Fullscreen HTML5Med Res (??MB)HTML4

PICAXE-18X 4-channel datalogger Pt.1: exploring the I2C bus This PICAXE-18X Datalogger is a highly versatile 4-channel data acquisition system. Based on one of the PICAXE series of microcontrollers, it’s easy to use and reprogram, enabling the end user to perform many different types of logging experiments. By CLIVE SEAGER B asically, this datalogger consists of four input channels that can be sampled and stored (logged) at user-defined intervals. One channel is dedicated for use with a digital temperature sensor, whereas the remaining three can be used as analog or digital inputs. Logging can be carried out at regularly spaced intervals (typically one minute to several hours), or an optional real-time clock (RTC) chip can be added to ensure accurate logging intervals over longer periods (once a week, once a month, etc). Use of the RTC will be covered in next month’s article. Data is saved in an onboard EEPROM memory chip. If desired, this memory chip can be upgraded for increased memory capacity. An optional memory expansion board can also be used to greatly increase memory capacity. Main Features • • • • • • • • Low-cost design Four logging channels One dedicated digital temperature sensor channel EEPROM data storage (easily expandable) PICAXE micro means simple programming 3 x AA battery operation, low power consumption Small footprint (approx. same size as 3 x AA cells) Optional real-time clock with lithium battery backup 72  Silicon Chip Once the “mission” is complete, data can be uploaded for analysis on a computer. Data can also be displayed (at the time of logging) on an optional liquid crystal display (LCD) if desired. The PICAXE-18X datalogger makes extensive use of the I2C bus for communication between ICs. Therefore, before proceeding any further let’s take a detailed look at the principles of the I2C bus. What is the I2C bus? The Inter-Integrated-Circuit (I2C) bus was originally developed by Philips for transferring data between ICs at the PC board level. The physical interface of the communication bus consists of just two lines – one for the clock (SCL) and one for the data (SDA). These lines are pulled high by resistors connected to the positive rail – see Fig.1. A value of 4.7kΩ is commonly used for these resistors, although the actual value used is not that critical. When either the master or slave ICs want to “transmit”, they signal their intent by pulling the lines low (0V). The IC that controls the bus is called the “master”. As in this project, the master is often a microcontroller. The other ICs connected to the bus are called “slaves”. There can be more than one slave on the bus, but each slave must have a different “address” so that it can be uniquely identified. In theory, up to 112 different addresses are possible, but most practical applications would generally have 1-10 slave ICs. A few interesting I2C slave devices include: www.siliconchip.com.au Table 1: Terms Used In this Article IC Integrated circuit or “chip” Master A microcontroller IC that ‘controls’ the operation of a circuit Slave An IC that’s controlled by the master IC Byte A number between 0 and 255 Register A memory location within the slave that stores 1 byte of data Register Address An address that points to a particular memory register Block Group of 256 registers EEPROM IC An IC that can store data (electrically eraseable programmable read only memory) RTC IC A slave IC that can maintain the date/time (real-time clock) 24LC16B: a 2k EEPROM memory chip (Microchip) DS1307: a real-time-clock chip (Dallas/Maxim) PCF8574: an 8-bit input/output expander (Philips) SP03: speech synthesiser module (Devantech) Why use the I2C bus? The I2C bus boasts the following advantages: (1) Major semiconductor manufacturers produce lowcost I2C compatible ICs. The range of ICs available is quite extensive: EEPROMs, real-time clocks, A-D converters, D-A converters, PWM motor/fan controllers, LED drivers, digital potentiometers, digital temperature sensors, etc. (2) Many of these ICs come in small 8-pin packages. This makes the circuit design very straightforward. (3) Multiple slave devices can be connected to the same bus, using only two microcontroller pins. (4) The bus design is very simple, using just two lines and two