PPPoU driver

In the era of IoT, when the goal is to address every device on the Internet with IP, even on the simplest microcontroller, may face a barrier in the form of the lack of a proper interface (Ethernet, Wi-Fi) the use of uart in conjunction with an appropriate adapter, be it USB, Bluetooth or optical/infrared uart may be the easiest to connect both worlds.

Almost every microcontroller has at least one uart, and may not have Ethernet MAC, Wi-Fi or Bluetooth, but uart/serial null-modem connection is possible always and the most legitimate and proven protocol to deliver IP world is PPP.

Build and set up the device

Before you start building Phoenix RTOS with network stack — LwIP, you need to adjust the custom build.project script, in function b_build_project() add the following lines:

b_log "Builing phoenix-rtos-lwip"
(cd phoenix-rtos-lwip && make $MAKEFLAGS $CLEAN all)

some targets may require adding also

b_install "$PREFIX_PROG_STRIPPED/lwip" /sbin

next it is required to add lwip to syspage programs with PROGS variable, e.g.:

PROGS=("dummyfs" "imxrt-multi" "lwip" "psh")

If phoenix-rtos-build buildsphoenix-rtos-lwip correctly, you can start the null-modem point-to-point connection in the Phoenix RTOS system. To enable the driver and up the interface just after Phoenix RTOS kernel starts, add e.g. the following line to the plo script:

app flash0 -x @lwip;/dev/uart3:115200:up xip1 ocram2

It is important that the entry is added with the imxrt-multi driver or with the appropriate platform serial driver. Driver that is used in the example is imxrt-multi available for imxrt targets, it will create an entry in /dev directory e.g. uart3 corresponding to the auxiliary uart port instance on i.MX RT1064-EVK board, where uart1 is already used for the systems console.

The configuration field :115200: is optional and represents the connection speed (value of 115200 bauds is the default baud rate and can be omitted). Of course, you can set other speeds, e.g. 9600, 230400 or 460800. Other connection parameters, such as parity and stop bits, are fixed to 8N1, because nowadays, other values are rarely used, especially for serial PPP connections.

Default route

By default, pppou driver will add the default route via itself. If the default route is not to be added, use the optional nodefault parameter, as in the example below.

app flash0 -x @lwip;pppou:/dev/uart3:115200:nodefault:up xip1

Setup device side

For example, on the imxrt platforms (memory map used in example is for i.MX RT1064) the plo script might look like this:

map itcm 0 58000 R+E
map dtcm 20000000 20028000 R+W
map ocram2 20200000 20280000 R+W+E
map xip1 70000000 70400000 R+E
kernel flash0
app flash0 -x @dummyfs xip1 dtcm
app flash0 -x @imxrt-multi xip1 dtcm
app flash0 -x @lwip;pppou:/dev/uart3:460800:up xip1 ocram2
app flash0 -x @psh xip1 ocram2
go!

Alternatively, phoenix-rtos-lwip can also be started with the command psh sysexec (NON-MMU targets) at any time:

sysexec ocram2 lwip pppou:/dev/uart3:115200:up

If provided the LwIP server has not already been running, and on MMU architectures either using the exec command or directly in psh command prompt.

Once the phoenix-rtos-lwip server has started, you can make sure by issuing the command ps, which should show that the lwip with the correct arguments is already running, in the example figure below you can see that the baud rate 460800 was used.

Setup host side

As what has been done above and both the device and host have been connected with a null-modem cable or using any uart-ttl-usb adapter, then on the host side you must also configure the connection, if it is Linux or BSD, you can use, for example this command (prepend pppd with sudo, doas or su command if root user rights are required):

pppd /dev/ttyUSB0 460800 10.0.0.1:10.0.0.2 lock local noauth nocrtscts nodefaultroute maxfail 0 persist

The device entry /dev/ttyUSB0 may differ between systems, the address pair 10.0.0.1:10.0.0.2 means local:remote that the host will get 10.0.0.1 address and the remote device with phoenix-rtos-lwip running will get 10.0.0.2 address, the ppp daemon will run in the background and wait continuously (maxfail 0 persist) for connections.

To check if the connection has been successfully established, use ping command on the host side:

Additional /dev entries will be created like ifstatus, route and pf.

Enabling IPv6 (dual stack)

Add the following line at the top of file phoenix-rtos-lwip/include/default-opts/lwipopts.h and rebuild.

#define LWIP_IPV6 1

Re-run pppd with +ipv6 flag, the remote side may be assigned with link-local address like fe80::55a0:6c87:7de3:611b

Debugging

Compile phoenix-rtos-lwip pppou driver with logging enabled, and then on host use the following command that enable full pppd debugging

pppd /dev/ttyUSB0 <speed> 10.0.0.1:10.0.0.2 lock local nodetach noauth debug dump nocrtscts nodefaultroute maxfail 0 holdoff 1

Replace <speed> with the baud rate of remote side and supported by the system, e.g. 9600, 115200, 230400 or 460800.