Running Pi-HATs with a Raspberry Pi Pico

Being a Pi-user since the very first Pi1, I own many different Pi-HATs. Some of them are in daily use, but many of them are sitting in the shelf. So I wondered if I could give them a second chance in combination with a Pico. And one of my major use-cases are e-ink displays for the Pi. These e-inks don't really match with the Pi, since they are optimized for low-power scenarios. But even the Pi-Zero drains batteries too fast to be a suitable partner for these kind of displays.

Although equipped with a full 2x20 pin socket, most HATs only use a few pins like power, ground, I2C or SPI. So using a bunch of jumper cables should already be sufficient. Although that is true and fine for initial tests, a good and solid connection is always the better alternative.

So I did some research and discovered a few adapter-boards on the market that might be suitable. But on closer inspection it turned out that most of them missed one important point: just mapping some arbitrary pins is not enough. So I decided to create my own adapter boards.

Hardware

One board uses the Pi form-factor, the second uses the Pi-Zero form-factor:

They fit into standard enclosures, but since the USB-connection is on the side they need an additional cutout. Changing available 3D-models should be a simple task. And the bigger adapter PCB has a footprint for a standard JST-2 battery connector exactly where the USB-power cutout is. Anyhow, this was not the major challenge when designing these boards.

The biggest challenge was the correct pin-mapping. The Pi has I2C, UART, two SPI and I2S. I did not take the last one into account so I ended up with two revisions. I2S needs two consecutive pins. On the Pi, that is GPIO18 and GPIO19, but they are not next to each other on the pin-header.

Another contraint was space. I did not want to route traces below the WLAN-chip and antenna of the Pico-W. In the end I had to make a compromise for the Pi-Zero adapter: the first revision maps both SPIs but not I2S, the second revision maps SPI0 and I2S but not SPI1. Which is not a big deal since I haven't found a HAT yet that actually uses SPI1.

I also don't map the ID-pins of the Pi. These are used to automatically configure the correct driver on the Pi. On the Pico, you don't run a generic OS but a specific program, so you have to take care about correct drivers already before when you put them on the CIRCUITPY-drive.

The Pi-adapter has more space. I added a SD-card reader and I broke out a number of pins. One of the drawbacks of many HATs is that they block the complete pin-header although they only use a few pins. Breaking out the pins is not strictly necessary since you can access all pins from the back anyhow.

Software

The second part of the project was porting the HAT-drivers to the Pico. For Adafruit HATs, that was fairly simple. Adafruit has CircuitPython support for almost everything they sell. And since Blinka brings CircuitPython to the Pi-SBCs, "porting" the drivers is a matter of using the correct pins.

On example: the speaker-bonnet. The learning guide (https://learn.adafruit.com/adafruit-speaker-bonnet-for-raspberry-pi) tells you it is using the I2S pins GPIO18, GPIO19 and GPIO21 on the Pi. After looking up the mapping for the adapters, you just plug in those pins into a small example program provided by a second guide: (https://learn.adafruit.com/mp3-playback-rp2040/pico-i2s-mp3) and off you go playing MP3 on the speaker-bonnet. This is actually much simpler on the Pico compared to the Pi, because you don't have to go through all the steps to install the relevant drivers.

For other HATs, you will usually find CircuitPython example code for the Pi using Blinka in the learning guide for the HAT. In this case, you can take the code as is and only replace the pin-numbers.

I also own a number of HATs from Pimoroni. They don't provide CircuitPython drivers, but at least for some of the HATs there are ready to use drivers for the builtin driver-IC. In only a few cases I had some real porting work to do. But once I found out how to translate CPython I2C/SPI-calls to CircuitPython, the porting was straightforward.

Project-Repository

You can find the project repository here: https://github.com/bablokb/pcb-pico-pi-base. The repo has KiCad design files as well as ready to use production files for my preferred PCB manufacturer.

Also in the repo are CircuitPython libraries and example code for all the HATs I tested or ported.

