This year on DORS/CLUC 2018 I decided to talk about device tree in Linux kernel, so here are blurb, presentation and video of that lecture.
You have one of those fruity *Pi arm boards and cheep sensor from China? Some buttons and LEDs? Do I really need to learn whole new scripting language and few web technologies to read my temperature, blink a led or toggle a relay?
No, because your Linux kernel already has drivers for them and all you need is device tree and cat.
Below is transcript of talk:
0:00:00.000,0:00:09.540 Hello. How are you? This is the participation part, come on 0:00:09.540,0:00:19.619 you're not the first I time here! OK, my name is Dobrica PavlinuÃ Â¡iÃÂ and I will try to 0:00:19.619,0:00:25.019 persuade you today that you can do with your Linux something which you might not 0:00:25.019,0:00:32.969 have thought of by yourself I hope in a sense in last year and a half I noticed 0:00:32.969,0:00:38.610 that I am using microcontrollers for less and less and that I'm using my 0:00:38.610,0:00:44.760 Linux more and more for more or less the same tasks and in that progress process 0:00:44.760,0:00:51.239 I actually learned something which I want to share with you today in a sense 0:00:51.239,0:00:57.059 my idea is to tell you how to do something with your arm single board 0:00:57.059,0:01:05.729 computer in this lecture we will talk mostly about Allwinner boards but if 0:01:05.729,0:01:12.119 you want a hint if you want to buy some arm computer please buy the board 0:01:12.119,0:01:17.070 which is supported by armbian. armbian is the project which actually 0:01:17.070,0:01:21.360 maintains the distribution for our boards and is currently the best 0:01:21.360,0:01:27.689 distribution for arms aside from me raspbian for Raspberry Pi but raspbian 0:01:27.689,0:01:32.909 supports only Raspberry Pi but if you have any other board please take a look if 0:01:32.909,0:01:37.860 there is an armbian port. if there isn't try to contribute one and if you are 0:01:37.860,0:01:41.850 just deciding which board to buy my suggestion is buy the one which is 0:01:41.850,0:01:47.070 already supported on the other hand if you already did something similar you 0:01:47.070,0:01:52.079 might have found some references on the internet about device three and it 0:01:52.079,0:01:55.619 looked like magic so we'll try to dispel some of that 0:01:55.619,0:02:03.180 magic today unfortunately when you start playing with it one of the first things 0:02:03.180,0:02:10.459 you will want to do is recompile the kernel on your board so be prepared to 0:02:10.459,0:02:15.810 compile additional drivers if they are not already included. armbian again 0:02:15.810,0:02:21.890 wins because it comes with a lot of a lot of drivers already included and 0:02:21.890,0:02:28.950 this year I will not say anything which requires soldering which might be good 0:02:28.950,0:02:35.190 for you if you are afraid of the heat but it will be a little bit more than 0:02:35.190,0:02:41.519 just connecting few wires not much more for a start let's start with the warning 0:02:41.519,0:02:46.980 for example you have a arms in arms small arm board and you want to have a 0:02:46.980,0:02:52.019 real clock in it you know the one which keeps the time when the board is powered 0:02:52.019,0:02:57.239 off has the battery and so on if you buy the cheapest one from China which is 0:02:57.239,0:03:02.879 basically for Arduino you will buy the device which is 5 volt device which your 0:03:02.879,0:03:10.170 arm single board computer isn't. you can modify the board removing two resistors 0:03:10.170,0:03:16.409 if you want to but don't tell anyone I2C, and will mostly talk about i2c 0:03:16.409,0:03:23.819 sensors here, should be 5 volt tolerant so if you by mistake just connected and 0:03:23.819,0:03:30.569 your data signals are really 5 volt you won't burn your board but if you are 0:03:30.569,0:03:35.129 supplying your sensor with 5 volts please double-check that your that is 0:03:35.129,0:03:39.060 sure to connect it to your board and nothing bad will happen this is the only 0:03:39.060,0:03:44.970 warning I have for the whole lecture in this