resistors. The disadvantages are as follows: (1) The I2C bus communication protocol is quite complicated. However, by using PICAXE-based microcontroller systems (or similar), simple BASIC-style commands can be used for all I2C data transfers. With this method, very little technical knowledge of the bus protocols is required. (2) Each slave IC will have unique setup parameters (eg, slave address) which must be extracted from the manufacturer’s datasheet. This is usually not too difficult once you know what you’re looking for! Slave configuration parameters Although all I2C slave devices work in roughly the same way, four parameters must be extracted from the manufacturer’s data sheets for each type of device. Parameter 1 – Slave Address: as already mentioned, each slave IC on the I2C bus must have a unique address. This is not a problem when using different types of ICs on the same bus, as most ICs have a different default slave address. The slave address is generally seven bits long, with www.siliconchip.com.au the 8th bit reserved to indicate whether the master wishes to write to (1) or read from (0) the slave. A 10-bit slave address is also possible but is rarely used and so is not covered in this article. For example, the data sheets for a particular device might define its default address as “1010000x”, with “1010000” being the address bits and “x” being the read/write bit. In a PICAXE system, the state of the read/ write (8th) bit is automatically set or cleared by the microcontroller as necessary for a read or a write operation. To connect two or more of the same types of ICs (eg, EEPROMS) on the same bus, an external addressing scheme is often employed. In the case of the popular 24LCxx series of EEPROMs, three external address pins (A2, A1 and A0) are provided. By connecting these pins to V+ or 0V on your circuit design, up to eight parts can be uniquely identified on the same bus. For these ICs, the slave address is defined in the datasheets as “1010dddx”, where “d” is 1 or 0 depending on the state of the external address pins A2 - A0. Parameter 2 -Bus Speed (100kHz or 400kHz): the maximum bus speed for data transfer between the master and slave is normally 400kHz. However, some parts will only work up to 100kHz, and so the manufacturer’s data sheet should be checked for each slave IC used. Note that this is the maximum speed – all parts can be run at the slower speed if desired. Parameter 3 – Register Address Size (Byte or Word): all data transfer from the master to the slave is a “write”, and this means that a byte of data is transferred from the master to a register within the slave IC. Conversely, all data transfer from the slave to the master is a “read”. Simpler slave devices have a maximum of 256 registers and so a “register address” of one byte length can be used to identify the particular register of interest. However larger devices, particularly memory EEPROMs, have more that 256 registers and so may need a “word’ (2-byte) register address instead. Fig.1: the I2C bus consists of just two signal lines, SDA (serial data) and SCL (serial clock). Note the pull-up resistors to the positive supply rail. Fig.2: a Microchip 24LC16B 16kbit EEPROM IC is used for data storage. It retains logged data even when powered off. January 2004  73 but only up 6 bytes from address 2 (10, 18, etc). If you don’t follow this rule, you’ll overflow the 8-byte page write boundary! I2C in the PICAXE system Hardware: all the PICAXE “X” parts (18X, 28X, 40X) include an on-chip I2C communications port. Two pins take on the SDA (serial data) and SCL (serial clock) functions when any of the I2C BASIC commands are used. For the PICAXE-18X system, SDA is leg 7 and SCL is leg 10. Software: communication with the slave device requires just three BASIC commands – i2cslave, readi2c and writei2c. i2cslave: the i2cslave command is used to set up the slave parameters for each slave IC. The syntax is: i2cslave slave_address, bus_speed, address_size where slave_address is the address (eg, %10100000); bus_speed is the keyword i2cfast (400kHz) or i2cslow (100kHz); and address_size is the keyword i2cbyte or i2cword as appropriate. All the bits are in the package; you just have to assemble the kit according to the instructions. Parameter 4 – Page Write