24 September 2012

Simple XBee ZB (Series 2) P2P Communications

Some folks have difficulty using the XBee Series 2 modules (now called XBee ZB) for simple point-to-point communication in transparent (AT) mode. For this reason, the XBee Series 1 modules (now called XBee 802.15.4) are often recommended for simple P2P applications, with the S2 modules being considered "too complicated".

I disagree with this recommendation because the XBee ZB modules are more capable (and are perfectly capable of simple P2P communication), their radios are a little better than the XBee 802.15.4 modules (both in transmitter power and receiver sensitivity) and they even cost a couple bucks less. With a pair of ZB modules, if you ever want to try mesh networking with three or more nodes, there's nothing else you need to do (except to buy more modules).

The downside is that the XBee ZB modules do require some configuration to establish a P2P link. (I have not experimented with the XBee 802.15.4 modules myself, and I have heard conflicting stories regarding whether a pair can be used as a P2P link "out of the box" without further configuration.)

Getting to the point, I was discussing an XBee ZB issue on the Arduino forum the other day, when I hit on what I think is the minimal amount of configuration required to establish a point-to-point, transparent mode (AT) link with two brand-new XBee ZB modules right out of the box.

First, two assumptions. One, we assume that the modules come from the factory with the Router AT firmware loaded. I think this is a pretty good assumption, all the ZB modules that I have came this way (and BTW, the interface speed is set to 9600 baud). Two, we assume that there are no other XBee networks operating nearby. This is probably a good assumption for folks with their first pair of XBees, but more on that later in case it's not a good assumption for you.

It turns out that there is only one step required to get the new pair of XBee ZBs talking! Using Digi's X-CTU program, load the Coordinator AT firmware on one of the two modules. (Every XBee ZB network needs to have exactly one coordinator, so the first thing when setting up a network is to satisfy this basic requirement anyway.)

This works because the modules' default value of zero for the network ID (PAN ID) causes the coordinator to select a random PAN ID and causes the router to join any PAN ID available. Hence there is a potential issue if there is already another XBee network operating, the router may join it instead of talking to the new coordinator. If this is the case, then there is an additional step: The PAN ID for both units needs to be set to some non-zero value different from that of the other network(s). I like to always set the PAN ID anyway, since what passes for normal around here seems to be a minimum of two separate XBee ZB networks in simultaneous operation, but if you just have that first pair of modules, then more than likely you don't have to worry about it (unless your neighbor hasn't come out of the closet about his XBee addiction).

The other thing that makes this work is that the default destination address (DH and DL parameters) will be zero for the router and 0x000000000000FFFF for the coordinator. Zero is a special address that causes the router to send its traffic to the coordinator. 0x000000000000FFFF is also a special address called the broadcast address. This means that the coordinator will send to every other node on the network.

To broadcast or not to broadcast

As it turns out, using the broadcast address is OK for a simple demonstration with two nodes, but in general, broadcast transmissions should be used sparingly because they cause a lot of network overhead, and this can be significant on larger networks. In the case of two nodes, it's easy enough to avoid and just have the coordinator address its traffic directly to the router. First determine the router's 64-bit (16 hex digit) address. In X-CTU, it's the SH (Serial number High) and SL (Serial number Low) parameters. It's also printed on the label on the bottom of each XBee. SH and SL are each 8 hex digits, and the high part will always be 0x0013A200 for XBees made by Digi International. The low part will be a unique number, for instance 0x406B85A5. Next, connect the coordinator to X-CTU and set its Destination address High (DH) and Destination address Low (DL) to be the router's address. This will cause all transmissions from the coordinator to be unicast transmissions rather than broadcast, and to be directed to the router.

Well this post ended up a bit longer than I thought it would. I hope you stuck in there, and I hope it was useful. I hope to expand on this in another post at a later date, with more details regarding loading firmware, setting parameters, and connecting the XBees, but I wanted to get the thought out there for now. Happy networking!

