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Pt.2: By CLIVE SEAGER* * Clive Seager is Technical Director of Revolution Education Ltd, the developers of the PICAXE system. PICAXE Goes Wireless Get your PICAXE projects talking over the airwaves using the latest high-tech 2.4GHz XBee modules! I N PART 1 LAST MONTH, we des­ cribed how to build and test a pair of wireless data communications nodes based on AXE210 project boards and XBee modules from MaxStream. This month, we look at some of the more advanced features of the XBee modules. By way of example, we then learn about some of these features during the construction of a wireless light and temperature sensor, based on one of the AXE210 boards. As part of the project, we also see how to receive and log the data from the wireless sensor to disk with the aid of the second AXE210 board and a PC. Finally, we dispense with the PC altogether and see how easy it is to get a PICAXE-to-PICAXE wireless link up and running! XBee networking Fig.1: here’s a screen shot of the XBee Setup wizard, showing the correct settings for board #1. Remember to swap the DL and MY values around when programming board #2! 88  Silicon Chip As demonstrated last month, sending and receiving data between two XBee modules is quite straightforward. In those examples, we relied on the XBee’s default settings. However, consider the case where more than one module is within receiving range. Unless we specifically want to send a “broadcast” message to all modules, then some means of addressing the data to the intended recipient is required. This is where the XBee’s intelligent networking features come into play. For a start, each module is factory programmed with a unique serial number, allowing it to be explicitly addressed. In addition, each XBee module can be identified with a programmable “nickname” for ease of use. Other powerful networking features include the ability to arrange modules into groups, or “private area networks” (PANs). By assigning a common group ID to all members, modules not part of that group are automatically excluded. So how do you set an XBee module’s nickname or group ID? Well for modules connected to a PC, it’s quite straightforward; you’d use the Programming Editor’s “XBee Setup” wizard. We’ll investigate this method in more detail shortly. You can also set these parameters using a microcontroller such as our PICAXE-18X. Although we don’t cover this method here, it’s simply a matter of sending the appropriate commands to the XBee module before sending or receiving data over the radio link. To do this, the module is first placed in command mode by sending a special sequence of characters. Once in command mode, the command(s) and any other parameters (such as the nickname mentioned earlier) are sent. A final “exit” command returns the module to idle mode, ready to send and/or received data over the airwaves. siliconchip.com.au Table 1: Module Settings Parameter Board #1 Board #2 Baud rate <BD> 2400 2400 Broadcast channel <CH> C C Network group <ID> 3332 3332 Destination nickname <DL> 4321 1234 My nickname <MY> 1234 4321 Before continuing, we strongly recommend that you download and review the XBee datasheet, available from www.maxstream.net. There you will find a detailed description of the XBee’s networking features and the various commands needed to make them work. In the following project examples, we’ll refer to a subset of the XBee’s commands and their related terms. An informative summary of these is given in Table 2. Putting it into practice If all the addressing information sounds a bit confusing, don’t worry – it should become clearer with a couple of examples! In this first of these, we will wire up a temperature sensor and a light dependent resistor (LDR) to the PICAXE chip on one of the AXE210 Connect boards. We’ll then transmit data gathered from the sensors via the on-board XBee module to a second XBee module connected to a computer, where is can be viewed on-screen or logged to disk. This example uses “nickname” (short) addressing. We’ve arbitrarily chosen address “1234” for board #1 and “4321” for board #2, so let’s begin by programming these addresses into the two XBee modules. Initially, we’ll configure both boards for “PC to XBee interface” use (mode #2), as described last month. To do this, insert the MAX3232 chip into its 16-pin socket on each AXE210 Connect board (remove any PICAXE chip) and make sure jumper J1 is in the “top” position). Next, start the PICAXE Programming Editor and open the “XBee Setup” window by selecting PICAXE -> Wizards -> AXE210 PICAXE Connect from the toolbar. The settings required for each siliconchip.com.au Fig.2: light and temperature sensors are wired to board #1 for the first project. This simplified circuit shows the connections, made via the row of solder pads next to the serial socket. Table 2: Introduction To XBee Buzzwords Broadcast Channel <CH> XBee modules can communicate on 16 different “broadcast channels”. All XBee modules in the same network must share the same channel