with external components like a MEM sensor for example over I2C or SPI. LPBAM and LPDMA can be used to transfer data from the peripheral into memory, and the MCU core will only wake up when the transfer is completed : Wakeup on Complete.
File:I2C SPI transfer.png
There is related examples in the STM32U5 Cube Firmware :
1) STM32Cube\Repository\STM32Cube_FW_U5_V1.2.0\Projects\NUCLEO-U575ZI-Q\Applications\LPBAM\LPBAM_I2C_TransmitReceive
2) STM32Cube\Repository\STM32Cube_FW_U5_V1.2.0\Projects\NUCLEO-U575ZI-Q\Applications\LPBAM\LPBAM_SPI_TransmitReceive

1.3.2. ADC or DAC conversion triggered by a low-power timer LPTIM

We can also trigger ADC with a periodic timer and store the data into memory while the rest of the device is sleeping

File:ADC trigger.png
There is a related example in the STM32U5 Cube Firmware :
STM32Cube\Repository\STM32Cube_FW_U5_V1.2.0\Projects\NUCLEO-U575ZI-Q\Applications\LPBAM\LPBAM_ADC_TempSense

Information

Check AN5816 for the STM32CubeMX LPBAM TempSense application description and building.

1.3.3. LPTIM PWM ration change, Input Capture, Pulse Counter

The LPTIM can be used for timing and for output generation while the STM32 device is in low-power mode. it provides the basic functions of the STM32 general-purpose timers with the advantage of a very‑low power consumption. Additionally, when configured in Asynchronous counting mode, the LPTIM keeps running even when no internal clock source is active.

File:LPTIM Trigger using LPBAM.png

There is a related example in the STM32U5 Cube Firmware :
STM32Cube\Repository\STM32Cube_FW_U5_V1.2.0\Projects\NUCLEO-U575ZI-Q\Applications\LPBAM\LPBAM_LPTIM_PWMGen

Information

Check AN4865 for Low-power timer (LPTIM) applicative use cases on STM32 MCUs and MPUs.

More complex applications can be built using several DMA channels or chaining operations from different peripherals on the same channel as it is the case in AN5834 which describes the LC sensor metering feature included in the STM32U5 Nucleo board NUCLEO-U575ZI-Q.

2. LPBAM Example Overview

To follow along with this example, you will need a NUCLEO-U575 board.
Before starting the hands-on, make sure you have installed the following software :

• STM32CubeMX 6.5.0 (minimum version)
• STM32CubeIDE 1.9.0
• Cube U5 software package

2.2. Objectives

The purpose of this example is to demonstrate how to :

• Run multiple queues with LPGPIO concurrently by using low-power timer LPTIM to trigger the sequence.
• Configure LPBAM scenario
• Measure the power consumption in STOP2 mode

2.3. STM32CubeMX LPBAM Configuration

To create an LPBAM application using the STM32CubeMX tool, the user needs the STM32CubeMX standard view. This sets up the main application and code generation, and the LPBAM view to build the LPBAM applications.

• Open STM32CubeMX and choose your board in the board selector menu :

File:board selection.png
Then choose start project with no peripheral initialization pushing on "NO" button. (To avoid generation of useless project code, the required peripherals are initialized later.)
File:peripheral initialization popup.png
To build an LPBAM application when TrustZone® is deactivated:

• Choose "without TrustZone activated".
• Click on OK.

File:trustzone disactivated.png
The STM32CubeMX tool entry point is always the standard view. Opening the project manager and saving the main project is recommended. For this application:

• Click on project manager panel.
• Name the project LPBAM_LPGPIO_LPTIM
• For the "Project Location", save the project with the other examples in the firmware package.

At this point, the project is configured and the .IOC file is saved under the selected path. To increase the system performance, enabling the ICACHE peripheral in one-way configuration is recommended. To do this, click on the "System Core" menu, then on the "ICACHE" peripheral, and change "Mode" to "1-way (direct mapped cache)".

File:ICache activation in STM32CubeMX.png
To reach the highest performance, configure the system clock to the highest value. PLL 1 configuration is as follows:

• PLLM = 1
• PLLN = 80
• PLLP = 2
• PLLQ = 2
• PLLR = 2

Choose Systick as a timebase

File:Choose SysTick as Timebase .png

2.3.1. LPBAM scenario & configuration

At this point, the main project system is configured. On the next step, we will build the LPBAM_LPTIM_PWMGen application. Click on the "LPBAM Scenario & Configuration" panel

File:LPBAM scenario Configuration.png

click on the “+ Add Application” option to add an LPBAM application
When adding an LPBAM application, the STM32CubeMX tool shows the LBPAM view. As for the standard view, it contains “LPBAM Scenario & Configuration”, “Pinout & Configuration” and “Clock Configuration” panels.

