Raspberry Pi Pico + LAN8720

Overview

This tutorial demonstrates how Mongoose Library can be used on an RP2040-based board using the Pico-SDK and MIP (a bare-metal embedded TCP/IP stack designed specifically for Mongoose). We implement a minimal static web server that responds "ok" to any URL

To connect to an Ethernet network, we use an add-on board containing a PHY, in this case a LAN8720

pic

We take the 50MHz reference clock from the LAN8720 and configure the RP2040 to use it (so it works at 50MHz); then we make use of several PIO state machines to perform the RMII tasks: send frames, receive frames, and control the PHY (SMI).

Hardware connections

The reference clock (labeled as RETCLK in the example LAN8720 board) can be connected to either GPIO20 or GPIO22; we've used the former one.

Most other pins you can change at will, bear in mind that because we use the PIO, each function requires consecutive GPIOs, see below

LAN8720 Raspberry Pi Pico
1, 2 VCC 3V3 36
3, 4 GND GND 38
5 MDC GPIO15 20
6 MDIO GPIO14 19
7 CRS GPIO8 11
8 nINT / RETCLK GPIO20 26
9 RX1 GPIO7 10
10 RX0 GPIO6 9
11 TX0 GPIO10 14
12 TX-EN GPIO12 16
13
14 TX1 GPIO11 15

Project structure

This example uses the Pico-SDK and compiles with its standard toolchain:

  • main.c: the main application file, contains Mongoose logic and clock initialization code
  • mongoose.c - a symlink to repo's mongose.c, the Mongoose Library source code
  • mongoose.h - a symlink to repo's mongose.h, the Mongoose Library header file
  • driver_rp2040_rmii.c - the low-level code for the MIP driver emulating a simple MAC controller
  • driver_rp2040_rmii.h - the driver header file
  • CMakeLists.txt - A cmake file that selects the source files and compilation options
  • Makefile - mainly pulls the SDK from its Github repo and calls cmake

Build and run

  • It is assumed you're using Linux or Mac as a workstation and you have cmake and make installed
  • Start a terminal in the project directory; clone the Mongoose Library repo, and run the make build command:
    $ git clone https://github.com/cesanta/mongoose
    $ cd mongoose/examples/rp2040/pico-rmii
    $ make build
    
  • Once the build succeeds, boot your Pico in USB Mass Storage Mode by holding down the BOOTSEL button while you plug it into the USB port. Once it is connected, then release the BOOTSEL button. It should automatically mount as a new USB disk in your machine. Now either manually copy or drag and drop the generated build/firmware.uf2 file into this new USB disk device.
  • The Pico will flash this code and reboot, unmounting this disk and running the flashed code.
  • Now open a second terminal, this will be our serial console to see the logs. Run a serial port software (we use picocom; make sure you configure it at 115200bps and to add a carriage return). Device name is usually /dev/ttyACM0. Wait a bit and plug your network cable:
    $ picocom /dev/ttyACM0 -i -b 115200 --imap=lfcrlf
    6213 1 mongoose.c:6757:onstatechange    Link up
    6217 3 mongoose.c:6840:tx_dhcp_discover DHCP discover sent
    621b 3 mongoose.c:6723:arp_cache_add    ARP cache: added 192.168.69.2 @   52:54:...
    6222 2 mongoose.c:6749:onstatechange    READY, IP: 192.168.69.243
    6227 2 mongoose.c:6750:onstatechange           GW: 192.168.69.1
    622b 2 mongoose.c:6752:onstatechange           Lease: 21600 sec
    

Try it out

Start a browser on http://IP_ADDRESS where IP_ADDRESS is the board's IP address assigned by your DHCP server. You should see ok.

You can also use curl:

$ curl 192.168.69.243
ok

How it works

Mongoose detects it is being compiled for an RP2040 architecture, so it will configure itself to call the proper SDK functions it needs, for example for its time base in milliseconds.

Once we init the networking stack, it will call our driver. The driver will use the PIO and some DMA channels to simulate an elementary MAC controller

MCU and board initialization

We call SDK functions to initialize the MCU clock and the stdio:

Mongoose initialization

Then we initialize Mongoose, this is no different from what we always do in any example.

MIP initialization

MIP has to be enabled to be compiled in, and so Mongoose will work in association with it. This is done in the CMakeLists.txt file by defining MG_ENABLE_MIP=1.

Then this networking stack has to be configured and initialized. This is done by calling mip_init() and passing it a pointer to a struct mip_if. Inside this structure:

  • have pointers to a struct mip_driver and any extra data that it could need. This structure is usually provided by the driver and we just reference it; in this case, we'll see the internals later
  • For DHCP: set ip as zero
  • For a static configuration, specify ip, mask, and gw in network byte order

In this example, we use DHCP:

Note that, we also need to specify a unique MAC address. For this example we chose a fancy unicast locally administered address.

Run Mongoose

Then we run Mongoose. This is no different from what we always do in any example, though note that it should be run after network initialization. The logic is standard: initialize the event manager (as we already did), start a listener on port 80, and fall into an infinite event loop:

The listener will call this function when it receives an HTTP request:

There we just reply "ok"

Under the hood

This subsection is only for those with enough curiosity or interested in writing their own driver for MIP. Feel free to skip ahead if this does not apply to you.

The struct mip_driver provides pointers to driver functionality. An init function called for driver initialization, a tx function to be called when MIP needs to send a frame, an rx function to be called when MIP is able to process a received frame, and an up function returning the state of the interface:

To know whether the interface is up or not, MIP calls the function we configured, and we ask the PHY:

To send a frame, MIP calls the function we configured. There, we check if the DMA controller has finished sending any prior frames, then copy the frame to its buffer and retrigger the transfer process. It will pass each byte to the PIO to be sent to the PHY:

If a driver has to be polled for data, then it will also implement an rx() function. Otherwise, if received data is asynchronous to MIP, Mongoose provides an internal lock-free queue and we can take advantage of it. Either the user or the driver code (as in this example) has to set the queue length to the required size, MIP will allocate the necessary memory for us.

When the PIO state machine in charge of taking data out of the PHY detects the end of a frame, it will generate an interrupt. We then will stop the DMA transfer and reconfigure it by switching buffers in a ping-pong configuration; then we'll push that frame to the MIP queue by calling mip_qwrite():

As MIP is a part of Mongoose, MIP will run on the next call to the event manager and then call our rx function; there we will get the frame by calling mip_qread(), check its CRC, and return it to MIP to be processed:

The init function, not shown here, initializes the DMA channels and loads the PIO programs; initializing all the software MAC controller subsections.

Custom application

In order to create your own Mongoose-enabled application you have several ways:

  1. The obvious way, is to add the required functionality to this example.

  2. If, for some reason, you can't use this example as a base (e.g.: you have your own big project to which you need to add Mongoose, or you'd rather use your preferred IDE), you can do the following:

    • Add relevant project files to your project
    • Add mongoose.c and .h files to your project
    • Add Mongoose-specific configuration flags, see the Makefile
    • Add the required preprocessor symbols:
      • MG_ENABLE_MIP=1
    • Now write code similar to that in main.c; for that you can read Mongoose documentation and follow our examples and tutorials