number. Different channels can be used for different networks for privacy reasons or to reduce interference. In addition, switching channels can sometimes be an effective means of reducing interference from competing transmitters, such as computer WiFi networks, microwave ovens and cordless telephones. Baud Rate <BD> This is the data rate on the XBee serial interface (ie, between the XBee and PICAXE or computer). When using a PICAXE micro, choose a baud rate of 2400 for maximum compatibility. Although slow by modern standards, this rate is adequate for most projects. Network Group <ID> XBee modules can be arranged in network groups (also called “Personal Area Networks”, or “PANs”). Each module will only respond to others in its group, as defined by a 16-bit ID number. Serial Number <SH> <SL> Each XBee module has a unique, factory-assigned 64-bit serial number (also called a “long address”). The serial number is printed on the bottom of the module and obviously cannot be changed. Data can be addressed to a specific device by including its 64-bit serial number. Nickname <MY> Optionally, an XBee module can be addressed by its “nickname” (also called a “short address”) rather than by serial number. Nicknames have the advantage of being userprogrammable and are only 16-bits long – so they consume less memory in a PICAXE. Using this method of addressing, any module in a network can be replaced with a new unit (which would have a different serial number) simply by programming it with the existing nickname. module are shown in Table 1. As you can see, all of the settings are the same, apart from the “my nickname” and “destination nickname” values, which have been swapped. This allows the two modules to correctly identify each other. Fig.1 shows a screen shot of the settings for the XBee module in board #1; remember to swap the DL and MY values around when programming board #2! After altering any parameter, note that the associated “Write” button must be clicked otherwise your changes May 2006  89 XBee Communications Checklist Fig.3: once you’ve fitted the sensors and downloaded the program in Listing 1, the board will immediately begin transmitting its data. Here’s the result, as received by the second board and displayed in the Editor’s terminal window. Fig.4: data gathered from the sensors can be saved to disk with this excellent freeware RS232 logging program from Eltima. Be sure to alter the serial port options to suit the PICAXE system. will not be saved. Once you’ve configur­ed both boards, it’s a good idea to perform a ping test to verify that the units are in fact communicating. Details of how to do this test were given in last month’s article. Light & temperature sensor We’ve chosen board #1 to operate as the remote temperature and lightsensing node, so remove the MAX3232 chip and plug the PICAXE-18X into its 18-pin socket. This is the “PICAXE to XBee interface” configuration (mode #1) described last month. Next, wire a light-dependent resistor (LDR) and DS18B20 temperature 90  Silicon Chip (1) All modules must use the same baud rate (recommended). (2) All modules must be on the same broadcast channel. (3) All modules must be in the same network group. (4) An XBee module can be configured to transmit to: • any other module, • a module(s) with a specific nickname, or • a specific module, using its 64-bit serial number. (5) An XBee module can be configured to receive data: • from any other module, • only when its nickname is used, or • only when its 64-bit serial number is used. sensor to input0 and input7 of the PICAXE, as shown in Fig.2. A row of solder pads along the top of the board provides easy access to the port input pins, as well as +5V and ground. As the circuit is extremely simple, it could be wired “point-to-point” with light gauge hook-up wire. Alternatively, you could use a small solderless breadboard for the job; see Pt.3 of the “PICAXE in Schools” series, published in July 2005 for ideas on breadboard use. Now connect board #1 to your computer and download the program shown in Listing 1 into the PICAXE chip. The “init” section of the program sets the serial pin high and then waits for 100ms. This gives the XBee time to wake up. The main loop then reads the light value (readadc on input0) and temperature value (readtemp on input1) and transmits the data every second. Disconnect board #1 from the computer and connect board #2 (fitted with a MAX3232). The data being transmitted by board #1 should now be displayed in two columns in the Terminal window (see Fig.3) – it’s that simple! Logging data PICAXE users often ask for serial datalogging software to allow readings from a project like this to be stored in a computer file, so that the data can be analysed later. Our favourite piece of software to do this is “RS232 Data Logger” from Eltima Software (www. eltima.com), as it’s free and very easy to use. To use the RS232 Data Logger, just highlight the COM port of interest, enter the filename and select the appropriate serial port options (see Fig.4). All you then need to