Buffer: all EEPROM memory chips require a “write time” to save the data in the chip. This is typically 5 or 10ms. When writing lots of data, this can cause a significant delay. To help overcome this issue, many ICs have a page write buffer that can accept more than one byte at once (typically 8, 16 or 32 bytes) so that all these bytes can be programmed at once. This means, for instance, in the case of 8 bytes that you only have one 10ms delay, rather than an 80ms delay. Note that page writes can only start at a multiple of the buffer size; they must not overflow the page buffer size. In effect, this means (for an 8-byte buffer) that you can write 8 bytes starting at address 0 (or 8 or 16, etc) writei2c: the writei2c command is used to write data to the slave. The syntax is: writei2c start_address,(data,data,data,data…) where start_address is the start address (byte or word as appropriate); and data is bytes of data to be sent (either fixed values or variable contents). Multiple bytes of data can be sent at once but care should be taken not to exceed the page buffer size. readi2c: the readi2c command is used to read data back from the slave into variables in the PICAXE. The syntax is: readi2c start_address,(variable, variable,…) where start_address is the start address (byte or word as appropriate); and variable is where the returned data is stored in the master (b0, b1, b2, etc) Example To write the text “hello” (actually five bytes of data – one byte for each letter) to a 24LC16B memory IC and Fig.3: here’s how the light and temperature sensors connect to the PICAXE micro. 74  Silicon Chip www.siliconchip.com.au Table 2: EEPROM Comparison Chart Device Registers Buffer Slave Speed Address 24LC01B 128 8 %1010xxxx i2cfast (400kHz) i2cbyte 24LC02B 256 8 %1010xxxx i2cfast (400kHz) i2cbyte 24LC04B 512 16 %1010xxbx i2cfast (400kHz) i2cbyte 24LC08B 1k (1024) 16 %1010xbbx i2cfast (400kHz) i2cbyte 24LC16B 2k (2048) 16 %1010bbbx i2cfast (400kHz) i2cbyte 24LC32A 4k (4096) 32 %1010dddx i2cfast (400kHz) i2cword 24LC65 8k (8192) 64 %1010dddx i2cfast (400kHz) i2cword 24LC128 16k (16,384) 64 %1010dddx i2cfast (400kHz) i2cword 24LC256 32k (32,768) 64 %1010dddx i2cfast (400kHz) i2cword 24LC512 64 (65,536) 128 %1010dddx i2cfast (400kHz) i2cword b = block address (internal to EEPROM); d = device address (configured by external pins A2, A1, A0); x = don’t care then read it back into variables, the program would be: i2cslave %10100000, i2cfast, i2cbyte writei2c 0,(“hello”) pause 10 readi2c 0,(b0,b1,b2,b3,b4) ‘set slave parameters ‘write the text ‘wait 10ms write time ‘read the data back again Many projects involve the storage of data. This may be data collected during a datalogging experiment or preconfigured data built into the circuit at the time of build (eg, messages in different languages to be displayed on an LCD screen). The PICAXE chips can generally store 128 or 256 bytes of data internally but some projects may require much more than this, and so an external memory storage IC is required. External EEPROM (Electrically Erasable Programmable Read-Only Memory) ICs can be used to store large amounts of data. Most EEPROMs store data in “blocks” of 256 registers, each register storing one byte of data. The simplest EEPROMs may only have one block of 256 registers, while more expensive EEPROMs can have up to 256 blocks, giving a total of 256 x 256 = 65,536 (64k) memory registers. The 24LCxx series EEPROMs (see Fig.2) are probably the most commonly used I2C EEPROM devices. Many manufacturers make these parts but we will only consider Microchip brand parts in this article because these tend to be readily available via mail order catalogs. These EEPROMs can be written to over one million times and the EEPROM also retains data when the power is removed. Pin 7 of the IC is a write-enable pin that can prevent the data being corrupted (keep the pin high to prevent data being changed). Often, this pin is connected to a microcontroller pin, so that the microcontroller can control when data can be written (pull pin low to enable writes). The cheapest EEPROMs (eg, Microchip parts ending in the letter “B”) only use a single byte register address, which by definition can only uniquely identify 256 registers. This means that the various blocks (if they exist) must