#sggeek

Xbee basics - FRAMES 

Just like every sentence has a structure, The way Xbee’s communicate also has a structure. This structure is called a FRAME. It looks typically like this :

7E 00 14 92 00 13 A2 00 40 CC 86 29 02 A4 01 01 00 10 00 00 10 F5

A sentence is usually identified by a capital letter in the beginning, ending with a full stop. For Xbee, the sentence structure is first consisting of a number of bytes. The start byte, is 0x7E (also referred commonly as 7E, i’ll use the “0x” prefix generously as most tutorials you see will do that too). 

The next 2 bytes indicate the length of the whole frame. So you can say that 7E is the capital letter of a sentence, while 00 14 indicates how long this sentence is. For the example above 00 14 adds up in Hex as 14, which is 18 in decimal ( the usual number system we use ). 

We can now start counting the number of bytes after 14. You’ll realize that there are actually 19 bytes!! Why this is because the last byte, F5 is actually called a CHECKSUM. Turns out , most electronic hardware are really smart, when they send data to each other, they try to make sure that the data is correct. There’s a way this checksum is calculated, but lets keep that at the back of our minds right now.

The next byte after the length, is 92. This is the the FRAME TYPE, it explains what the rest of the frame might be. For example, if your 4th byte is 92, it means it is getting IO input from one of the pins that is connected to the Xbee. There is a list of different Frame type numbers, each frame type has a different meaning.   For the full list of meanings, check it out here : https://docs.digi.com/display/XBeeZigBeeMeshKit/Frame+structure So you can say that the Frame type is like telling us the type of sentence this is.   After frame type comes the 64bit address of the Xbee. Its like a serial number of sorts. It usually starts with 00 13 A2 00 (As most Xbees will have the same first 4 bytes) then for the above example, the next 4 bytes are 40 CC 86 29. These 8 bytes (64 bits) make up the serial number of the Xbee! 

The next 2 bytes are an internal IP of sorts. 02 A4 is what the coordinator Xbee issued to this Xbee. So if you think of the Coordinator Xbee as your home WIFI router, it issues any device connected to it an IP address. Your phone/ computer for example, will have an internal IP of 192.168.1.12. This is an internal IP within your home LAN which is different from how you look like to the rest of the internet. The same is happening to Xbee. 02 A4 is a random “IP of sorts” that has been issued to it. 

The rest of the bytes 01 01 00 10 00 00 10. Is telling you what pins you’ve configured to receive data or transmit data. Xbee has about 10pins depending on which model you get, and these pins can capture either a HIGH LOW , digital in/out put, as well as ANALOG, maybe temperature or humidity in/output. These bytes are an indication if out of those pins, which ones are turned on and which ones are turned off, you can set it all in XCTU, you probably just need to know the last byte. Which is typically the data that you’re trying to capture. For our case, its 10! which means I’ve received a HIGH on one of my digital pins. 

the last byte F5 is the checksum. Remember when I told you to keep checksum at the back of your mind. Well, right now, we’ve almost reached the end of this frame. The checksum adds up all the bytes from the 4th byte to the 2nd last byte. We’ll tentatively call this amount CKS. The equation is so :

0xFF - 0xCKS = <checksum value>

For our case all the bytes from byte 4 to 2nd last byte , taken away from FF is actually F5. So our sentence has officially ended.