example I showed you a really 0:03:44.970,0:03:49.829 simple way in which you can take the sensors run i2cdetect, detect its 0:03:49.829,0:03:59.370 address in this case it's 68 - and then - load 1 kernel module and all of the 0:03:59.370,0:04:03.889 sudden your Raspberry Pi will have battery backed clock, just like your laptop does 0:04:03.889,0:04:12.569 but how did this journey all started for me? about two years ago I was very 0:04:12.569,0:04:18.060 unsatisfied with the choice of pinouts which you can download from the internet 0:04:18.060,0:04:23.820 I was thinking something along the lines wouldn't be it wouldn't it be nice if I 0:04:23.820,0:04:27.060 could print the pinout for any board I have 0:04:27.060,0:04:36.360 with perfect 2.54 millimeters pin spacing which I can put beside my pins 0:04:36.360,0:04:43.440 and never make a mistake of plugging the wire in the wrong pin and we all know 0:04:43.440,0:04:48.000 that plugging the wire in the wrong pin is always the first problem you have on 0:04:48.000,0:04:54.120 the other hand you say oh this is the great idea and you are looking at your 0:04:54.120,0:04:58.320 pin out which is from the top of the board and you are plugging the wires 0:04:58.320,0:05:02.160 from the bottom of the board and all of the sudden your pin out has to be 0:05:02.160,0:05:06.660 flipped but once you write a script which actually displays the pin out it's 0:05:06.660,0:05:12.950 trivially easy to get to add options to flip it horizontally or vertically and 0:05:12.950,0:05:18.870 create either black and white pin out if you are printing it a laser or color pin 0:05:18.870,0:05:26.639 out if you are printing it on some kind of inkjet so once you have that SVG 0:05:26.639,0:05:31.680 which you can print and cut with the scissors and so on it's just a script on 0:05:31.680,0:05:38.370 your machine so you could also have the common line output and then it went all 0:05:38.370,0:05:44.070 south I started adding additional data which you can see on this slide in 0:05:44.070,0:05:50.250 square brackets with the intention of having additional data for each pin for 0:05:50.250,0:05:56.250 example if I started SPI I want to see that this pin is already used so I will 0:05:56.250,0:06:01.830 not by mistake plug something into the SPI pins if I already have the SPI pin 0:06:01.830,0:06:08.150 started if I have the serial port on different boards your serial might be 0:06:08.150,0:06:14.880 UART4 on this particular CPU but it's the only serial in your Linux system so 0:06:14.880,0:06:21.270 it will be /dev/ttyS0 for example so I wanted to see all the data 0:06:21.270,0:06:27.180 and in the process I actually saw a lot of things which kernel know and I didn't 0:06:27.180,0:06:33.690 so today talking to you about it of course in comment line because you might 0:06:33.690,0:06:36.990 rotate your board well plug in the wires you can also do 0:06:36.990,0:06:45.540 all the flips and things you you already saw in the in the graphic part so let's 0:06:45.540,0:06:50.190 start with the sensor okay I said cheap sensor for eBay we'll get the cheap 0:06:50.190,0:06:54.840 sensors from eBay but this sensor is from some old PowerPC Macintosh. It was 0:06:54.840,0:06:59.580 attached to the disk drive and the Macintosh used it to measure the 0:06:59.580,0:07:04.890 temperature of the disk drive. you know that was in the times before the smart 0:07:04.890,0:07:12.060 had a temperature and I said hmm this is the old sensor, kernel surely 0:07:12.060,0:07:16.800 doesn't have support for it, but, oh look, just grep through the kernel 0:07:16.800,0:07:21.900 source and indeed there is a driver and this was a start I said hmm 0:07:21.900,0:07:29.580 driver in kernel, I don't have to use Arduino for it - now that I know that 0:07:29.580,0:07:35.070 driver is there and I have kernel module compiled, we said that prerequisite 0:07:35.070,0:07:40.950 is that we can compile the kernel, what do we actually have to program or do to 0:07:40.950,0:07:46.260 make this sensor alive? not more than this a echo in the middle of the slide 0:07:46.260,0:07:52.200 you just echo the name of the module and