14 September 2012

Yet Another Real-Time Clock

Real-Time Clocks (RTCs) are popular add-ons to microcontroller projects. I am no exception, I have a lot of them kicking around. The most common RTC seems to be the Maxim DS1307. A lot of my RTCs are DS1307s. It's a real workhorse chip, and easy to use. Still, it has disappointed me in a couple ways. For one, it's not always as accurate as I would like. I've used inexpensive no-name crystals and I've used more expensive ones from the top-shelf distributors. Sometimes they don't even seem to operate within the crystal's specs (typically ±20ppm). Not sure why this is, and it certainly could be my fault, but there it is. Another thing is that sometimes I have a 3.3V microcontroller circuit, but the DS1307 requires 5V.

Another alternative is the Maxim DS3231 (or its SPI relative, the DS3234). I love this chip, it is so cool. The integrated, temperature-compensated crystal makes it very accurate (±2ppm from 0°C to +40°C), and it will operate on 3.3V. This addresses all of my gripes with the DS1307. But it is more expensive, nearly $9 from Mouser in single quantities as I write this, where the DS1307 is about half that. (Yes, I know DS1307s can be had for significantly less from other sources.) But the DS3231 is harder to find at discount prices, and is only available in a surface-mount package if that makes a difference to you.

Enter the Microchip MCP79412. It operates off a crystal similar to that used by the DS1307, so the basic accuracy is about the same. But, it can be calibrated by setting an internal register. It will operate down to 1.8V. And while it does require a few more external passive components than the DS1307, it costs only $1.23 in single quantities. I popped for ten and paid $0.98 per copy. Again, it is only available as a surface-mount component.

The MCP79412 has some other cool tricks up its sleeve as well, including alarms, tracking power outages, and EEPROM in addition to SRAM. I've detailed these in the table below.

To summarize, I've been tinkering with the MCP79412 on and off for the last couple months and have come to like it quite well. I designed a breakout board for it (pictures below) and wrote an Arduino library to support it. I've only used three of the ten chips so far, but they have all operated well within the ±20ppm spec of the crystal I chose. One unit seems to be within 2ppm, and so hardly needs trimming.

If you are also using this chip, or would be interested in it, I'd love to hear from you!

PS: For a comprehensive example using the MCP79412, see my Power Outage Logger project.

Real-Time Clock Comparison
Feature MCP79412 DS1307
On-Chip Calibration ±127 ppm N/A
Alarms Dual alarms (single output) N/A
Power Fail/Restore Timestamps Yes N/A
Unique ID 64-bit ID N/A
EEPROM 128 bytes N/A
Battery-Backed SRAM 64 bytes 56 bytes
Vcc 1.8 - 5.5V 4.5 - 5.5V
I2C Interface Clock Frequency 400 kHz (Vcc ≥ 2.5V) 100 kHz
Square-Wave Output 1, 4096, 8192 or 32,768 Hz 1, 4096, 8192 or 32,768 Hz

MCP79412 RTC Breakout Board, Top
MCP79412 RTC Breakout Board, Bottom

04 September 2012

Warning! One Million Ohms

An electronic version of an old joke known among physicists and engineers.

Amuse your friends and confuse your enemies! Keep the uninitiated away from your workbench or desk and out of your lab!
  • Great conversation piece or gag gift
  • Big, scary 1,000,000 Ω resistor in the middle of the board
  • Pre-programmed AVR microcontroller (ATtiny85)
  • Arduino-compatible, hackable open-source hardware and software Can be re-programmed with an ICSP programmer, using either the Arduino integrated development environment or WinAVR
  • Runs on two AA batteries (not included)
Pressing the SELECT button turns the circuit on and causes the red LEDs to flash. To change the flashing speed and pattern, press SELECT again. Hold SELECT down to turn the circuit off, or it will automatically turn itself off after five minutes.