Next Steps

What I might do in the future is to create a similar adapter PCB for the Feather form-factor. While in my current designs the Pico sits inbetween the PCB and the HAT, with the Feather I would probably make the Feather plug in from behind.

The second thing I am working on is to support the new Waveshare ESP32-S3-Pico. This is an ESP32-S3 in the Pico form-factor with identical physical dimensions and identical pin-layout. This breakout is interesting since it gives me a device with far more memory than the Pico provides. And I don't have to create new adapter boards. First results look promising.

CircuitPython Board Definition Files

Since I have two form-factors and two revisions each with their own pin-mapping, looking up the mapping is cumbersome. So I also created my own CircuitPython versions that do the mapping for me. So board.GPIO18 will always map to the correct pin on the Pico, regardless which PCB I use.

With two form-factors, two revisions and now three devices (Pico, Pico-W, EPS32-S3-Pico) I have a total of 16 combinations, thus potentially 16 CircuitPython versions. A lot to maintain, but not all combinations are actually in use (yet).

Using Github Codespaces for CircuitPython Development

Introduction

If you wan't to contribute to CircuitPython, one of the hurdles you need to take is the installation of the development environment.

There is a nice guide from Dan Halbert https://learn.adafruit.com/building-circuitpython which walks you through all the necessary steps.

There are a few problems though:

• you will need to download and install a lot of software-packages. Some of them might even need other versions than those that the packet-manager of your distribution provides. Or they conflict with other projects you are working on.
• If you use a different flavor of Linux, you cannot just copy and paste the commands from the guide but also have to change commands and package-names.
• Your software-environment is bloated. Disks are very large these days, so this is not the main problem, but backups take definitely longer (I assume that you do backup your computer).

You could use a dedicated development machine or a virtual machine, but setting this up is again additional work.

Github Codespaces are a solution for all of these problems. A Codespace is a sort of virtual Linux-system. Technology wise it is a Linux container running within docker in the cloud. If you have a Github account, you can create such a system within seconds. You just head to https://github.com/codespaces and create a codespace from one of the templates (the "Blank" template is just fine).

The interface to the codespace is the web-version of "Visual Studio Code" (VSC), so you have a state-of-the-art editor, terminals, git and so on - all from within your browser. As an alternative, you can install VSC on your local machine, add the codespace-extensions from the VSC-marketplace and connect from your local VSC to you codespace. This is higly recommended, since the browser version is sometimes sluggish.

Since codespaces use ressources in the cloud, Github charges for using them. The good news is that the free plan of every account has 120 CPU-hours and 15GB storage per month included. The minimal machine has 2 CPUs, so this boils down to 60 hours per month. This should be enough unless you are a professional developer.

Automatic Setup for CircuitPython

At this point, you could just create an empty codespace from the template and follow the guide from Dan. I actually recommend that you do that once, since you will learn about the different tools you need to install.

For regular use, it is much simpler to let Github do all this work. For this reason the CircuitPython repository has predefined codespace configurations for most of the ports.

So the normal workflow would look like this:

• create a fork of https://github.com/adafruit/circuitpython
• create a new development branch within your fork
• clone this branch into a codespace
• go for a coffee-break: the initial setup will take about 10 minutes
• edit and build your own version of CircuitPython
• add, commit and push any changes back to your branch
• create a pull-request for upstream

You can find detailed instructions for the third step in the Readme: https://github.com/adafruit/circuitpython/blob/main/.devcontainer/Readme.md

Daily Use

Once you have created your codespace, you can keep it and use it whenever you want. Codespaces have two states: "active", i.e. running or "stopped". In the latter state you are only charged for the storage, so don't forget to stop your codespace after you finished your work. Github will automatically stop your codespace after 30 minutes of inactivity. In your accout settings you can change this value to something shorter. Also, Github will delete unused codespaces after 30 days of inactivity. But you will be prompted before this happens.