Information

The naming chosen in the project should be reused in code-generated APIs and variables. This ensures consistency between STM32CubeMX LPBAM tool views and LPBAM generated application code.

Warning

It is recommended to carefully choose application naming for clear and readable generated code.

To do so :

• Change the name of the application from LpbamAp1 to PWMGen.
• Change the name of the "Scenario" to "MultiValue".
• Change the name of the Queue to PWMUpdate

File:Application and scenario naming.png
To reach the lowest power consumption regarding the hardware target, you need to shut off all unused resources through the PWR peripheral. To do so :

• Click on the "Pinout & Configuration", then on "Power and Thermal", then on "PWR".
• Enable the power-down for all the SRAMs, except SRAM4 and ICACHE.

File:PWR configuration.png

Now configure the LPBAM peripherals To do so, in Pinout & Configuration:

• Click on Timers then "LPTIM1". The "LPTIM1 Mode and Configuration" window appears
• Configure the following changes :

In the “User Constants” panel, click on "add "and create three "Constants":

Information

Check AN5816 to know how to calculate the user constants

At this point, the LPTIM only needs the LPTIM clock configuration before it can generate a PWM signal.

File:LPTIM clock config.png

The next step is to configure the LPGPIO : PA1

The next step is to configure the DMA channel that ensures the transfer of LPTIM signal to trigger the LPGPIO. In LPBAM mode, the DMA channel must be configured in linked-list mode

When DMA transfer is done in an infinite loop, the DMA channel execution mode should be configured in circular mode:

• Left-click on "System core", then on "LPDMA1". Enable the "Linked-List Mode" for "CH0".
• Click on "CH0". Then change the "Execution Mode" to "Circular" so that ADC conversion tasks are run in an infinite loop.

Next, go to "NVIC Settings" and enable "LPDMA1 SmartRun Channel 0 global interrupt". All LPBAM peripherals are now configured and ready for scenario queue building.

Information

Note that the LPDMA is chosen because it is functionnal down to Stop2 mode.

2.3.2. Scenario queue building

Warning

Respecting the order of function execution : first function, first execution is mandatory to configure the queue parameter.

STEP1 : The LPTIM1 configuration sequence for the first node is as follows:
1. Go to LPTIM1 and click on the "+" in front of Start
2. Change the function name to "Start_1".
3. Change the start Mode as "Continuous Mode"
5. Under "Trigger Configuration", keep "The Function execution is" as "Not conditioned by a trigger".

STEP2 : configure the LPTIM1 configuration sequence for the second node as follows:
1. Go to LPTIM1 and click on the "+" in front of PWM.
2. Change the function name to "PWM_1".
3. Enable the period update state, and insert 3270 as the period value.
4. Enable the pulse update state, and insert 1630 as the pulse value.
5. Under "Trigger Configuration", keep "The Function execution is" as "Not conditioned by a trigger".

STEP3 : we will configure the LPGPIO that will be triggered by the previous LPTIM signal, We will create 2 nodes; one for the Set and the other for the Reset

1. Go to LPGPIO1 and click on the "+" in front of "Write Pin".
2. Change the function name to "Write_Pin_1".
3. Choose PA1 as pin Name .
4. Choose Set as pin State.
5.Under "Trigger Configuration", change "The Function execution is" from "Not conditioned by a trigger" to "Triggered on the Rising edge of the hardware Signal".
6. Change "Trigger hardware Signal is" from "EXTI line 0" to "LPTIM CH1".

STEP 4 : For the second node, only change the function name to "Write_Pin_2" and choose Reset as pin State. At this point we will configure the LPTIM1 with 50ms period to trigger the LPGPIO, we chose to configure the LPTIM with 2 different frequencies to show the independence of the queue. To do so, repeat the steps 2.3.4 and only change the period and pulse values in step 2 as following :

• Enable the period update state, and insert 1630 as the period value.
  
• Enable the pulse update state, and insert 815 as the pulse value.
  

At this point, your queue should look like this :
File:Queue.png

At this point, the "LPBAM LPTIM_PWMGen" application is built in the STM32CubeMX LPBAM tool. It is then recommended to check the LPBAM design via the “Check LPBAM design” button. The STM32CubeMX tool checks, in the background, the consistency of the built application. It returns detected issues in the LPBAM log. File:check LPBAM DESIGN.png

2.5. Power Consumption measurement

As the LPBAM targets the lowest possible power consumption, it is recommended that all unused pins are set to analog mode:
1. Click on "Pinout" and choose "Set unused GPIOs".
2. Set the highest number of GPIOs available (111 in this application).
3. Change "GPIO Type" from "Input" to "Analog".
File:Pinout Configuration.png

• Click on "GENERATE CODE" or CTRL+ S

3. References

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