The Learning Curve 

To be honest the Xbee has been a steep learning curve and this few weeks has been one of the toughest yet. This post is more of a “note-to-self“. Being faced with new hardware, and a gargantuan task of learning about it and programming it into a viable product. There have been many-a-time I thought of giving up. Thoughts of  “I am probably not the most qualified person to take up this job” came up.  It got me thinking about how we should all proceed when faced with such a task. Not only, how to continue learning, but how to pace yourself and come up with strategies when u know you probably will not be able to understand everything at one go.  First, you have to understand how you learn. For me, ( i’m not sure if it resembles you too ) but you first start out with a really simple tutorial. The tutorial promises a very interesting effect with very minimal work. Basically just copy the code and copy the set-up and everything will work. And true enough 100% of the time it does.  Then I start to expand this knowledge. I’ll copy it a few times, change an attribute here and there, see if it works. This is my exploration phase. Where I am still using the tutorial’s code, but I just switch things around. Unsure of where to place what and how the same code will still work but with some of my own input. Then at stage 3 (the Head-banging phase as I call it), I’ll say “ i need it to do XXX YYY” and I’ll try to copy code here and there, switch attributes to variables. Then this is mostly when I hit my first brick wall. Often I’ll see too far and forget to realize that I have yet to grasp the basics yet. Yes yes, the code works in the tutorial, but my own case is rather specific, and furthermore, I have to understand how to WRITE the code properly first. My brain tells me to stop and read, but my fingers will just keep on typing , substituting “int” for “byte” for “unsigned int” indiscriminately just to see what works. And if it works, I’ll totally forget about it and go on to some other things. So , after stage 3, (which usually drags on for quite awhile) I’ll move on to the RTFM ( Read the Fuckin’ Manual ) stage. There are too many things that I don’t know and the only way around it is RTFM. This stage is usually the longest, as I have to know what I don’t know, which sometimes... you really don’t know because you have yet to know it yet. =/ This is where I’m stuck now. In between wanting to do something, and the lacking of enough knowledge to execute it properly. I try often to keep asking forums, friends, and RTFM at the same time. But there really is a big gap between knowing somethings and final delivery. And in forums, I even have a problem writing down the problem because I have no idea how to describe it and hence, back to RTFM. 

TIL (Today I learned) -- XBee and SgGeek 

I’ve been asked a few times these few weeks about what I was going to do with SgGeek, and the truth is, I have NO IDEA!! SgGeek was a natural progression. It started out with finding out about IoT ( Internet of Things ) from Instructables, then getting an arduino kit, then a raspberry pi, then setting up media servers for myself, then to friends, simple coding on python, finding out about networking etc. Thereafter it moved on to Retrogaming for myself, and then for friends, and then for anyone else who wanted it.  I just went with whatever interests me and at the heart of it all, it was because I wanted to know more about tech and blinking lights haha (> __<) . So SgGeek, will always be about learning. Whatever business that spins out of it, whatever money I make from this, it wasn’t exactly intentional, but they certainly help me on my journey for knowledge. On top of it, there’s also a sense of fulfillment when I deploy some open-source project on a client’s house. Or I see that they are really happy with a project I made. Or I make new friends, they all add up. 

I was quite lucky to meet a friend/teacher recently who introduced me to Xbee. Therefore these few months, I will be learning and writing about this uber interesting device. A short introduction is, Xbee is a tiny tiny microcontroller ( about 3x3cm ) that is able to emit and receive radio signals over a long range (abt 30-90m). Sometimes you see Routers saying 802.11b or 802.11ac things like that, ya? Those are radio frequency standards. WIFI is a kind of radio wave, 802.11a is a kind of radio wave standard that is used to transfer data to through your router. Xbee is also a kind of radio emitter and receiver but they operate in the 802.15.4 standard. 

But why have a bulky router when you have this small one that can also send and receive radio waves? The trade off is bandwith for power consumption and flexibility. Xbee is extremely low powered, very small (mobile), and can be deployed easily with battries! (yes a battery operated router is possible).Technically, If you have a friend who’s 180m away from you, you’ll need 3 XBee Pros, running on batteries in btwn you 2, and you can wirelessly control anything in his house, lag-free. But the down side is bandwidth. The current iteration can transmit 100 kbit/s, which, through an online calculator, is about 12.5kbps. Your 3mb(3000kb) mp3 file transfer might take... about 4 minutes? (an eternity ... i know right?!)

As such Xbees are usually deployed for embeded devices. Often seen in home automation, eg: when owner comes near an xbee, maybe with a PIR(passive infra-red) sensor, turn a light on / play some music / start the dishwasher / burn the house etc.  

Getting a DD-WRT Router cheaply

Being my virgin try on flashing my own router’s firmware, I did some research. you can check out DD-WRT’s website, there’s a huge list of routers that you can flash their firmware and with ease, really. Each router has their own wiki page which explains how to do it. So here’s the checklist on how I selected my router.

1) Firmware must be on DD-WRT Website

2) Must have Gigabit LAN ports.

3) musts be under $100

4) Must be tested and proven in Singapore’s fiber environment

Number 4 is kind of just a safety step as while I was shopping in Challenger, I saw quite a few routers with stickers like “Singtel fiber supported” or “works on malaysia fiber network”. Being a little noob, I wanted to be extra sure.

But to cut the story short, I found the TP-Link Archer C2 $89  and D-Link DIR816L for $79

The only issue is, they both don’t run DD-WRT. The other TP-Link router which can be DD-WRT-ed is Archer C8 and C9 which costs around $200 - $300.