Tag Archives: embedded

Reading DHT11 temperature sensor on Raspberry Pi under FreeBSD

Connecting sensor to the RPi

DHT-11 is a very cheap temperature/humidity sensor which is commonly used in the IoT devices. It is not very accurate, so for the accurate measurement i would recommend to use DHT21 instead. Anyway, i had DHT-11 in my tool box, so decided to start with it. DHT-11 using very simple 1 wire protocol – host is turning on chip by sending 18ms low signal to the data output and then reading 40 bytes of data. Details about the protocol could be found in the specification. To read data from the chip it should be connected to the power (5v) and gpio pin. I used pin 2 as VCC, 6 as GND and 11 as GPIO (it is GPIO17, see pinout):

FreeBSD support

There is no support for this device out of the box. I found some sample code on the github, see lex/freebsd-gpio-dht11 repository. This code was a good starting point, but soon i found 2 issues with it:

  1. Results are very unreliable, probably due to gpio decoding algorithm.
  2. Checksum is not validated, so sometime values are bogus.

Initially i was thinking to fix this myself, but later found kernel module for this purpose, 1 wire over gpio. This module contains DHT11 kernel driver (gpio_sw) which implements DHT-11 protocol in the kernel space and exporting /dev/sw0 for the userland. Driver compiles on FreeBSD11/ARM without any changes. Use make install to install the driver.

Putting all the things together

  • To specify GPIO pin with a sensor put hint.gpio_sw.0.pin=17 into /boot/loader.conf.
  • I found that gpio_sw.ko needs to be loaded after kernel initialization, or device is not created. So i have added /sbin/kldload gpio_sw.ko to the /etc/rc.local file.
  • To make /dev/sw0 available for the non-root users you should add devfs_system_ruleset="localrules" to the /etc/rc.conf and add into /etc/devfs.rules this section:
add path 'sw0' mode 0644
  • Sample program (test.c) shows non human-readable data, use something like printf("h:%d.%d %%, t:%d.%d C\n",Buf[0],Buf[1],Buf[2],Buf[3]); if you want to get humidity and temperature, e.g.
root@rpi-b:/home/freebsd/gpio_sw # ./test
h:22.0 %, t:12.0 C

That is it, after reboot you should have working /dev/sw0 device.

Solving problems with LUA

Final goal was to add this sensor to the domoticz software. It is using LUA scripting to extend it functionality, e.g. to obtain data from non-supported or non standard devices. So, i decided to read /dev/sw0 from the LUA. I wrote simple test script:

file = io.open ("/dev/sw0", "rb")
out = file:read (5)

However script was always returning nil and i had to use truss tool to understand the problem. This is part of the trace:

open("/dev/sw0",O_RDONLY,0666)             = 3 (0x3)
fstat(3,{ mode=crw-r--r-- ,inode=96,size=0,blksize=4096 }) = 0 (0x0)
ioctl(3,TIOCGETA,0xbfbfe28c)             = 0 (0x0)
read(3,0x2065b000,4096)              ERR#22 'Invalid argument'

As you could see – LUA trying to read 4096 bytes, despite the fact that we specified 4. And driver checks this and returns the error. To fix this i did a small patch to avoid error and make LUA happy:

-- gpio_sw/gpio_sw.c    2014-05-12 11:26:51.000000000 +0000
+++ gpio_sw.mod/gpio_sw.c   2017-01-14 14:10:33.736813000 +0000
@@ -273,9 +273,10 @@
   duprintf("read - start, uio_resid=%i\n", uio->uio_resid);
   struct gpio_sw_softc *sc = cdev->si_drv1 ;
-  if ( uio->uio_resid >= sc->BufSize) return EINVAL ;
+  if ( uio->uio_resid >= sc->BufSize) sc->Len=sc->BufSize-1 ; // to work with buffered IO
+  else sc->Len=uio->uio_resid;
   if ( ! sc->GpioStatus) return ENXIO ;
-  sc->Len = uio->uio_resid ;
   int i = 0 ;
   while ( i < sc->Len)

After modification script works as it should:

[root@rpi-b /home/freebsd]# lua52 test.lua

Now chip is connected to the domoticz and reporting actual temperature/humidity.

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ICMP Watchdog in the Ubiquiti Networks devices

About watchdog

I am using wireless devices from the Ubiquiti Networks. Usually everything works fine, but in rare cases of software/hardware bug it would be great to automatically restart device when needed. AirOS provides this functionality, it is called “ping watchdog” and is located in the web interface, “services” tab. However there is no a lot of documentation about how it works, so i decided to research this. Screenshot of the watchdog interface with default values provided below: Screen Shot 2016-07-18 at 08.38.52.

Under the hood

Ubnt AirOS is OpenWRT based OS with ssh enabled, so we can ssh to the device to find how this watchdog works. If ping watchdog is enabled in the web interface you should see something like this in the process list:

/bin/pwdog -d 300 -p 300 -c 3 -m 300 -e /bin/support /tmp/emerg /etc/persistent/emerg.supp emerg 0; reboot -f

This “pwdog” service is a custom busybox applet which is based on busybox ping implementation with modifications to implement watchdog functionality. I been able to find it source code on the github.

So there is detailed description of the pwdog service logic:

  1. On system start it waits -d seconds (300 by default), to allow initialization of the hardware and software. I would not recommend to reduce this value, or you will have a chance that device will never start. In the web interface it is “Startup Delay:” value.
  2. After initial delay it will send ICMP ping to the specified host (last parameter) and will wait -p seconds (300 by default, “Ping Interval:” in the web interface). After this step 2 will be repeated.
  3. If there is no reply -c times (by default – 3) pwdog will run command specified in the -e argument (/bin/support /tmp/emerg /etc/persistent/emerg.supp emerg 0; reboot) or just reboot if it is not specified. In this example watchdog also saves support information. In the web interface you can modify this value using “Failure Count To Reboot.:” parameter.
  4. There is also -m parameter which defines low memory threshold. It is enabled by default and is not configurable via web interface.

