Strobotics

My sample order of the RFM22 were waiting for me when I arrived home late last night.

Here is a picture of the RFM22 (Left) next to the RFM12B (Right).  Pretty much the same physical size, but more pins, and a lot of very small discrete components.

So What’s New?

These new transceiver modules from HopeRF appear to be based on Silicon Labs Si4431 and Si4432 RF Chips.  Silicon Labs purchased Integration, from where the RFM12B were based on their EzRadio Series of transceiver chips.

It looks like there are a heap of new feature available, (too many to list here), including some onboard lower MAC smarts called EzyMac, while not as nice as the 802.15.4 MAC layer it looks like it can simplify things such as:

• Automatically adding pre-amble and sync bytes.
• Automatic packet size – you just push bytes into the Tx FIFO and it will create the packets of a fixed size and send for you.
• Built in basic frequency hopping.
• Built in Data Whitening, Manchester Encoding, and CRC

Some more niceties:

• 64Byte FIFO.
• Onboard A/D, allows access to read such things as an onboard Temperature Sensors, Voltage buses.
• x2 configurable GPIO ports.
• More Rx Sensitivity than the RFM12B.
• More Tx power than the RFM12B
• Lower Operating voltage.
• 8bit RSSI value.
• Three different modulation schemes to select from.

Some not so niceties:

• More current draw on Rx than the RFM12B.
• More current draw on Tx than the RFM12B (understandable seeing it also has a higher Tx Power).
• Costs about twice as much as the RFM12B ~ $USD6.00 in sample quantities direct from HopeRf vs ~$USD3.00 for the RFM12B.
• Heaps more commands/Registers to learn.
• No onboard encryption.

Feature comparison:

For the next version of the widget,  I’m planning to have an on board boost regulator.  This design will allows the widget to run from just about any voltage source as low as 0.7v (so NiMh will be fine) and up to 5.5v, this boost regulator in conjunction with a standard LDO regulator will give me a nice input voltage range of 0.7v – 12V.  Before I finalise the design I wanted to test things to see how well it’ll work. 

I ordered in a couple of battery holders from Polou with an integrated boost regulator, since the widgets are designed for 3.3v operation I ordered this model http://www.pololu.com/catalog/product/796,  that provides a 3.3v 100mA supply from a single AA battery, I’m not too sure what the minimum voltage is, but is running fine on a single NiMh cell.

Well today they arrived and I now have one hooked up to my solar powered widget.  Installation was simple as the small pcb on the rear of the battery holder allows me to direct access to the cell contacts, so I soldered the solar cell +ve and –ve on the respective contacts.

From some previous tests with my solar powered widget, I was finding that the batteries would start charging when the solar cell output reached around 2.8 –2.9v, this is fine for during the day with the solar cell in direct sunlight, now with the single cell I’m finding that inside under artificial light I’m charging at around 1.5v, with plenty of overhead left.

You can see below the voltage output of the boost regulator is ~3.3v (3.322 to be exact) and the voltage from my solar cell is ~1.52v, this is inside under some standard down lights while writing this post at the kitchen table (actually one of the down lights is dead so I could expect a little more output from the cell)

Just to try something different I removed the battery, seeing if the regulator could run direct from the solar cell, unfortunately it doesn’t, I doubt the cell in the current light conditions can provide enough start up current for the regulator,  I will however try it in the natural sunlight tomorrow.

With the addition of the boost regulator, and the widget in max power savings between sensor reading (when asleep widget is drawing 1.5uA, which is most of the time) then the widget should run indefinitely from the single cell….or maybe I could just get away with a solar cell and a super cap now there’s a future project….

The other day while playing around with the ADC on the widget I had some problems with noise causing the reading to bounce around, It needed some form filtering to remove the noise.   Normally almost everyone seems to take a number of readings and then average the result, unfortunately this takes time.

