There’s a reasonably cheap chinese USB thermometer called TEMPer. I got mine from Brando for 12 Euros. It’s a USB-to-serial chip from WinChipHead. It’s DTR, RTS, and CTS lines are used to connect a LM75 I²C temperature sensor. To talk to the LM75, you need some bit-banging driver.

I’ve put together a command line utility for the TEMPer that can program the built-in thermostat (TEMPer has a LED connected to that output) and print out temperature measurement data. It does it’s job, and might serve as an example on how to do I²C over a simple interface.

Fortunately, ppp(8) offers a configuration parameter to influence the IP addresses negotiated with the peer (ifaddr), and suggesting a different address will make the configuration work.

If you get this log output from ppp, you need to configure address selection:

IPCP: deflink: RecvConfigAck(4) state = Req-Sent
IPCP:  IPADDR[6] 10.68.235.57
IPCP:  PRIDNS[6] 193.189.244.197
IPCP:  SECDNS[6] 193.189.244.205
IPCP: deflink: State change Req-Sent --> Ack-Rcvd
IPCP: deflink: RecvConfigReq(47) state = Ack-Rcvd
IPCP:   [EMPTY]
IPCP: deflink: SendConfigAck(47) state = Ack-Rcvd
IPCP:   [EMPTY]
IPCP: deflink: State change Ack-Rcvd --> Opened
IPCP: deflink: LayerUp.
IPCP: myaddr 10.68.235.57 hisaddr = 0.0.0.0
Warning: iface add: ioctl(SIOCAIFADDR, 10.68.235.57 -> 0.0.0.0): Destination address required
Error: ipcp_InterfaceUp: unable to set ip address

Here’s my complete ppp.conf, with the ifaddr line included:

u3g:
	set device /dev/cuaU0.0
	set speed 115200
	set ifaddr 10.0.0.1/0 10.0.0.2/0 255.255.255.0
	set authname internet
	set authkey  internet
	set log local phase ipcp
	set dial "ABORT BUSY TIMEOUT 2 \
		\"\" \
		AT OK-AT-OK \
		AT+CFUN=1 OK-AT-OK \
		AT+CMEE=2 OK-AT-OK \
		AT+CSQ OK \
		AT+CGDCONT=1,\\\"IP\\\",\\\"internet\\\" OK \
		AT+CGACT? OK-AT-OK \
		AT+CGATT? OK \
		AT+CGCLASS? OK \
		AT+COPS? OK \
		ATD*99***1# CONNECT"
	set crtscts on
	nat enable yes
	add default HISADDR
	disable dns

This applies to both 7-stable and 8-stable (with both the old and the new USB stacks). I’ve used u3g(4) on both occasions. The original ppp.conf for 3G modems is based off this one from Nick Hibma.

This is how it looks like when using ifaddr:

IPCP:  PRIDNS[6] 10.11.12.13
IPCP:  SECDNS[6] 10.11.12.14
IPCP:  PRINBNS[6] 10.11.12.13
IPCP: MS NBNS req 130 - NAK??
IPCP:  SECNBNS[6] 10.11.12.14
IPCP: MS NBNS req 132 - NAK??
IPCP: deflink: SendConfigReq(2) state = Req-Sent
IPCP:  IPADDR[6] 10.0.0.1
IPCP:  COMPPROTO[6] 16 VJ slots with slot compression
IPCP: deflink: RecvConfigReq(50) state = Req-Sent
IPCP:   [EMPTY]
IPCP: deflink: SendConfigNak(50) state = Req-Sent
IPCP:  IPADDR[6] 10.0.0.2
IPCP: deflink: RecvConfigRej(2) state = Req-Sent
IPCP:  COMPPROTO[6] 16 VJ slots with slot compression
IPCP: deflink: SendConfigReq(3) state = Req-Sent
IPCP:  IPADDR[6] 10.0.0.1
IPCP: deflink: RecvConfigNak(3) state = Req-Sent
IPCP:  IPADDR[6] 10.42.237.110
IPCP:  IPADDR[6] changing address: 10.0.0.1  --> 10.42.237.110
IPCP: deflink: SendConfigReq(4) state = Req-Sent
IPCP:  IPADDR[6] 10.42.237.110
IPCP: deflink: RecvConfigAck(4) state = Req-Sent
IPCP:  IPADDR[6] 10.42.237.110
IPCP: deflink: State change Req-Sent --> Ack-Rcvd
IPCP: deflink: RecvConfigReq(51) state = Ack-Rcvd
IPCP:   [EMPTY]
IPCP: deflink: SendConfigAck(51) state = Ack-Rcvd
IPCP:   [EMPTY]
IPCP: deflink: State change Ack-Rcvd --> Opened
IPCP: deflink: LayerUp.
IPCP: myaddr 10.42.237.110 hisaddr = 10.0.0.2
PPP ON freebsd-current>