the i2c address the i2c addresses we saw 0:07:52.200,0:07:56.070 it before we can get it with i2cdetect by just connecting the sensor and 0:07:56.070,0:08:02.280 the new device will magically appear it will be shown in the sensors if you have 0:08:02.280,0:08:06.840 lm-sensors package installed but if you don't you can always find the same 0:08:06.840,0:08:14.160 data in /sys/ file system which is full of wonders and as we'll see in non 0:08:14.160,0:08:19.350 formatted way so the first two digits actually the last three digits digits 0:08:19.350,0:08:26.640 are the decimal numbers and the all the other are the the integer celsius in 0:08:26.640,0:08:34.470 this case. but you might say - I don't want to put that echo in my startup 0:08:34.470,0:08:40.380 script! or I would like to have that as soon as possible I don't want to depend 0:08:40.380,0:08:43.980 on the userland to actually start my sensor and believe 0:08:43.980,0:08:48.710 it or not because your smart phones have various sensors in 0:08:48.710,0:08:52.790 them, there is a solution in the Linux kernel for that and it's called the device tree 0:08:52.790,0:08:58.700 so this is probably the simplest form of the device tree which still doesn't look 0:08:58.700,0:09:06.020 scary, but it i will, stay with me, and it again defines our module the address 0:09:06.020,0:09:12.020 which is 49 in this case and i2c 1 interface just like we did in that echo 0:09:12.020,0:09:16.700 but in this case this module will be activated as soon as kernel starts up 0:09:16.700,0:09:24.590 as opposed to the end of your boot up process. one additional thing that kernel 0:09:24.590,0:09:31.000 has and many people do not use is ability to load those device trees 0:09:31.000,0:09:35.750 dynamically the reason why those most people don't use it is because they are 0:09:35.750,0:09:41.270 on to old kernels I think you have to have something along the lines of 4.8 0:09:41.270,0:09:47.510 4.8 or newer to actually have the ability to load the device trees live 0:09:47.510,0:09:54.470 basically you are you are using /sys/kernel/config directory and this script 0:09:54.470,0:10:00.620 just finds where you have it mounted and loads your device tree live word of 0:10:00.620,0:10:09.200 warning currently although it seems like you can do that on Raspberry Pi the API 0:10:09.200,0:10:14.270 is there the model is compiled, everything is nice and Diddley, kernel 0:10:14.270,0:10:18.350 even says that device tree overlay is applied, it doesn't work on the raspberry 0:10:18.350,0:10:24.950 pi because raspberry pi is different but if you gave a any other platform live 0:10:24.950,0:10:31.670 loading is actually quite nice and diddley. so now we have some sensor and it 0:10:31.670,0:10:37.310 works or it doesn't and we somewhat suspect that kernel 0:10:37.310,0:10:43.040 developers didn't write a good driver which is never ever the case if you 0:10:43.040,0:10:48.260 really want to implement some driver please look first at the kernel source 0:10:48.260,0:10:52.220 tree there probably is the implementation better than the one you 0:10:52.220,0:10:56.930 will write and you can use because the kernel is GPL you can use that 0:10:56.930,0:11:00.970 implementation as a reference because in my 0:11:00.970,0:11:05.890 small experience with those drivers in kernel they are really really nice but 0:11:05.890,0:11:10.960 what do you do well to debug it? I said no soldering and I didn't say but 0:11:10.960,0:11:15.010 it would be nice if I could do that without additional hardware. Oh, look 0:11:15.010,0:11:20.890 kernel has ability to debug my i2c devices and it's actually using tracing. 