I came across this simple low pass filter on EDN.  A Low pass filter works by blocking the higher frequency (in this case noise) and allowing the low frequency signal (in this case the underlying signal) to pass. hence the name.  Perfect in cases where measuring slow moving signals, like temperature etc.  The algorithm is fast, simple to implement and works a treats.  It can be made to filter faster for a more response to change or slower to give more filtering.

#define FILTER_SHIFT 3

int32_t filter_reg;

int16_t filter_input;

int16_t filter_output;

filter_input = analogRead(0);                 // read ADC

// low pass filter to get rid of noise

filter_reg = filter_reg - (filter_reg >> FILTER_SHIFT) + filter_input; 

filter_output = filter_reg >> FILTER_SHIFT;   

The algorithm is a classified as a leaky integrator, where the latest reading have more weight than the older reading.  So you take a single reading each cycle of the sensor loop and feed that into the filter, the output is used you would normally use the raw value.

The response of the filer can be adjusted by the FILTER_SHIFT, to make it more responsive, then lower this value, to give better filtering and hence a slower response then increase this value.  For my application I found that a value of 3 to 4 worked well, but will depend on the underlying application.

I suggest you read the full article for an in depth description.

In my previous blog entry I described adding a solar cell to the widget and how I could measure the voltage produced by the Solar Cell, however I still need to monitor actual battery voltage easily, and preferably with not a lot of additional hardware.

Problems reading ones own voltage.

Measuring ones own voltage has a couple of problems:

1. You can’t just measure VCC directly, i.e. you can’t connect an analog pin to VCC.  The reason is the battery voltage varies, and since the default AREF is tied to VCC you will always read VCC unless you use some form of a reference not tied to VCC.

2. I can also use a reference diode and a resistor between VCC and GND, to form an external reference voltage, the diode has a known voltage drop across it so this can be measured using the default AREF.  We can calculate the battery voltage by measuring the volt drop across the diode.  While this will work, the circuit will always consume a small amount of power, even when not being used, remember I want to get as much life out of my batteries as possible.  So it rules this out as a solution.

3. For the same reasons as 1) I can’t use a simple voltage divider and the default AREF.  As the voltage varies so too will my readings.

4. I can use a simple voltage divider and the internal 1.1v bandgap reference for my AREF, the same as I used for my solar cell voltage monitor in the previous blog entry.  Again this will work, but also as in 2) this will always consume  some power even when not being used, so is ruled out as a solution in this case.

No Extra components?

We can however measure VCC voltage with no external components by simply reading the value of the internal bandgap reference voltage (1.1v) which is always fixed regardless of the battery voltage,  and by using the default AREF which is tied to VCC, so will vary as the battery voltage changes.  We now have a way of measuring the voltage change against a fixed reference.

V_BAT =  (VREF * 1024) / analogRead(14);

Where:

You might see in the above formula, that analogRead() function is reading channel 14.  But analogRead() only supports channels 0-7 right? 

Correct, but what we are doing is to read the internal ADC channels after we make a small change to the Arduino source code.

Changes to “wiring_analog.c”

To take an ADC measurement of  the internal bandgap we need to switch to ADC MUX channel 14 (23.8.1 in reference manual).   However to do this with the arduino wiring library we need to make changes to “wiring_analog.c” as per http://code.google.com/p/arduino/issues/detail?id=10.  This change allows us to read from all ADC MUX channels, internal and external.

Example

Once you’ve made the change to “wiring_analog.c” then use the following example sketch to read your VCC/Battery Voltage.

uint16_t raw_bandgap = 0;      // Internal bandgap reference

float volt_battery = 0.0;

void setup(){

  Serial.begin(57600);

}

void loop(){

  // Read Battery voltage

  analogReference(DEFAULT);                          // use VCC as AREF

  raw_bandgap = analogRead(14);                      // read and discard first result after changing reference (see 23.5.2 of manual)

  raw_bandgap = analogRead(14);                      // measure internal bandgap voltage

  volt_battery = (1.1 * 1024) / raw_bandgap;         // calculate voltage

  Serial.print(volt_battery);

  Serial.println(" v_bat");

  delay(1000);                    

}

When this method won’t work.

