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PICAXE-18X 4-channel datalogger Pt.2: adding a real-time clock Last month, we examined the datalogger circuit, described how to build the basic module and detailed a basic datalogging mission using the light and temperature sensors. This month, we add a real-time clock to the datalogger and show you how to use it. By CLIVE SEAGER U SING THE BASIC datalogger hardware, logging can be carried out at regularly spaced intervals (up to several hours) using simple programmed time delays. For example, the datalogger program presented last month used the “Pause” instruction to generate a 60,000ms (1 minute) sampling interval. However, time delays generated in this way are not entirely accurate, due mainly to manufacturing tolerances within the PICAXE micro’s internal resonator. This becomes particularly evident when longer sampling periods are called for. To avoid this problem, you can add a real-time clock (RTC) IC to the basic datalogger module. DS1307 Real-Time Clock chip The Maxim/Dallas Semiconductor DS1307 is an accurate real-time clock in an 8-pin DIP package. It automatically maintains the current time and date, including corrections for leap years and months with less than 31 days. A standard low-cost 32.768kHz watch crystal connects between pins 1 & 2 to provide an accurate time base. An optional 3V lithium coin cell can also be connected to pin 3, ensuring that the device keeps functioning even when main circuit power is removed. The IC automatically detects removal of the main power source and switches to the lithium cell when required. Less than 1µA of current is consumed by the chip in this mode, meaning a cell life of 10 years or more. The DS1307 has two additional features of interest. Pin 7 is an open collector output that can be programmed to flash at a 1Hz rate. This allows an LED to be attached as a seconds indicator in clock applications. The IC also contains 56 bytes of general-purpose RAM, which can be used as extra memory by the PICAXE micro. Fig.1: this diagram shows how the DS1307 real-time clock chip is connected to the PICAXE-18X 4-Channel Datalogger circuit. www.siliconchip.com.au Installing the RTC Installation of the RTC upgrade on the datalogger PC board is very straightforward February 2004  73 Table 1: DS1307 Registers time/date data is in BCD (binary-coded decimal) format, which makes it very easy to interpret using normal hex notation. For example, 11:35am will contain $11 in the hours register and $35 in the minutes register. Address Register (all BCD) 00 Seconds (00-59) 01 Minutes (00-59) Setting the Time/Date 02 Hours (00-23) 03 Day of Week (01-07) 04 Date (01-31) 05 Month (01-12) 06 Year (00-99) To initialise the chip after the circuit is first powered up, the current time must be written to the registers. The example program that follows sets the time to 11:59:00 on Thursday 25/12/03 using the “writei2c” command. This is carried out by loading the registers in order from address 00 upwards (seconds then minutes, hours, etc.) 07 Control (set to $10) 08-$3F General-purpose RAM and should take you less than a minute! First, insert the DS1307 IC into the empty 8-pin IC socket (IC2), making sure that you have the pin 1 (notched) end oriented correctly. Next, slip the CR2032 lithium cell into its holder with the positive (+) side facing up. That’s it! Note however that the chip will not operate until the current time/date is set. I2C Slave Parameters The following I2C slave details can be found in the DS1307 datasheet (available from www.maxim-ic.com): slave address address size bus speed 1101000x 1 byte 100kHz This means that the PICAXE i2cslave command is as follows (see last months article for further explanation): i2cslave %11010000, i2cslow, i2cbyte The DS1307 registers are defined in Table 1. All the 74  Silicon Chip i2cslave %11010000, i2cslow, i2cbyte writei2c 0, ($00, $59, $11, $03, $25, $12, $03, $10) end After this program is downloaded, the green LED (LED1) should flash once every second. Using the DS1307 Reading the time and date from the DS1307 is best shown by example. The program given in Fig.5 acts as an “alarm clock” datalogger, checking the time every 23 seconds. When the time is exactly 07:00, the temperature and light sensors are read and stored in EEPROM. The program runs for 30 days, after which the red LED comes on to show that the mission is complete. Datalogger Wizard Of course, this is a relatively simple example. ProFig.2: the pinouts for the DS1307 real-time clock chip. A 3V lithium cell connected to pin 3 will keep the device functioning even when main circuit power is removed. www.siliconchip.com.au Fig.5: Datalogger Program Fig.3: installing the RTC upgrade on the Datalogger PC board is easy – just plug DS1307 IC into the empty 8-pin IC socket (IC2) and slip the CR2032 lithium cell into its holder. grams that make full use of the datalogger’s resources are considerably more complicated. However, the PICAXE Programming Editor software includes a “Wizard” which will automatically generate the more complex BASIC code for you with just a few mouse clicks! To use the Wizard: (1). Start the Programming Editor software (v3.5.1 or later). (2). Select View -> Options and choose PICAXE-18X mode. Click OK. (3). Select PICAXE -> Wizards -> AXE110 Datalogger -> Start New Datalogger Mission. (4). Chose the desired options and click on the “OK” button. The Datalogger Wizard dialog is shown in Fig.4. Most of the Wizard’s options are self-explanatory and as you can see, two timing options make use of the DS1307 clock upgrade. One sets an accurate timing interval while the other sets an alarm clock style time/date for logging. Once the “OK” button is clicked, the Wizard gener- main: high 5 let b13 = 0 ‘write protect EEPROM ‘reset address counter loop: i2cslave %11010000, i2cslow, i2cbyte sleep 10 readi2c 0, (b0, b1, b2) if b2 <> $07 then loop if b1 <> $00 then loop ‘set DS1307 slave ‘wait 23 sec ‘read sec, min, hour ‘if hour not 07 loop ‘if min not 00 loop high 3 low 5 ‘LED green ‘write enable readadc 0,b3 i2cslave %10100000, i2cfast, i2cbyte writei2c b13,(b3) pause 10 ‘read light value from 0 ‘set block 0 parameters ‘write the value ‘wait EEPROM write time readtemp 7,b4 i2cslave %10100110, i2cfast, i2cbyte writei2c b13,(b4) pause 10 ‘read temp value from 7 ‘set block 4 parameters ‘write the value ‘wait EEPROM write time high 5 low 3 ‘write protect EEPROM ‘LED off pause 60000 ‘wait 1 minute let b13 = b13 + 1 if b13 > 30 then stop goto loop ‘increment address ‘30 days up? ‘no so loop stop: high 2 goto stop ‘LED red ‘loop forever ates a BASIC program to perform a complete mission based on the options you have selected. This is displayed on-screen and can be edited just like any other BASIC program if desired. Note that once a datalogger program is downloaded, the mission starts immediately. It’s not possible to restart a mission by pressing the RESET button. Instead, you must download the program again. Summary The DS1307 is an easy-to-use real-time clock that can be used to add long-term accuracy to your datalogging missions. It is a low-cost versatile addition to your PICAXE datalogger system. The DS1307 real-time clock and CR2032 lithium cell can be purchased together (Part No. AXE034) from www.microzed.com.au. The full datalogger kit is available as Part No. AXE110. Next month: expanding your datalogger’s memory, displaying information on an LCD and adding a humidSC ity sensor. About the Author Fig.4: the Datalogger Wizard does all the hard work for you – just select the options you want and click the OK button to load the code. www.siliconchip.com.au Clive Seager is the Technical Director of Revolution Education Ltd, the developers of the PICAXE system. February 2004  75