0:11:20.890,0:11:26.140 the same thing I'm using on my servers to get performance counters. isn't that 0:11:26.140,0:11:30.340 nice. I don't have to have a logic analyzer. I can just start tracing and 0:11:30.340,0:11:40.120 have do all the dumps in kernel. nice, unexpected, but nice! so let's get to the 0:11:40.120,0:11:45.790 first cheap board from China so you bought your arm single base computer 0:11:45.790,0:11:52.030 single board computer and you want to add few analog digital converters to it 0:11:52.030,0:11:57.070 because you are used to Arduino and you have some analog sensor or something and 0:11:57.070,0:12:05.620 you found the cheapest one on eBay and bought few four five six because they 0:12:05.620,0:12:10.990 are just the $ each, so what do you do the same thing we saw earlier you just 0:12:10.990,0:12:17.800 compile the models say the address of the interface and it will appear and 0:12:17.800,0:12:23.770 just like it did in the last example so everything is nice but but but you read 0:12:23.770,0:12:30.790 the datasheet of that sensor and the sensor other than 4 analog inputs also 0:12:30.790,0:12:38.860 has 1 analog output which is the top pin on the left denoted by AOUT, so you 0:12:38.860,0:12:42.850 want to use it oh this kernel model doesn't is not very 0:12:42.850,0:12:46.900 good it doesn't have ability to control that of course it does but how do you 0:12:46.900,0:12:50.680 find it? my suggestion is actually to search 0:12:50.680,0:12:57.190 through the /sys/ for either address of your i2c sensor, which is this 0:12:57.190,0:13:05.790 case is 48, or for the word output or input and you will actually get all 0:13:05.790,0:13:12.400 files, because in linux everything is a file, which are defined in driver of this 0:13:12.400,0:13:16.620 module, and if you look at it, there is actually out0_output 0:13:16.620,0:13:24.630 out0_output file in which you can turn output on or off so we are all 0:13:24.630,0:13:29.910 golden. kernel developers didn't forget to implement part of the driver for this 0:13:29.910,0:13:38.910 sensor all golden my original idea was to measure current consumptions of arm 0:13:38.910,0:13:44.490 boards because I'm annoyed by the random problems you can have just because your 0:13:44.490,0:13:49.470 power supply is not powerful enough so it's nice actually to monitor your power 0:13:49.470,0:13:55.740 usage so you will see what changes, for example, you surely... we'll get that, remind 0:13:55.740,0:14:01.250 me to tell you how much power does the the additional button take 0:14:01.250,0:14:05.130 that's actually interesting thing which you wouldn't know if you don't measure 0:14:05.130,0:14:10.950 current so you buy the cheapest possible eBay sensor for current, the right one 0:14:10.950,0:14:16.560 is the ina219 which is bi-directional current sensing so you 0:14:16.560,0:14:21.690 can put it between your battery and solar panel and you will see 0:14:21.690,0:14:26.160 whether the battery is charging or discharging, or if you need more channels 0:14:26.160,0:14:34.020 I like an ina3221 which has 3 channels, the same voltage 0:14:34.020,0:14:40.500 but 3 different channels, so you can power 3 arm single computers from one 0:14:40.500,0:14:47.670 sensor if you want to. and of course once you have that again the current is in 0:14:47.670,0:14:53.260 some file and it will be someting... 0:14:53.260,0:14:56.490 But, I promised you IOT? right? nothing 0:14:56.490,0:14:59.070 I said so far is IOT! where is the Internet? 0:14:59.070,0:15:05.100 where are the things? buzzwords are missing! OK, challenge 0:15:05.100,0:15:08.550 accepted! Let's make a button! you know it's like a 0:15:08.550,0:15:15.029 blink LED. So buttons, because I was not allowed to use soldering iron in 0:15:15.029,0:15:21.990 this talk, I'm using old buttons from old scanner. nothing special 3 buttons, in 0:15:21.990,0:15:25.350 this case with hardware debounce, but we don't care. 0:15:25.350,0:15:29.270 4 wires 3 buttons. how hard can it be? 0:15:29.270,0:15:36.240 well basically it can be really really simple this is the smallest 0:15:36.240,0:15:42.638 font so if you see how to read this I congratulate you! in this case I am 0:15:44.580,0:15:51.360 specifying that I want software pull up, in this first fragment on the top. 0:15:51.360,0:15:57.840 I could have put some resistors, but you said no soldering 0:15:57.840,0:16:03.540 so here I am telling to my processor. please do pull up on 0:16:03.540,0:16:09.540 those pins. and then I'm defining three keys. as you can see email. connect and 0:16:09.540,0:16:15.660 print. which generate real Linux keyboard events. so if