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Digi-Temp automatically displays temperatures on its own readout or on your PC. Up to eight sensor temperatures are dis­played at intervals of one second. Digi-Temp monitors eight temperatures This little device will monitor & display the tempera­ture at eight different locations at 1-second intervals. And you can use it to log those temperatures into your computer for air conditioning or process control. The temperature range is from -50°C to 99.9°C. By GRAHAM BLOWES 80  Silicon Chip Digi-Temp is a self-contained temperature monitor which can be used by itself or in conjunction with your computer for con­trol applications. In concept, it is similar to those el-cheapo indoor/outdoor temperature sensors which are frequently adver­ t ised. Those units are thermistor based and their accuracy seems quite variable, which is to be expected; after all they are cheap. The accuracy of some units, would you believe, is also affected by temperature! Digi-Temp has none of those problems, being a purely digital device. It can transmit the data from each sensor to the Rain Brain sprinkler controller (published in the January 1996 issue of SILICON CHIP) and to your PC. If data transmission is not needed, no problem! Just power it with a 12VDC plugpack and you have a standalone unit that can be used anywhere, as it has its own LED display. It could be installed in your car or on a bookshelf at home. Digi-Temp is a no-frills project. It is just a plastic box with a 4-digit readout. There is just one PC board which fits snugly inside the box. There are no switches to operate. You just plug it in and it automatically cycles through the temperatures at eight different locations. There is also a 25-pin D socket for connection to the serial port of your computer. The data transmission is an all ASCII string which can be received on a normal communications program, such as Telix or the Windows terminal program. I have written a simple Qbasic program that could form the basis of a simple data logger on your PC. As can be seen from the block diagram in Fig.1, the DS1820 temperature sensors simply connect onto a single wire bus (plus supply lines) wherever a device is needed. Temperatures from -50°C to +99.9°C can be displayed on this unit. The accuracy of each device is ±0.5°C with a display resolution of 0.1°C. Best news of all is that the unit does not require calibration of any sort; just build it and go! Digi-Temp uses a Z86E08 micro­ controller to communicate with the DS1820 temperature sensors and with external devices such as your PC and the Rain Brain sprinkler controller referred to earlier. The data transmitted from the DS1820 has a checksum attached to it, so any errors in transmission are detected. The same method of checksum verification is used when the data is re-transmitted to your PC. The Rain Brain will ignore any data where the CRC (cyclic redundancy check) is wrong, as will the Qbasic program mentioned earlier. Further, if the Z86E08 detects a CRC error in any of the DS1820s, it flashes the number of the offending sensor for a few seconds, then resets itself and interrogates the single wire bus +V G Fig.1: block diagram for the Digi-Temp. Up to eight DS1820 tem­perature sensors can be daisy-chained together. This process is quite tricky, so I recommend you get a copy of the data sheet to get the full picture. It is possible to identify 75 different one-wire devices per second. Dallas Semi­con­duct­or has a web site at http://www. dalsemi.com/ The DS1820 counts the number of clock cycles that an oscil­ l ator with a low temperature coefficient goes Fig.2: this is memory map for the DS1820 through during a period digital temperature sensor. determined by a high temperature coefficient oscilto re-establish contact with all the lator. The low temperature DS1820s connected. coefficient means that it is unaffected by temperature, whereas the high temDS1820 temperature sensors perature coefficient oscillator varies Made by Dallas Semiconductor according to the temperature around it. Corporation, the DS1820s are clever Once a temperature conversion little beasties. Each device has its is completed, the device places the own unique 64-bit ROM number. resulting 16-bit, sign-extended two’s The first eight bits form the family complement binary number (-55 to code, the next 48 bits is a unique +125) into the scratchpad RAM, ready ID number, and the last eight bits is for the master to read it (when a ‘READ the CRC checksum of the previous SCRATCH’ command is issued). This 56 bits. The DS1820 has nine bytes number has a resolution of 0.5°C. of