Storage size is a minor problem, since Github does not charge for the storage that the standard Linux image uses. A fully operational codespace for the espressif-port e.g. has about 2.4GB, so the 15GB limit will be enough for a number of codespaces.

Further Reading

Codespaces are a powerful tool with many features not covered here. To find out more, read the documentation: https://docs.github.com/en/codespaces.

Final Note

The scripts for the automatic setup of codespaces are not maintained by the core CircuitPython developers. As CircuitPython evolves the buildsystem will change and the scripts might stop working. In this case, it is best to create an issue.

Have you ever wanted to trick your friends into letting you shut off their Chromebook, Well now you can.

• First you need to install circuit python for the trinkey
• Second you need to add the libraries to the trinkeys
• finally copy the code and save

The libraries you need are neopixel.mpy and adafruit_hid

With CircuitPython 8.x.x the new cyw43 library was introduced.  The CYW43 supports controlling the power and efficiency of the CYW43439A0 WiFi chip on the Raspberry Pi Pico W microcontroller and other microcontrollers.  The pins controlling 3 minor functions were transferred to the CYW43439A0 because Raspberry Pi Pico had used all the RP2040 pins.  So the CYW43439A0 WiFi module had to be shoehorned between the RP2040 and the green LED, SMPS_MODE, and VBUS_SENSE functions.  These three functions and the CYW43439A0 will be discussed here.

There are four sections to the new cyw43 internal library.  This is new to CircuitPython starting with version 8.0.0.  The features that import cyw43 will give you access to are:

• CYW43439A WiFi power management
• Access to and control of the onboard Green LED
• Access to and control of the SMPS_MODE option (PFM, PWM)
• Access to whether power is being supplied by way of the USB port by reading VBUS_SENSE

Four WiFi power management modes are predefined with the following:

• PM_STANDARD, 0xa11142: Is the standard power management mode. It enables power management and sets the power conservation timer to 200ms, 0x14.
• PM_AGGRESSIVE, 0xa11c82: It enables power management and sets the power conservation timer to 2000ms, 0xc8. This mode provides optimal power usage at the cost of performance.
• PM_PERFORMANCE, 0x111022: It enables power management and sets the power conservation timer to 20ms, 0x02. This mode uses more power to increase performance.
• PM_DISABLED, 0xa11140: Power management is disabled in this mode.  The power management timer is set to 200ms, 0x14.  Note: CircuitPython sets this mode at power on and reset because it provides the best connectivity reliability.
• Alternate Power Settings: There does not seem to be a standard name for what each means.  These are from the MicroPython documentation.
    PM_NONE = 0x000010 # Like PM_DISABLED above
    PM_POWERSAVE = 0x000011 # More power savings than PM_PERFORMANCE above
    PM_PERFORMANCE = 0xA11142 # Same as PM_STANDARD above

To see what each bit of the 12 bit word does, check the first document referenced at the end of this document.

The following code snippet shows how to set the mode to the STANDARD mode.

    try:
        import cyw43  # Also tests for Raspberry Pi Pico W
        cyw43.set_power_management(cyw43.PM_STANDARD)
    except ImportError:
        cyw43 = None
    print(hex(cyw43.get_power_management())) # Will show the HEX value.

The challenge for the designers of the Raspberry Pi Pico W to add WiFi services they needed to repurpose three feature pins.  The green LED, SMPS_MODE, and VBUS_SENSE control lines are used.  To access them on the Pico W, the following code snippets may help you.

To drive the green LED we must now go through the WiFi module.  Fortunately, board.LED does that for us.

    # Green LED, WL_GPIO0, CYW0
    GreenLED = digitalio.DigitalInOut(board.LED)
    GreenLED.direction = digitalio.Direction.OUTPUT
   
    GreenLED.value = True # Turn green LED ON
    GreenLED.value = False # Turn green LED OFF
    print('Green LED ', GreenLED.value) # Test for green LED state

SMPS_MODE sets the way the power management chip converts the supplied power to the 3.3 volts the RP2040 MCU and other devices require.  SMPS_MODE controls the PS (Power Save) pin on the RT6150 buck/boost power manager. 