be identified in a different way. The 24LC16B has eight blocks, the other EEPROMS have less (see Table 2). www.siliconchip.com.au The way these cheap EEPROMs overcome this address problem is by merging the block address into the slave address. This means, in effect, that a single 24LC16B appears on the I2C bus as eight different “slaves”, each slave having a unique address and containing 256 registers. At first glance, this method of addressing seems rather awkward but it does keep manufacturing costs to a minimum. The down-side to this simplification is that only one part can be used per bus (the external IC pins A2-A0 are not actually physically connected within Fig.4 – Datalogger Program main: high 5 'write protect EEPROM for b1 = 0 to 59 'start for…next loop high 3 low 5 'LED green 'write enable readadc 0,b2 i2cslave %10100000, i2cfast, i2cbyte writei2c b1,(b2) pause 10 'read light value from 0 'set block 0 parameters 'write the value 'wait EEPROM write time readtemp 7,b3 i2cslave %10100110, i2cfast, i2cbyte writei2c b1,(b3) pause 10 'read temp value from 7 'set block 3 parameters 'write the value 'wait EEPROM write time high 5 low 3 'write protect EEPROM 'LED off pause 60000 next b1 'wait 1 minute 'next loop stop: high 2 goto stop 'LED red 'loop forever January 2004  75 Fig.5: the complete circuit diagram for the datalogger. Although shown here, the DS1307 (IC2) and piezo buzzer (PZ1) are optional components not supplied with the the basic kit. these cheaper ‘B’ parts). The more expensive EEPROMS (24LC32 upwards) use a word register address and so the block address can be incorporated within the normal register word address. This means that the EEPROM appears on the I2C bus as a single slave and so up to eight identical devices can be connected to the bus by configuring the external A2-A0 address pins accordingly. Using eight of the commonly available 24LC256 EEPROMs will give a huge 2Mb of memory! Simple datalogger circuit The program listing in Fig.4 shows how the 24LC16B is used in a real-world application – in this case, as part of the datalogger board. A DS18B20 digital temperature sensor and a LDR light sensor are read once every minute and the results saved in the 24LC16B EEPROM. A simplified portion of the datalogger circuit showing how the sensors are connected is shown in Fig.3. 76  Silicon Chip Light readings are saved in the first block (000) of the memory, whereas temperature readings are saved in the fourth block (011). A for...next loop is used to repeat the process 60 times, and the for…next loop counter value (b1) is used as the address to save the data within the appropriate memory block. Once the experiment is complete, the stored data must be retrieved from EEPROM. Normally, this is achieved by connecting the datalogger to a computer and uploading the data. With the PICAXE-18X system, this is easily achieved with the use of a “Wizard” built into the PICAXE Programming Editor software. This process is explained in detail later in this article. Circuit details Fig.5 shows the full PICAXE-18X Datalogger circuit. Note that this circuit includes some optional components (eg, the DS1307 real-time clock) that will be covered in future articles. Table 3 shows the input/output www.siliconchip.com.au pin arrangement of the PICAXE-18X microcontroller. Input sensors The datalogger has four input channels, as follows: Input 0 is normally used for a miniature light sensor (LDR – Light Dependent Resistor). The miniature LDR is connected via the two screw terminals in terminal block CT6. This input is pre-configured as a potential divider with a 10kΩ pull-down resistor. Input 7 is pre-configured for use with a DS18B20 digital temperature sensor. This is connected via terminal block CT5. The flat side of the sensor faces down when connecting the sensor into the terminal block. Digital temperature sensors give precise readings in degrees Celsius and so are much more accurate than traditional thermistor-based circuits. Inputs 1 and 2 are arranged for connection to your own sensors (analog or digital). Each input pin and +V and 0V (GND) are connected to terminal blocks CT3 and CT4. No pull-down resistors are present on the board and so should be connected externally if required. Memory The datalogger is supplied with a