If you have a voltage regulator or a protection diode inline between your battery and VCC, then this will only show the voltage at VCC, not the true battery voltage. 

If you have a voltage regulator then you will always read what it puts out, and not give a true indication of battery voltage.  I guess you could still use the method as a form of low battery measurement.  i.e. when the battery falls below regulation values I guess the regulator will fail to regulate and you should get a lower than normal reading.  however I haven’t tested this theory.

If you only have a protection Diode then measure the volt drop across it and compensate in your calculations, it will still give you a good indication of battery voltage.

Update:

Some other uses for this method is for auto-calibration of sensors as described by ladyada(you need to go to the bottom of the page)

One of the tasks I want to use the widget boards for is a Wireless Sensor Network around the house for measuring environmental values.  The sensors that I want to live outdoors will need to be self sufficient in terms of power, so I wanted to see if I could charge the batteries via solar, of course this can be done, but again I want to do it as cheaply and as simply as possible, I also want the widget board to monitor the solar voltage and report it back.

I already had a 3.6v 0.2w cell that I picked up some time ago from Sure Electronics for about $10 for 5.  The spec for this cell is max voltage (under load) is 3.6v and max current is 60mA, more than enough to trickle charge x2 AA NIMH batteries. 

The Batteries should not require much of a top up as the node will be sleeping most of the time, from the power saving discussion @ JEELabs, by adding the solar, I should be able to just leave the node without having to change batteries (replacing failed batteries aside).

To measure the voltage produced by the solar cell I need a voltage divider that would produce 1.1v max @ 3.6v.

Why 1.1v I hear you ask?  The reason for the A/D full scale input voltage of 1.1V is that I’ll be using the ATMega168 internal bandgap reference as the AREF source for the onboard A/D and not the default AREF. 

There are a couple for reasons for this.  The internal bandap reference itself is a 1.1v voltage reference, because the battery voltage may change over time (remember I’m not using any onboard voltage regulators) so as the VCC / battery voltage changes, so would my A/D voltage readings unless I use a reference voltage.   Now as the reference is 1.1v this means that my input must not go above 1.1v else I will go over scale and get an incorrect reading.

I’m not going to bore you with the math so a quick calculation using an online voltage divider calculator (http://www.daycounter.com/Calculators/Voltage-Divider-Calculator.phtml)

Input Voltage = 3.6v

Output Voltage = 1.0v  (remember I don’t want to go over 1.1V so I’ve given myself some headroom)

Gave the values for R1 = 26K and R2 = 10K

Luckily I had both these values, however for the 26K I only had  one in 5% tolerance, the 10K I had as 1% (I like to use low tolerance resistors for voltage dividers so my error is minimised),upon testing the 26K resistor actually read 26.4K on my meter, so after plugging these real values in my actual FS Vout is 0.989v, pretty close to 1.0V.

A blocking diode is also required to prevent any damage to the Solar cell by the batteries, see the circuit and photo below.

(Click for larger image)

(Click for larger image and notes)

Arduino Sketch to read the voltage produced by the solar cell, next step is to find a waterproof housing and add some environmental sensors. 

Another benefit of taking voltage readings of the solar cell is I now have a light sensor.

int raw_solar = 0;        // Raw Solar FS ~ 1.0v

int raw_bandgap = 0;      // Intenal bandgap reference

float volt_solar = 0.0;

void setup(){

  Serial.begin(57600);

  analogReference(INTERNAL);  // select internal reference for AREF

}

void loop(){

  raw_solar = analogRead(0);                         // read solar voltage raw values

  volt_solar = (raw_solar * 1.1 * 3.617) / 1024;     // Scale ADC input.  (voltage divider r1= 26.4k  r2 = 10k  vi=vo/(r2/(r1+r2)) = vo/.2777 =  vo * 3.617

  Serial.print(volt_solar);                          // display raw solar voltage reading

  Serial.println("v");  

  delay(1000);                    

}

The other day when playing around with different fuse settings on the Widget Board, I incorrectly wrote the wrong values, oops one bricked board   However all is not lost.