do is click on the “Start logging” button! Once the logging is complete, click on the “Stop logging” button. The file created can then be opened in Excel and many other applications, depending on your requirements. Light and temperature warning This second project demonstrates wireless PICAXE-to-PICAXE communication. Board #1 remains unchanged, while board #2 must be fitted with a PICAXE chip and two LEDs. These will be used to indicate the state of the temperature and light readings received from board #1. Begin by fitting the PICAXE chip to board #2 (don’t forget to remove the MAX3232!). The two LEDs are connected to output0 and output1 of the PICAXE, as shown in Fig.5. A row of solder pads along the bottom of the board provides easy access to the port output pins and ground. That done, connect board #1 to your computer and download the new program shown in Listing 2. As you can see, we no longer have the “#” character before the variable names, because we are now transmitting raw byte data, rather than ASCII characters. Two “$55” characters also precede each transmission; this is a simple means of ensuring that the receiver accepts only valid data. Now connect board #2 to your computer and download the receiving program in Listing 3. This program waits for valid data from the transmitter and then switches the LEDs according to the predefined temperature and light threshold values. Initially, the debug command can siliconchip.com.au Parts List For Project Examples Fig.5: two LEDs and their current limiting resistors are connected to board #2 for the second project. The connections are made via the row of solder pads situated between the two reset switches. 1 DS18B20 digital temperature sensor 1 light-dependent resistor (LDR) 2 3mm or 5mm LEDs light-duty hook-up wire Resistors (0.25W 5%) 1 10kW 2 330W 1 4.7kW The temperature sensor (Part No. DS18B20) and LDR (Part No. SEN002) are available from MicroZed Computers, phone 1300 735 420 or browse to www.microzed.com.au be used to display the received values on-screen. As shown, the program uses “40” as the light threshold value and “20” as the temperature threshold. You may need to tweak these slightly, depending on your ambient conditions. Reducing power consumption If you are designing your own project based on the AXE210, you are likely to be considering batteries as the power source. Two simple improvements to the project board are immediately obvious. The first is to power the whole circuit from a 3V battery pack, allowing you to bypass the two relatively inefficient voltage regulators. Of course, you’ll need to disable the two voltage divider circuits on output6 & output7 when the PICAXE is running on the lower 3V supply. If you’ve already built the boards, then the easiest way to achieve this is to remove one 10kW resistor (the one connected to 0V) from each divider string. You can also use the “sleep” function of the XBee module to reduce power consumption. By installing jumper J4 on the board, the SLEEP pin of the XBee module is connected to output6 of the PICAXE chip. This then allows you to place the XBee module in low-power sleep mode with a low command and wake it up when needed with a high command. Note that to allow the XBee module to enter low power mode using the external SLEEP input, you must first configure it to do so via the advanced siliconchip.com.au Program Listings settings in the XBee Setup wizard. Click the “>” button to see the advanced settings. You’ll note that two pin-controlled options are available under the “Set Sleep Mode” heading – “Pin Doze” and “Pin Hibernate”. The difference between these two options is summarised as follows: Option Current Drain Wakeup Time Pin Doze <50mA 2ms Pin Hibernate <10mA 13.2ms Increasing range Need more range? A higher-powered version of the XBee module called the “XBee Pro” is also available. It boasts a transmit power of 100mW, versus the 1mW of the standard XBee. The two modules are pin-for-pin compatible, so no changes are required to the AXE210 board to use the uprated module. But before spending more money on the “Pro” module, check out Stan Swan’s 2.4GHz gain antennas in this issue. They cost virtually nothing, are fun to build and can increase range by two times or more! Summary The XBee modules make serial communication between PICAXE pro­jects a breeze. Additionally, their range can be increased significantly for little cost using home-brew antennas. For more information on the XBee modules, point your browser to www. SC maxstream.net Listing 1 symbol TAB = 9 init: high 7 pause 100 serout 7, T2400, ("Light",TAB,"Temp",CR,LF) main: readadc 0,b0 readtemp 1,b1 serout 7, T2400, ( #b0,TAB,#b1,CR,LF) pause 1000 goto main Listing 2 init: high 7 pause 100 main: readadc 0,b0 readtemp 1,b1 serout 7, T2400, ($55,$55,b0, b1) pause 1000 goto main Listing 3 main: serin 7, T2400, ($55,$55),b0, b1 debug test_LDR: if b0 > 40 then LDR_high low 0 goto test_temp LDR_high: high 0 test_temp: if b1 > 20 then temp_high temp_low low 1 goto main temp_high: high 1 goto main May 2006  91