Enabling Pin Alarms For Deep Sleep On NRF52840 With CircuitPython

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Hey guys! Let's dive into a cool project that combines the power of the nRF52840 microcontroller, the flexibility of CircuitPython, and the usefulness of an IMU (Inertial Measurement Unit). We're aiming to enable the alarm.pin.PinAlarm(preserve_dios) feature to wake up a board from deep sleep. This is particularly useful for projects where you need your device to conserve power while still being responsive to external events. I'll be using a Seeed Studio XIAO nRF52840 Sense board, which is a tiny, awesome piece of tech with a built-in LSM6DS IMU.

Understanding the Challenge: Deep Sleep and GPIOs

So, the deal is, I'm using a Seeed Studio XIAO nRF52840 Sense board. This little guy has an onboard LSM6DS IMU, which is powered by a GPIO pin on the nRF52840 MCU. The goal is to make this IMU wake the board from deep sleep using an alarm.pin.PinAlarm. This is a common requirement for many projects, especially when you need to monitor sensor data or respond to external events without constantly running the microcontroller.

The problem arises because, during deep sleep, the GPIOs (General Purpose Input/Output pins) are typically turned off to conserve power. That means your IMU can't signal the board to wake up. While the alarm.pin.PinAlarm works fine for waking up from light sleep, it doesn't function as expected when the device is in deep sleep mode because the pins aren’t monitored. However, the nRF52840 MCU has features that can maintain GPIO states during sleep, allowing wake-up from pin changes, and it’s all about figuring out how to leverage them in CircuitPython. This feature is known as preserve_dios, which allows the pins to retain their state during deep sleep.

Diving into alarm.pin.PinAlarm and preserve_dios

In CircuitPython, the alarm module provides a straightforward way to manage sleep and wake-up events. The alarm.pin.PinAlarm allows the board to wake up from a specified pin change. The magic lies in the preserve_dios parameter. It essentially tells the system to maintain the GPIO state during deep sleep. By default, this is disabled, but by using it we can achieve our goal of making the IMU wake the board.

To implement this, you'll need to set up your GPIO correctly and then use the alarm.pin.PinAlarm with the preserve_dios parameter set to True. The specifics of how to configure the pin and the alarm depend on your project setup, but the core concept remains the same. First, you identify the pin connected to the IMU's interrupt or wake-up signal. Then, you configure this pin as an input and enable the internal pull-up or pull-down resistor, depending on your circuit design. Finally, you create a PinAlarm object, specifying the pin and the edge (rising, falling, or both) that should trigger the wake-up. By enabling preserve_dios, you ensure that the GPIO state is maintained during deep sleep and that the pin change can wake the board.

Setting up Your Code

Let's get into the code part. Here's a simplified example to get you started. This code assumes you have the board module imported. You’ll need to modify it based on your specific hardware setup and the pin connected to your IMU.

import alarm
import board
import time

# Replace with your IMU's wake-up pin
wake_pin = board.D5

# Create a PinAlarm with preserve_dios set to True
pin_alarm = alarm.pin.PinAlarm(pin=wake_pin, value=True, pull=True, preserve_dios=True)

# Sleep until the pin changes
alarm.light_sleep_until_alarms(pin_alarm)

print("Board woke up from deep sleep!")

Explanation of the Code

  • import alarm: Imports the necessary alarm module to manage sleep and wake-up events.
  • import board: Imports the board module, which gives you access to the microcontroller's pins.
  • wake_pin = board.D5: Defines the pin connected to your IMU's wake-up signal. Make sure to replace board.D5 with the correct pin on your board.
  • pin_alarm = alarm.pin.PinAlarm(pin=wake_pin, value=True, pull=True, preserve_dios=True): Creates a PinAlarm object with the following parameters:
    • pin: Specifies the pin to monitor.
    • value: Monitors for a rising edge, change to False for a falling edge.
    • pull: Enables the pull-up resistor on the pin. If the IMU provides an active low signal, this will pull the pin high until the IMU asserts the wake signal.
    • preserve_dios=True: Crucially, this enables the preserve_dios parameter, ensuring the pin's state is maintained during deep sleep.
  • alarm.light_sleep_until_alarms(pin_alarm): Enters deep sleep until the pin_alarm triggers. This function will block the execution until an alarm is triggered and the board wakes up.
  • print("Board woke up from deep sleep!"): Prints a message once the board has woken up. This is a simple check to confirm that the wake-up worked.

Hardware Considerations

Before you start, ensure your hardware is properly connected. The IMU's wake-up pin should be connected to a GPIO pin on your nRF52840 board. Also, make sure the IMU is properly powered and initialized. The preserve_dios functionality relies on the pin being able to detect a change, so the hardware setup needs to facilitate this. This includes ensuring the correct voltage levels and using appropriate pull-up or pull-down resistors if needed. In many cases, the IMU will have an interrupt pin that signals a wake-up event. You'll connect this pin to a GPIO on your nRF52840 and configure it as the wake-up source.

Important: Always check your board's datasheet and the IMU's documentation for specific pinout details and operating voltages. Incorrect connections can damage your hardware.

Testing and Debugging

After writing your code, upload it to your board and test it. The best way to test this is to put your board into deep sleep and then trigger the wake-up event. The print statement after the sleep() function will verify it has worked. If the board doesn't wake up, there are a few things you can check:

  • Wiring: Double-check all your wiring. Ensure the IMU's wake-up pin is correctly connected to the GPIO pin on your board.
  • Pin Configuration: Verify that the pin is configured correctly in your code. Make sure you're using the right pin number and that the value argument matches the signal level expected by your IMU (e.g., high for a rising edge, low for a falling edge). Also, check the configuration of pull-up/pull-down resistors.
  • IMU Configuration: Confirm your IMU is set up to generate the wake-up signal. You might need to enable the appropriate interrupt or motion detection features in your IMU's configuration. Consult the IMU's datasheet and any associated CircuitPython libraries.
  • Power: Make sure your IMU and your board are getting enough power, especially during deep sleep. Low voltage can cause unexpected behavior. Ensure the power source can supply enough current for the IMU and the microcontroller.
  • Code Errors: Debugging involves looking at the code and using a debugger to identify potential problems. Double-check the code for syntax errors and logical mistakes. Add print statements before and after sleep to identify the execution flow.

Conclusion: Unleashing Deep Sleep Capabilities

By enabling preserve_dios and using alarm.pin.PinAlarm, you can successfully make your nRF52840 board wake up from deep sleep using a GPIO pin. This is a powerful technique for building low-power projects, especially those that incorporate sensors like the LSM6DS IMU. Remember to adjust the code based on your specific hardware setup, paying close attention to the pin configurations, IMU settings, and power supply. Once you've got it working, you can create incredibly efficient and responsive projects! Happy coding, guys!