you are in X and 0:16:15.660,0:16:20.730 press that key, it will generate that key, I thought it would it would be better to 0:16:20.730,0:16:26.400 generate you know the magic multimedia key bindings as opposed to A, B and C 0:16:26.400,0:16:31.940 because if I generated a ABC and was its console I would actually generate 0:16:31.940,0:16:39.140 letters on a login prompt which I didn't want so actually did it and it's quite 0:16:39.140,0:16:47.310 quite easy. In this case I'm using gpio-keys-polled which means that my CPU 0:16:47.310,0:16:52.500 is actually pulling every 100 milliseconds those keys to see whether 0:16:52.500,0:16:57.150 they their status changed and since the board is actually connected through the 0:16:57.150,0:17:01.890 current sensing - sensor I mentioned earlier I'm 0:17:01.890,0:17:11.209 getting additional: how many mA? every 100 milliseconds, pulling 3 keys? 0:17:13.010,0:17:19.350 60! I wouldn't expect my power consumption to rise by 60 milliamps 0:17:19.350,0:17:24.480 because I am pulling 3 keys every 100 milliseconds! but it did and because I 0:17:24.480,0:17:32.430 could add sensors to the linux without programming drivers, I know that! why am I 0:17:32.430,0:17:40.450 using polling because on allwinner all pins are not interrupt capable so in 0:17:40.450,0:17:44.520 the sense all pins cannot generate interrupts on allwinner 0:17:44.520,0:17:48.940 raspberry pi in this case is different on raspberry pi every pin can be 0:17:48.940,0:17:54.279 interrupt pin on Allwinner that is not the case. so the next logical question is 0:17:54.279,0:17:58.510 how do I know when I'm sitting in front of my board whether the pin can get the 0:17:58.510,0:18:05.440 interrupt or not? my suggestion is ask the kernel. just grap through the debug 0:18:05.440,0:18:10.720 interface of the kernel through pinctrl which is basically the the thing 0:18:10.720,0:18:19.570 which configures the pins on your arm CPU and try to find the irq 0:18:19.570,0:18:25.840 will surely get the list of the pins which are which are irq capable take in 0:18:25.840,0:18:30.370 mind that this will be different on different arm architectures so 0:18:30.370,0:18:34.419 unfortunately on allwinner it will always look the same because it 0:18:34.419,0:18:41.020 is allwinner architecture. actually sunxi ! but on the Raspberry Pi for 0:18:41.020,0:18:45.880 example this will be somewhat different but the kernel knows, and the grep is 0:18:45.880,0:18:51.490 your friend. so you wrote the device three you load it either live or some 0:18:51.490,0:18:57.279 something on some other way you connect your buttons and now let's try does it really 0:18:57.279,0:19:03.580 work? of course it does! we will start evtest which will show us all the 0:19:03.580,0:19:08.350 input devices we have the new one is the gpio-3-buttons, which is the same name 0:19:08.350,0:19:14.559 as our device three overlay and we can see the same things we saw in 0:19:14.559,0:19:20.230 device three we defined three keys with this event but we free-of-charge 0:19:20.230,0:19:25.990 got for example keyboard repeat because this is actually meant to be used for 0:19:25.990,0:19:31.630 keyboards our kernel is automatically implementing repeat key we can turn it 0:19:31.630,0:19:35.919 off but this is example of one of the features which you probably wouldn't 0:19:35.919,0:19:43.330 implement yourself if you are connecting those three keys to your Arduino but but 0:19:43.330,0:19:46.120 but this is still not the internet-of-things 0:19:46.120,0:19:49.789 if this is the Internet it should have some kind of 0:19:49.789,0:19:56.299 Internet in it some buzzwords for example mqtt and it really can just 0:19:56.299,0:20:01.759 install trigger-happy demon which is nice deamon which listens to 0:20:01.759,0:20:06.830 input events and write a free file configuration which will send the each 0:20:06.830,0:20:15.139 key pressed order MQTT and job done i did the internet button without a line 0:20:15.139,0:20:23.419 of code the configuration one side note here if you are designing some some 0:20:23.419,0:20:30.919 Internet of Things thingy which even if you are