This is useful for testing the PPP driver.

With a USB UART available as /dev/ttyUSB1 connected to pins (e.g.) 5,22,18,21 on the device:

$ sudo pppd nodetach noauth crtscts lock proxyarp 192.168.150.1:192.168.150.2 /dev/ttyUSB1 115200

Then on the device:

import machine, network

u = machine.UART(1, baudrate=115200, tx=5, rx=22, cts=18, rts=21, flow=3)
p = network.PPP(u)
p.active(1)
p.connect()

On success, pppd will print out

Using interface ppp0
Connect: ppp0 <--> /dev/ttyUSB1
local  LL address fe80::ad92:0d2c:01a6:4e6d
remote LL address fe80::2d30:5ecb:8702:97ba
local  IP address 192.168.150.1
remote IP address 129.168.150.2

To enable authentication, change noauth to auth servername and add an entry to `/etc/ppp/pap-secrets:

user servername pass 192.168.150.2

Then pass the authentication details to connect:

p.connect(authmode=p.AUTH_PAP, username="user", password="pass")