single 24LC16B EEPROM memory chip. This can store 2048 byte readings (eight blocks of 256 bytes). This usually enables 512 readings for each of the four sensors. If desired, the memory capacity can be increased by replacing this EEPROM with a 24LC256 EEPROM (Part No. MIC050). This can store 32,768 bytes of data (128 blocks of 256 bytes). Details on how to expand the memory of the system will be covered in a future article. Fig.6: follow this diagram closely when assembling the board. Take care with the orientation of the 100µF capacitor, the three ICs and the two LEDs. between outputs 2 and 3. Switching output 2 high and output 3 low will produce a green colour. Switching output 2 low and output 3 high will produce a red colour. Table 3: Input/Output Pin Configuration Analog Input 0 LDR light sensor (CT6) Power supply Analog Input 1 Spare sensor input 1 (CT3) The Datalogger is designed to run from a 3 x AA battery pack (3 x 1.5V = 4.5V with alkaline cells). If using rechargeable cells, a 4 x AA pack should be used (4 x 1.2V = 4.8V). The positive (red) wire should be connected to V+ on terminal block connector CT7. The negative (black) wire should be connected to GND. When connecting the wires it is recommended that the bare wire is bent back over the insulation and then the screw tightened on both. This gives a more secure connection. If a plugpack is used, it must be a high-quality regulated type, with an output voltage between 4.5V to 5V DC only. Unregulated plugpacks are unsuitable, as they generate excessively high output voltages under lightload conditions. Connection of a higher voltage source (eg, a 9V PP3 battery) or accidentally reversing the power supply connections will damage the ICs and digital temperature sensor, as there is no on-board voltage regulation. Analog Input 2 Spare sensor input 2 (CT4) Digital Input 6 Datalink serial input Digital Input 7 DS18B20 digital temperature sensor (CT5) Output 0 Piezo sounder (optional - PZ1) Output 1 I2C SDA Output 2 Bi-colour LED (red) Output 3 Bi-colour LED (green) Output 4 I2C SCL Output 5 EEPROM Write Enable (active low) Output 6 Serial LCD (optional) Output 7 Datalink serial output Serial cable connections The datalogger includes two sockets for connection to a PC’s serial port via the appropriate cable (PICAXE part AXE026). Socket CT1 (“Run”) is used for reprogramming the PICAXE chip, whereas socket CT2 (“Datalink”) is for transferring mission data. LED outputs The datalogger has a bi-colour LED (LED2) connected www.siliconchip.com.au January 2004  77 Note that the green LED (LED1) is connected to the square wave output of the optional DS1307 RTC chip, not the PICAXE chip. It will automatically flash on and off every second when the DS1307 chip is inserted (and initialised by the time/date wizard). Use of the DS1307 RTC is covered in next month’s article. Construction All parts mount on a small, double-sided PC board. Refer to the silkscreen overlay printed on the top side of the board Fig.7: to retrieve the logged data from EEPROM, you must first download a as well as the overlay diagram (Fig.6) for small BASIC program using the Datalink Wizard. This screen shot shows component placement and orientation. the Wizard’s default options. To ease the assembly task, install the smallest components first. Leave the connectors (CT1 CT8) and battery holder (BT1) until last. Parts List Assembly is quite straightforward, with attention to the following important points: 1 PICAXE-18X Datalogger PC board • Make sure that you have the notched (pin 1) end of 1 miniature LDR (connects to CT6) the IC sockets oriented as shown. 1 32.768kHz miniature watch crystal (X1) • The 2.5mm stereo sockets (CT1 & CT2) have align2 3.5mm stereo sockets (CT1, CT2) ment pins on the underside that must be ‘clicked’ into 3 3-way terminal blocks (CT3, CT6) position on the PC board prior to soldering. 2 2-way terminal blocks (CT6, CT7) • The metal can of the crystal (X1) should be soldered 1 5-way right-angle header socket (CT8) to the top of the PC board to secure it in position. 1 miniature pushbutton switch (S1) • IC2, PZ1, BAT1 and CT9 are optional parts not re1 CR2032 cell holder (BAT1) quired at this stage. Their use will be covered in future 1 3 x AA battery holder articles. 