only one who will use it it's a 0:20:30.919,0:20:33.889 good idea but if you are doing it for somebody else 0:20:33.889,0:20:41.119 please don't depend on the cloud because I wouldn't like for my door to be locked 0:20:41.119,0:20:45.590 permanently with me outside just because my internet connection isn't working 0:20:45.590,0:20:52.999 think about it of course you can use any buttons in this case this was the first 0:20:52.999,0:20:57.070 try actually like the three buttons better than this one that's why this 0:20:57.070,0:21:02.419 these buttons are coming second and this board with the buttons has one 0:21:02.419,0:21:08.840 additional nice thing and that is the LED we said that will cover buttons and 0:21:08.840,0:21:15.379 LEDs right unfortunately this LED is 5 volts so it won't light up on 3.3 volts 0:21:15.379,0:21:24.139 but when you mentioned LEDs something came to mind how can I use those LEDs 0:21:24.139,0:21:29.029 for something more useful than just blinking them on or off we'll see you 0:21:29.029,0:21:34.039 later then turning them on or off it is also useful you probably didn't know 0:21:34.039,0:21:40.580 that you can use Linux triggers to actually display status of your MMC card 0:21:40.580,0:21:47.720 CPU load network traffic or something else on the LEDs itself either the LEDs 0:21:47.720,0:21:51.859 which you already have on the board but if your manufacturer didn't provide 0:21:51.859,0:21:56.679 enough of them you can always just add random LEDs, write device tree and 0:21:56.679,0:22:01.559 define the trigger for them, and this is an example of that 0:22:01.559,0:22:08.219 and this example is actually for this board which is from the ThinkPad 0:22:08.219,0:22:14.019 ThinkPad dock to be exact, which unfortunately isn't at all visible 0:22:14.019,0:22:20.169 on this picture, but you will believe me, has actually 2 LEDs and these 3 0:22:20.169,0:22:27.489 keys and 2 LEDs actually made the arm board which doesn't have any buttons on 0:22:27.489,0:22:36.609 it or status LEDs somewhat flashy with buttons. that's always useful on the 0:22:36.609,0:22:41.889 other hand here I would just want to share a few hints with you for a start 0:22:41.889,0:22:47.320 first numerate your pins because if you compared the the picture down there 0:22:47.320,0:22:54.579 which has seven wires and are numerated which is the second try be the first 0:22:54.579,0:23:00.579 nodes on the up you will see that in this case i thought that there was eight 0:23:00.579,0:23:06.999 wires so the deducing what is connected where when you have the wrong number of 0:23:06.999,0:23:14.229 wires is maybe not the good first step on the other hand we saw the keys what 0:23:14.229,0:23:21.639 about rotary encoders? for years I was trying to somehow persuade Raspberry Pi 0:23:21.639,0:23:28.149 1 as the lowest common denominator of all arm boards you know cheap slow and 0:23:28.149,0:23:33.789 so on to actually work with this exact rotary encoder the cheapest one from the 0:23:33.789,0:23:41.589 Aliexpress of course see the pattern I tried Python I try the attaching 0:23:41.589,0:23:48.579 interrupt into Python I tried C code and nothing worked at least didn't work 0:23:48.579,0:23:56.859 worked reliably and if you just write the small device tree say the correct 0:23:56.859,0:24:01.809 number of steps you have at your rotary encoder because by default it's 24 but 0:24:01.809,0:24:08.589 this particular one is 20 you will get perfect input device for your Linux with 0:24:08.589,0:24:11.579 just a few wires 0:24:12.390,0:24:20.260 amazing so we saw the buttons we saw the LEDs we have everything for IOT except 0:24:20.260,0:24:25.840 the relay so you saw on one of the previous pictures this relay box it's 0:24:25.840,0:24:31.980 basically four relays separated by optocouplers which is nice and my 0:24:31.980,0:24:39.670 suggestion since you can't in device three you can't say this pin will be 0:24:39.670,0:24:45.790 output but I want to initially drive it high or I want to initially drive it low 0:24:45.790,0:24:52.330 it seems like you can say that it's documented in documentation it's just 0:24:52.330,0:24:56.440 