With a second working Widget Board and a simple Arduino sketch , I used the following procedure to recover from my mishap.

First I installed the following sketch onto a working Widget Board

void setup() {

  pinMode(3, OUTPUT);  

}

void loop() {

  while(1) {

    PIND |= _BV(3);

  }

}

What this sketch does is generate a 1.2MHz clock signal on the INT1 port of the Widget Board. (Click the picture below to see the the full trace )

Next I attached this signal to the XTAL1 pin (Pin 7) of the AtMega168v on the bricked board (I just held it against the crystal pin). 

While holding this connection on the pin, I then fired up AVR-DUDE and rewrote the correct fuse settings, after confirming that the fuse values could be read back I then remove the wire and everything was back to normal.

So all is not lost

Version 1.0 of the boards is progressing to the point where I’ll be starting work on v1.1, integrating user feedback and any issues on the tracker, I’ve put a few up there so please check them out, comment or add your own.  Issue Tracker.

I’m extremely happy to say that everything has progressed really well, there are not any show stoppers with the board and as they stand are very usable.  The range of the RF with the units I’ve tested is brilliant, mainly inside, I’m hoping to do some tests over the weekend outside.

The rest of my components finally arrived yesterday and I now have fully complete boards, the low profile crystals look really slick, alas I’ve decided not to keep them in the next board revision and opted for a more generic and cheaper HC49 type crystal.   There are a few more changes, but check out the Issue Tracker.

In the Wild

A few of the extra PCB I sent out are finally arriving at their intended destinations and hopefully everyone will have time and skills to build them.  For those that have already built them I have received some great feedback.  Looking forward to their test results.

Arduino

Last night I installed the crystals on all my boards and they are now running after re-programming the fuse bits.  I’ve also re-compiled the lily pad boot loader for 8Mhz and 10Mhz along with updating the Arduino board files, I will have them online in SVN shortly.

I’m also assuming that you know how to program your fuse bits and they are correctly programmed.  The fuse values I’m using are found below in the boards.txt file. I don’t have to remind you that programming incorrect fuse values can potentially BRICK your board.  If you’re unsure please ask on the discussion group.

You need to have the correct boot loader up and running on your Strobit first.  I’ve yet to test the boot loader programming from the Arduino environment as I do all mine manually using avrdude, but here are the hex files, the 8Mhz is for if you don’t use a crystal, the 10Mhz is if you have the crystal installed.

• Strobit 8Mhz bootloader (download)
• Strobit 10Mhz bootloader (download)

Below is the boards.txt found in the Arduino hardware directory, just copy and paste into this the boards.txt and restart the Arduino environment. 

Then select the Widget Board of choice from the TOOLS|Board menu.

##############################################################

strobit1.name=Strobit Wireless Widget (10 MHz) Atmega168

strobit1.upload.protocol=stk500

strobit1.upload.maximum_size=14336

strobit1.upload.speed=19200

strobit1.boot loader.low_fuses=0xE6

strobit1.boot loader.high_fuses=0xDF

strobit1.boot loader.extended_fuses=0x00

strobit1.boot loader.path=strobit

strobit1.boot loader.file=strobitBOOT_168_10MHZ.hex

strobit1.boot loader.unlock_bits=0x3F

strobit1.boot loader.lock_bits=0x0F

strobit1.build.mcu=atmega168

strobit1.build.f_cpu=10000000L

strobit1.build.core=arduino

##############################################################

strobit.name=Strobit Wireless Widget (8 MHz) Atmega168

strobit.upload.protocol=stk500

strobit.upload.maximum_size=14336

strobit.upload.speed=19200

strobit.boot loader.low_fuses=0xE2

strobit.boot loader.high_fuses=0xDF

strobit.boot loader.extended_fuses=0x00

strobit.boot loader.path=strobit

strobit.boot loader.file=strobitBOOT_168_8MHZ.hex

strobit.boot loader.unlock_bits=0x3F

strobit.boot loader.lock_bits=0x0F

strobit.build.mcu=atmega168

strobit.build.f_cpu=8000000L

strobit.build.core=arduino

##############################################################

I had a chance to grab a few photos of the progress so far, also helps that most of my components that I had on order arrived today so the boards has been kitted out with all the headers.  Now I can plug the prototype personality in, still waiting on my crystals, switches, and diodes.