When PS is low (0) (the default on Pico) the regulator is in Pulse Frequency Modulation (PFM) mode, which, at light and moderate loads, saves considerable power by only turning on the switching MOSFETs occasionally to keep the output capacitor topped up.  PFM outputs a constant wave shape but at different frequencies depending on the load.  Higher load >> higher frequency.  An inductor is used to store power between the HIGHs to smooth out the ripple introduced by turning the power converter MOSFETs ON and OFF.

Setting PS high (1) forces the regulator into Pulse Width Modulation (PWM) mode.  PWM mode forces the SMPS to switch continuously, which reduces the output ripple considerably at light loads (which can be good for some use cases) but at the expense of much worse efficiency.  PWM outputs a constant frequency with a varying portion of HIGH versus LOW states.  With a higher current demand, the output will increase the duration of the HIGH state and reduce the duration of the LOW state.  An inductor is used to store power between HIGH states to smooth out the 3.3 volts sent to the circuits.  Improved ripple is achieved, but worse efficiency at light loads will occur.  It may improve Analog to Digital Conversion and other stability critical operations.

Note: Under heavy load conditions the switcher will be in PWM mode irrespective of the PS pin state.

    # SMPS_MODE, WL_GPIO1, CYW1, 0=PFM Mode (Default, Best Efficiency),
    # 1=PWM Mode (Improved ripple, worse efficiency at light loads)
    SMPS = digitalio.DigitalInOut(board.SMPS_MODE)
    SMPS.direction = digitalio.Direction.OUTPUT
    SMPS.value = False # PFM Mode, most efficient
    print('SMPS_MODE ', SMPS.value)

By reading the value from VBUS_SENSE you can determine if power is being supplied through the USB connector.  If the value is True (1) power is being provided from the USB connector.  If it is False (0) power is coming from some other source.  It may be ~2.0 volts to 5.5 volts applied to the VSYS pin or 3.3 volts applied to the 3V3 pin.  Applying 3.3 volts to the 3V3 pin is not advised unless it is exceptionally well regulated.  Momentary startup overvoltage spikes can be lethal to your MCU.  If power is applied to VSYS an onboard diode connected between VBUS and VSYS prevents back feeding power to the PC USB port.

    # VBUS_SENSE, WL_GPIO2, CYW2
    # True = we have power from USB
    # False = power via VSYS or 3V3
    VBUS = digitalio.DigitalInOut(board.VBUS_SENSE)
    VBUS.direction = digitalio.Direction.INPUT
    print('VBUS_SENSE', VBUS.value)

Source documents:

https://docs.circuitpython.org/en/latest/shared-bindings/cyw43/index.html

https://datasheets.raspberrypi.com/picow/pico-w-product-brief.pdf

Why Do You Want A HID Report Descriptor?

The USB specification includes a section on Human Interface Devices (HID). These devices range from keyboards, mice, joysticks, audio controls to medical controls, eye tracking and LED lighting. CircuitPython has support for USB HID with built-in defaults for a keyboard, mouse and consumer control and associated libraries. Dan Halbert has an excellent guide to get started with this.

A USB report descriptor tells the host machine the how to talk to your device. But what if you want to communicate to a device that no one else has written a report descriptor for? Perhaps you are building a new joystick, LED indicator or medical ultrasound device. Then you will have to write your own report descriptor.

Simple Descriptor Example

Initially when you look at report descriptors they seem complicated but it is best to think of them as a description of one or more reports that can be sent to or from your device. Each report gives the current state you wish to communicate. For example a joystick report may give the X and Y axis, a throttle value and if any of a dozen buttons are pressed. The joystick may also have a second report defined that allows the host machine to light up one or more indicator lights.

Below is an example report descriptor for a joystick that has 5 buttons. This descriptor only defines one report with each button taking up 1 bit, and 3 padding bits. While this is readable it is hard to create. There is an easier way.