Exercise 2 – Reducing CPU activity by connecting peripherals with DPPI
This exercise shows how to use the PPI system to optimize current consumption for events involving more than one peripheral. The system replaces the CPU engagement when one peripheral triggers the action on another peripheral. Thanks to that, the CPU can remain in IDLE mode, reducing current consumption. The higher the event frequency, the more you can gain from using PPI.
Note
This exercise has limited support for the following SoCs:
nRF54LV10: The SoC is not equipped with the PWM peripheral.
nRF54LS05A and nRF54LS05A: The SoCs only have one Timer peripheral in the peripheral power domain. This exercise does not cover the peripheral connection across the power domains.
A handful of steps in this exercise are marked as optional as they require additional hardware:
Logic analyzer or oscilloscope
For the PPI demonstration, the application with the following system architecture was prepared.
The device continuously produces aPWM signal on a GPIO pin while the CPU sleeps. The only peripheral power domain is always active. The base current consumption is around 149 µA.
The device has additional GPIO input and output.
The device produces a signal pattern on each input event.
Two timers are required to generate the predefined signal pattern.
Important
The current consumption benefits come from eliminating the CPU activity on each high frequency trigger action.
To achieve similar current consumption reduction it is enough to use:
Single GPIO input
Single GPIO output
The input and output connected via DPPI
The current consumption benefits increases along with the frequency of the input.
To demonstrate the PPI configuration to execute complex behavior involving many peripherals without CPU activity the exercise includes complete workflow:
TIMER1 a TIMER 2 to produce the LED pattern
GPIO INPUT (with PWM loop-backed to it ) or System Timer (in first steps) as the workflow trigger
Like in previous exercises, all steps are described using the nRF54L15 DK, but you can use all supported nRF54L Series DKs. In addition, you will need a Power Profiler Kit II (PPK2) in the source meter configuration.
1. Build and flash the base code to your DK and observe the power consumption.
Run the Power Profiler application and ensure the DK is powered over the PPK2, as we have done in previous exercises.
The CPU is in IDLE mode and the power domain is active (due to constant PWM output)
2. Observe the current consumption on the system timer event.
In this step, we will use the system timer (the timer service provided by the operating system, not the separate Timer peripheral)
Input: System timer (k_timer)
Signal Pattern Output: LED 1
The following diagram describes the behavior of the device:
The system timer event enables Timer 1, which toggles the GPIO with a configured interval. It also starts Timer 2 (both are started upon the same input event), which disables toggling (by disabling Timer 1) after a configured period of time. This way, the pattern contains a defined number of GPIO cycles on each system timer event.
Initially, all the events are performed using CPU and timer interrupts. See how the DPPI system can eliminate the need to use interrupts (and waking the CPU) to achieve the same results.
2.1 Enable the system timer to trigger the pattern once every 250 ms in prj.conf.
Copy
CONFIG_USE_TIMER_TRIGGER=y
Kconfig
2.2 Build and flash the application to your DK.
Observe that the LED is blinking.
2.3 Observe CPU wake-up events.
We can identify when the CPU wakes up every 250 ms and the following events when the LED pattern is executed.
2.6 (Optional) Observe the pattern using an oscilloscope or a logic analyzer with the probe connected to P1.10 (LED1 pin). There should be two GPIO events (toggle) with 50 ms interval.
Copy
configGPIO_EVENT_PERIOD_USint"GPIO event period in microseconds"default50000configGPIO_EVENTS_CNTint"Number of GPIO events in one cycle"default2
Kconfig
3. Enable DPPI.
It replaces the interrupt handlers with direct connections within the peripherals used in the flow. This way, you keep the CPU in the IDLE state when the event is triggered. In the previous steps, the system timer was used to trigger the output pattern event. As the system timer uses the CPU for its operation, we can now trigger the pattern by using an external signal connected to the GPIO input.
For the input, you can use any external clock signal (for example, DDS signal generator, another DK), but as you have a PWM output on your device, use it as an input trigger.
Note
Please note that the maximum PWM period for nRF54L15 is : Max_period = (2^15 / (16 MHz / 128)) = 32768 / 125,000 Hz ≈ 262.144 ms. In this application the PWM period is configured to 250ms.
3.1 Enable the DPPI connection in the prj.conf file:
Copy
CONFIG_USE_PPI_WORKFLOW=y
Kconfig
3.2 Disable the system timer as the pattern trigger
Copy
CONFIG_USE_TIMER_TRIGGER=n
Kconfig
3.3 Connect the PWM output (P1.11) with the pattern GPIO input (P1.13) using a jumper connector.
You can, for example, use the one that was dismounted from the current measurement connector to attach the PPK2.
3.4 Build and flash the application to your DK and observe the results.
The events are no longer present. There is no improvement in the current consumption.
The DPPI reduced only a few short events, which do not play a big role in total current consumption. Consider a case when the input event triggers the pattern more frequently. In addition, modify the pattern to use much shorter intervals.
4. Modify the pattern frequency to see the benefits of using DPPI.
4.1 Configure the PWM period to 100 µs.
Configure the PWM period to 100 µs by adding the following line in the prj.conf file:
Copy
CONFIG_PWM_OUTPUT_PERIOD_US=100
Kconfig
4.2 Configure the pattern period.
Now, you need the pattern to have a much shorter interval. Configure it to 10 µsfor a single GPIO change interval (there are six of them for the one input event), by adding the following line in the prj.conf file
Copy
CONFIG_GPIO_EVENT_PERIOD_US=10
Kconfig
4.3 Increase the number of events in the pattern.
Add the following line in prj.conf
Copy
CONFIG_GPIO_EVENTS_CNT=6
Kconfig
4.4 Build and flash the application to your DK and observe the result.
The LED is on (blinking so fast you do not see the change), and the average current increased to 157 µA. Optionally, we can verify the pattern by using an oscilloscope os logic analyzer in the next step.
4.5 (Optional) Verify the output using an oscilloscope or a logic analyzer (like in step 2.6).
Now, there should be six GPIO events (toggle) with a 10 µs interval.
4.6 Disable the DPPI.
Disable the DPPI connection again to see how much average current consumption you were able to reduce by using this system:
Copy
CONFIG_USE_PPI_WORKFLOW=n
Kconfig
4.7. Build and flash the application to your DK and observe the results.
The average current consumption has increased significantly. This time, the device consumes ~540 µA compared to ~158 µA (with DPPI). You can optionally verify that the output signal looks the same in both cases.
Note
DPPI can be used for a wide range of peripherals, and the timer and GPIO are only a few of them. Feel free to experiment with triggering other peripherals. You can find an example of triggering the ADC sampling by a timer over DPPI in lesson 6 of the nRF Connect Intermediate course.
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•Support for nRF54LS05 DK (Available through the early access sampling program) •Support for the nRF54LM20B with Axon NPU for Edge AI applications
Bluetooth LE updates
•Quality of Service module is now production-ready. •New experimental features for RF testing (Direct Test Mode) and low-latency packet handling (LE Flushable ACL).
MCUboot & Partition Manager
•Single-Slot DFU and RAM Load mode are both promoted to fully supported •Partition Manager is officially deprecated in favor of Zephyr's devicetree-based partitioning.