This gives an idea of size. (I don’t have big hands either)  This board has a 900MHz 2dB GSM SMA antenna, prototype personality board installed and battery holder for CR123A Lithium battery. 

Different battery options, using x2AA with a switched battery pack soldered onto the board.

The SMA connector is a standard PCB straight SMA connector that has been soldered onto the edge of the board.  The board have been designed for the Sparkfun PCB edge mounted SMA connector (SKU: WRL-00593), however I found that a standard PCB SMA connector will do the same job, the distance between the centre pin and the ground pins is the same thickness as the PCB and all the pins line up with the Sparkfun footprint, the centre pin fits perfectly and so do the two bottom ground pins, only difference is the two top ground pins don’t get soldered.

One of the LEDs flashing….Its Alive!!!!

Today I did some testing on the RF side of things,  nothing scientific, just walking around the house seeing if it would dropout or report bad checksums, I’m happy to reports all is working as expected (using the 915Mhz RFM12B module)

The tests are done by using the RFM12B Example sketch found in the JeeLab RFM12B library by Jean-Claude over at Jeelab (RFM12B Arduino Library)

This library runs unmodified on the wireless widget board, just follow the instructions in the README about setting up node ids.

From the preliminary testing using some compact 1/2 wave GSM 900MHZ/1800MHZ antennas (http://www.dealextreme.com/details.dx/sku.5237)  I’m easily getting all the way around the house with no dropouts, have yet to do outside tests.

Today I constructed another 2 boards, very easy to do all done in around 30 minutes.  This now brings the total to 3.

Applying the boards with solder paste by hand, manually placing the components, bake them in the toaster oven together.  This time I got the LEDs the right way around and the right amount of solder paste on everything, no bridges on the CPU pins and looks like the right amount of fillet on the RFM12B module, very little cleanup work required, it’s amazing how little solder paste is required.

Add another 5 minutes to hand solder on the SMA connectors and program the boot loader on each, voila all done in around 30 minutes.

Now I have a potential “Network” I will start testing the RF side of things.  Currently I’m using the 915MHz modules.

Today I got a chance to do some testing on the boards. 

State of play so far:

1. Checked for shorts on power – OK
2. Did a board level continuity test – everything as per schematic.
3. Power at the correct levels and at the right places.
4. Found that my USBASP programmer needs sw2 set and now all working, I can now read and set fuses etc.
5. Arduino lillypad bootloader successfully loaded and working – Whoot!
6. ASCII sketch loaded via Arduino development environment, serial port tested at 9600, all working.
7. Jean claudes RFM12B Test Sketch loaded and working.  Can configure device and some random data is being read back from RFM12B, need a second board operational to fully test the RF side of things.

TODO:

1. Build a second device now that the basic circuit is working and verified.
2. Fully test RFM12B and wireless comms.
3. Test I/O i.e. Analogue and Digital ports, LEDS  etc
4. Customise boot loader to use onboard LEDS.
5. Document all of the above for others on the project’s website.

All in all I’m pretty happy

The PCBs arrived last week, unfortunately not in time for the long weekend so I haven’t really had a change to do anything until today and from my initial observations I’m happy with the results.

RFM12B PCB

2.4GHz MRF24J40MA PCB

Prototype PCB

As I mentioned on the Strobit-general Google List and the SPOT-development Google list,  they seem to have multiplied from when I placed my order to when I received them,  I calculated I would get approx 12 back of each board, in the end I received 32 of each board, go figure…..so if anyone wants a couple of free blank PCB then contact me and I can send you some if you cover the postage costs.  Please use the contact form to let me know.