1 18-pin IC socket Using the Datalogger 2 8-pin IC sockets To program the datalogger for a simple mission, begin Semiconductors by launching the Programming Editor software (v3.5.1 1 PICAXE-18X microcontroller (IC1) or later). Make sure that you are in PICAXE-18X mode 1 DS18B20 digital temperature sensor IC (connects from the View -> Options menu and type in the program to CT5) listed in Fig.4. 1 24LC16B EEPROM (IC3) Connect the programming cable to the “Run” socket 1 5mm green LED (LED1) and apply power to the module. Now select PICAXE -> 1 5mm bi-colour (red & green) LED (LED2) Run to download the program into the PICAXE chip. Capacitors The program starts immediately after the download, 1 100µF 16V PC electrolytic recording temperature and light levels at one-minute in2 100nF 63V MKT polyester tervals. The bi-colour LED flashes green as each reading is taken. After one hour has elapsed (60 samples), the Resistors (0.25W 1%) red half of the bi-colour LED illuminates to indicate the 2 22kΩ 4 4.7kΩ end of the mission. 3 10kΩ 2 470Ω Also required (not in kit) PICAXE Programming Editor Software (v3.5.1 or later) PICAXE download cable (Part No. AXE026) 3 x AA alkaline cells Obtaining kits and software Note: the design copyright for this project is owned by Revolution Education Ltd. Complete kits (Part No. AXE110) for this project are available from authorised PICAXE distributors – see www.microzed.com.au or phone MicroZed on (02) 6772 2777. The PICAXE Programming Editor software can be downloaded free of charge from www.picaxe.co.uk or ordered on CD (Part No. BAS805) from your local distributor. 78  Silicon Chip Retrieving data from a mission The Datalink communications utility within the Programming Editor software is used to retrieve mission data from the datalogger module. This utility saves the data in CSV (comma-separated variable) formatted files, which can later be opened with any spreadsheet application (eg, Microsoft Excel) for further analysis. The utility also includes the option of automatically drawing a graph of the data as it is uploaded. To use the Datalink communications utility, a small BASIC program must first be running in the PICAXE microcontroller. This program reads the data from EEPROM and transmits it (via the Datalink connector and serial cable) to the computer, where it is processed by the Datalink software utility. This BASIC program can be automatically created www.siliconchip.com.au There’s no need to change the Wizard’s default options (see Fig.7). Simply click on the “OK” button to download it to the datalogger module. Note that the Datalink utility uses the standard PICAXE programming cable to retrieve the data from the datalogger module. However, the cable must be inserted into the Datalink socket, not the PICAXE “Run” socket. Using the Datalink utility Wait until the datalogger mission is complete (status LED red) before using the Datalink utility. When it’s done, the procedure is as follows: (1) Download the Datalink program via the Datalink Wizard. (2) Connect the PICAXE cable to the Datalink socket on the datalogger. (3) Select File -> New within the Datalink Window. The data will then be uploaded, with the data simultaneously visible on screen. Once the data upload is complete, choose the File -> Save As menu to save the data as a CSV-formatted text file. Summary Fig.8: after running the Datalink Wizard, open the Datalink utility from the Programming Editor’s main menu (or press F9). It’s then just a matter of selecting File -> New and following the prompts to initiate the data upload. and downloaded to the datalogger using the Datalink Wizard. From the Programming Editor, select PICAXE -> Wizard -> AXE110 Datalogger and choose the “Retrieve Unknown Data” option to launch the Wizard. www.siliconchip.com.au The PICAXE system provides a very economical method of implementing a high specification datalogging system. As the datalogging mission is programmed by the end user, there is no limit to the function of the system – the function of the datalogger can be easily modified and customised as required. Next month, we will show you how to add a DS1307 SC real-time clock to the Datalogger! About the Author Clive Seager is the Technical Director of Revolution Education Ltd, the developers of the PICAXE system. January 2004  79