Below i tested how it works in the command line, with modified parameters:

XM.v5.6.6# /bin/pwdog -d 1 -p 3 -c 3 -m 300 -e /usr/bin/echo -v
pwdog[993]: pwdog: do_now=0, initial_sleep=1, timeout=3, retry_count=3, low_mem=300 exec=`/usr/bin/echo`
pwdog[993]: PING Watchdog is checking (
pwdog[993]: Missed 1 ping replies in a row.
pwdog[993]: Missed 2 ping replies in a row.
pwdog[993]: Missed 3 ping replies in a row.
pwdog[993]: 4 ping replies missed. Executing `/usr/bin/echo`.


ICMP watchdog in AirOS is not a very smart service and default configuration does not look optimal for me – in fact its enough to miss only 3 ICMP packets to start reboot process. Also it will fire only after 15 (300*3) minutes of the link failure. So i would probably recommend to increase number of counts and decrease ping interval. Also i am thinking about porting apinger to this device, because it provides much more advanced icmp check functionality.

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Monitoring WAN status on OpenWRT using Alarm Pinger


I am connected to the Internet using wireless link which is sometime not very stable. I decided to monitor status of the link to make sure that I am aware of the problem. Initially i tried to monitor link with Monit or Nagios + fping, but results were not very good, this software is not designed for continues monitoring with very small interval. So I decided to find some alternatives.

About Alarm Pinger

I was using Alarm Pinger (apinger) with pfSense distribution — it was used to monitor WAN links to switch between them if needed.

Alarm Pinger (apinger) is a little tool which monitors various IP devices by simple ICMP echo requests. There are various other tools, that can do this, but most of them are shell or perl scripts, spawning many processes, thus much CPU-expensive, especially when one wants continuous monitoring and fast response on target failure. Alarm Pinger is a single program written in C, so it doesn’t need much CPU power even when monitoring many targets with frequent probes. Alarm Pinger supports both IPv4 and IPv6.

This tool supports multiply monitoring targets, external scripts, email notification, daemon mode. Only problem was that tool was not available as OpenWRT package. So i decided to port it.

OpenWRT port

After few tests I found, that code can be compiled with only few minor patches (autoconf related). You can grab Makefile for package from this pull request. Hopefully it will be integrated in the official packages feed soon. Update: port merged.
Port provides init.d script and sample configuration. In the feature I am also planning to make Luci integration to show link status from the web interface.

To buid package on Turris I would recommend to use my turris buildroot docker image.

## Service configuration

I am using very simple configuration to monitor status of the Wireless link using pings to the ISP gateway:

# we need to use root because "rainbow" tool fails to work from other uid. 
user "root"
group "root"

# status file with link quality information
status {
    file "/tmp/apinger.status"
    interval 1s
# command to run, with alarm type and reason
# if used with multiply targets %t needs to be added
alarm default {
    command on "/root/gateway.sh %A %r"
    command off "/root/gateway.sh %A %r"
# This alarm will be fired when target doesn't respond for 30 seconds.
alarm down "down" {
    time 30s
# This alarm will be fired when responses are delayed more than 80ms
# it will be canceled, when the delay drops below 50ms
alarm delay "delay" {
    delay_low 50ms
    delay_high 80ms
# This alarm will be fired when packet loss goes over 5%
# it will be canceled, when the loss drops below 3%
alarm loss "loss" {
    percent_low 3
    percent_high 5
target default {
    interval 1s
    avg_delay_samples 10
    avg_loss_samples 50
    avg_loss_delay_samples 20
    alarms "down","delay","loss"
# ISP Gateway host to monitor. You can define many targets in case of MultiWAN. 
target "" {
    description "ISP Gateway"

Also I am using simple script to change WAN LED color in case of problems:


DEF_COLOR=33FF33 # see https://gitlab.labs.nic.cz/turris/rainbow/blob/master/turris.c

logger "event: $@"
# read data from status file
STATUS=`grep  "Active alarms:" /tmp/apinger.status`

case "$@" in
"delay ALARM")
  touch /tmp/apinger.delay.flag
"delay alarm canceled")
  rm -f /tmp/apinger.delay.flag
"down ALARM")
  touch /tmp/apinger.down.flag
"down alarm canceled")
  rm -f /tmp/apinger.down.flag
"loss ALARM")
  touch /tmp/apinger.loss.flag
"loss alarm canceled")
  rm -f /tmp/apinger.loss.flag
# link is down
if [ -e /tmp/apinger.down.flag ]; then
# loss or delay
if [ -e /tmp/apinger.loss.flag -o -e /tmp/apinger.delay.flag ]; then
# no active alarms found

This works pretty good – if line is down – WAN color is red, if it is unstable or congested – yellow. We can also monitor link status manually:

root@turris:~# cat /tmp/apinger.status
Fri Apr 10 12:39:24 2015

Description: ISP Gateway
Last reply received: #2876 Fri Apr 10 12:39:23 2015
Average delay: 3.247ms
Average packet loss: 0.0%
Active alarms: None
Received packets buffer: ################################################## ###################.


I am planning to extend functionality of the script with some cool features:

  • Integrate with Luci to show status in the web interface.
  • Add support for the failover switch to the LTE channel if link is down (and LTE dongle connected).
  • Enable rrdtools support provided by apinger.
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