Hot Out of the Oven

Today I assembled the first one!  Didn’t take long at all due to the minimal component count.  If anything I need to add more solder paste to the RFM12B footprint, but seems to have taken.

RFM12B PCB Assembled

I’ve partially assembled the board, enough to give me basic functionality so I can do some tests, while the rest of my components I’ve ordered (SMA connectors etc) arrive.  Unfortunately my stash of 0603 capacitors were actually 0805 ones in disguise, so I’ve had to use these even though the footprints are for 0603.   In the photo you can see some of them installed on their side.   Same goes for my bead, I only had some 0805 in my stash but have 0603 on order, just couldn’t wait  

I’ve enabled all the solder jumpers so I don’t need to install the switch and the BAV diode for the time being.

Changes for next board revision:

A couple of things stand out with this board revision, nothing major but niggly enough to warrant changes or at least thinking about them.

Crystal Footprint – One thing so far, I’m kicking myself for not making the crystal footprint a HC-49 SMD, I thought about it before I send the files off for fabrication, instead I opted for the sleek 0503 ceramic smd crystal, problem is they are more expensive and not as easy to come by, so I’ve added this as a change for the next board revision.  In future I’ll stick with the stock standard low cost crystal HC-49C footprint.

Switch – Do I really need it?  The onboard switch seemed like a good idea at the time, but in reality do I really need it?  I’m glad I had the fore thought to add a solder jumper to bypass it. Honestly I will probably more than likely use a switch remotely (i.e. on the side of an enclosure) or on the battery pack itself rather than on the PCB.  I may just leave the ability to have a remote switch and remove the onboard footprint all together.

LDO regulator onboard? – I’m still up in the air about this one.  Initially I wanted to keep things as low cost as possible (and I still do), and then add additional functionality via the personality boards so keeping the core board to bare minimum components, as I have only intended these to run from battery or from USB, not from a wall wart or A/C plug pack.  I have added some reverse protection with the BAV Scotty diode, but this won’t protect the board if someone connects a 9vDC source.  Anyway I think  I will wait and see how I go with testing and community feed back.

Jean-Claude over at JeeLabs has been playing with the RFM12 and has noted some differences between the RFM12B and the RFM12.

http://jeelab.equi4.com/2009/05/06/rfm12-vs-rfm12b-revisited/

PCB designs were finally sent off to Gold Phoenix yesterday, a bit of a delay as I had been wanting to complete a couple of other designs to fill up the panel before sending off.  Three different designs, Strobit RFM12B core, Experimental 2.4Ghz core and a prototype personality (think arduino prototype type shield) have now been merged using gerber merge into a 10” x 14.5” panel.  Hopefully they should be back early next week.

As one does while in bed at 3am and unable to sleep, I was thinking of the Strobit Triggr (which is probably why I couldn’t get to sleep in the first place) an idea presented itself for Arduino type shields (in particular Strobit Triggr personality boards) where they could be given some real personality.

Currently Arduino shields provide the hardware interface to the world for the base Arduino board.  On the Arduino resides the code that provide the functionality i.e the personality, in the form of a sketch.  The sketch and the shield have a one to one mapping, i.e. a sketch for one shield will not work with another shield type and visa-a-versa.  So whenever a shield is changed, the base Arduino boards needs re-programming with a new sketch to give it it’s personality from the host PC/MAC this provides the new functionality associated with the shield.

The idea is for the shields to have onboard flash memory that contains the actual Arduino sketch associated with it.  When the Arduino is powered up, it then checks for this flash first, if present then it proceeds to load  from here, reprogramming itself with the new code, reboots and now has the functionality associated with the shield.  If there is no flash available (a shield with no personality e.g. current shields) then the Arduino proceeds to load as per normal.

This way you could easily swap a shield/personality board without having to reprogram the Arduino, it keeps the functionality of the shield with the shield, thus giving shields a personality.