not implemented there on every arm architecture so you can write it in 0:24:56.440,0:25:02.440 device three but your device tree will ignore it. you but you can and this might be 0:25:02.440,0:25:07.240 somewhat important because for example this relay is actually powering all your 0:25:07.240,0:25:11.220 other boards and you don't want to reboot them just because you reboot the 0:25:11.220,0:25:16.270 machine which is actually driving the relay so you want to control that pin as 0:25:16.270,0:25:24.310 soon as possible so my suggestion is actually to explain to Linux kernel that 0:25:24.310,0:25:30.310 this relays is actually 4 LEDs which is somewhat true because the relay has the 0:25:30.310,0:25:36.460 LEDs on it and then use LEDs which do have the default state which works to 0:25:36.460,0:25:41.410 actually drive it as soon as possible as the kernel boot because kernel will boot 0:25:41.410,0:25:45.640 it will change the state of those pins from input to output and set them 0:25:45.640,0:25:52.000 immediately to correct value so you hopefully want want power cycle your 0:25:52.000,0:25:56.410 other boards, and then you can use the LEDs as you would normally use them in 0:25:56.410,0:26:01.420 any other way if LEDs are interesting to you have in 0:26:01.420,0:26:06.340 mind that on your on each of your computers you have at least two LEDs but 0:26:06.340,0:26:10.810 this caps lock and one is non lock on your keyboard and you can use those same 0:26:10.810,0:26:15.190 triggers I mentioned earlier on your existing Linux machine using those 0:26:15.190,0:26:20.980 triggers so for example your caps lock LED can blink as your network traffic 0:26:20.980,0:26:29.380 does something on your network really it's fun on the other hand if you have a 0:26:29.380,0:26:36.370 Raspberry Pi and you defined everything correctly you might hit into some kind 0:26:36.370,0:26:42.370 of problems that particular chip has default pull ups which you can't turn on 0:26:42.370,0:26:49.059 for some pins which are actually designed to be clocks of this kind or 0:26:49.059,0:26:54.220 another so even if you are not using that pin as the SPI clock whatever you 0:26:54.220,0:26:59.110 do in your device tree you won't be able to turn off, actually you will turn 0:26:59.110,0:27:04.240 off the the setting in the chip to for the pull up but the pull up will be 0:27:04.240,0:27:08.140 still there actually thought that there is a hardware resistor on board but 0:27:08.140,0:27:14.980 there isn't it's inside the chip just word of warning so if anything I would 0:27:14.980,0:27:21.309 like to push you towards using Linux kernel for the sensors which you might 0:27:21.309,0:27:25.570 not think of as the first choice if you just want to add some kind of simple 0:27:25.570,0:27:32.080 sensor to to your Linux instead of Arduino over serial port which I did and 0:27:32.080,0:27:38.049 this is the solution which might with much less moving parts or wiring pie and 0:27:38.049,0:27:43.090 in the end once you make your own shield for us but if I you will have to write 0:27:43.090,0:27:49.480 that device tree into the EEPROM anyway so it's good to start learning now so I 0:27:49.480,0:27:53.890 hope that this was at least useful very interesting and if you have any 0:27:53.890,0:28:02.140 additional questions I will be glad to to answer them and if you want to see 0:28:02.140,0:28:07.780 one of those all winner board which can be used with my software to show the pin 0:28:07.780,0:28:12.460 out here is one board which kost actually borrowed me yesterday and in which 0:28:12.460,0:28:18.549 yesterday evening I actually ported the my software which is basically just 0:28:18.549,0:28:22.330 writing the definition of those pins over here and the pins on the header 0:28:22.330,0:28:26.950 which I basically copy pasted from the excel sheet just to show that you can 0:28:26.950,0:28:32.830 actually do that for any board you have with just really pin out in textual file 0:28:32.830,0:28:36.370 it's really that simple here are some additional 0:28:36.370,0:28:38.300 links and do you have any questions? 0:28:39.420,0:28:41.420 [one?]