scratchpad RAM plus two bytes Greater resolution can be obtained by of EEPROM. The EEPROM bytes are performing the calculation shown in linked to programmable alarm trip equation 1 below. points (upper and lower). This calculation uses the values left The device has a repertoire of 11 in the counters, once a conversion com­mands, five of which are ROM has been completed. Fig.2 shows the functions while the other six are MEM- memory map of the DS1820. ORY functions. The most complex Circuit details command is called ROM SEARCH. This process enables all the connected Fig.3 shows the circuit diagram of devices to be identified by a process the Digi-Temp. IC1 is a Z86E08 microof elimination. processor clock­ed by an 8MHz crystal Equation 1 Temperature = temperature read - 0.25 + [(count per °C - count remain)/count per °C] where temperature read = (16-bit number from temperature MSB and LSB)/2 January 1997  81 Fig.3: the Z86E08 programmed microprocessor is the heart of the circuit. It interrogates each of the temperature sensors and displays their values on the 4-digit readout. It can also send the information to a PC via an RS232 port. which is inter­nally divided by two for all internal timing. Both internal timers of the Z8 are used; one to multiplex the 7-segment LED displays via Q3-Q6 at a 1kHz scan rate and the other for general timing duties. IC2 converts the BCD output of port 2 (bits 0 to 4) to the 7-segment code for the LEDs. Op amp IC4b and Q1 form a voltage-to-current converter. The input voltage applied to pin 3 of IC4a will cause an equivalent voltage to be dropped across the 150Ω emitter resistor for Q1. Using this circuit means a fixed amount of current is always drawn from the supply, no matter (theoretically) what the resist­ ance 82  Silicon Chip of the wires between the Rain Brain and the Digi-Temp. This method also allows minimal disturbance to the 5V supply provided by the regulator, IC5. Links LK2 and LK3 provide baud rate selection, although in practice 9600 baud seems to work very well, even over distances of 100 metres. Link LK1 was intended to be used when the Digi-Temp was operated without the LED displays when connected to the Rain Brain controller. This mode, however, is not used so it can be left out (pin 8 high). If the Digi-Temp is only to be used to transmit data to a PC, then the LED displays and associated hardware can be left off the PC board. R14 is the 4.7kΩ pull-up resistor associated with the DS1820 sensors. The sensors are open Drain, meaning that if the internal FET of any of the connected sensors is switched on, then a logic 0 is presented to P27. Software Because there is only one wire for both transmit and re­ceive operations, timing is critical. The timing is divided into two main groups, ‘read’ slots and ‘write’ slots. Refer to Fig.4 for details of these slots. When IC1 comes out of RESET, port pin P27 is configured as an output. It sends a RESET pulse out to all the DS1820s connect­ed to the single wire bus. The RESET signal is a logic 0 between 480µs and 960µs long. After this, P27 is set as an input. All connect- Above: all the components except for the LED displays are mounted on this side of the PC board. Note that the final version differs slightly from the unit shown here. ed DS1820s respond simultaneously with a presence signal. The presence pulse is a logic 0 between 60µs and 240µs long. IC1 then issues a ‘ROM SEARCH’ command. This process is the repetition of a three step routine: read a bit, read the comple­ment of the just read bit, then write a bit back to the sensor(s). IC1 performs this routine on every bit of the DS1820 ROM. After one complete pass (64 cycles), IC1 knows the contents of the ROM in one DS1820. The rest of the connected DS1820s are identified through additional passes. The following is a simpli­fied version of an example in the data sheet. Say we have four devices with the following ROM code seg­ments: ROM1  00110101... ROM2  10101010... ROM3  11110101... ROM4  00010001... The search process is as follows: (1). IC1 issues a RESET to the DS1820(s). All connected DS1820s Fig.4: because there is only one wire for both transmit and receive operations to the DS1820 sensors, timing is critical. The timing is divided into two main groups, ‘read’ slots and ‘write’ slots, as shown here. January 1997  83 Fig.5: this is the component overlay for the double-sided PC board. Note that this board is slightly different from that shown in the photos. respond with a simultaneous presence pulse. (2). IC1 issues the ROM SEARCH command. (3). IC1 reads a bit. Each DS1820 will place the value of the first bit of its respective ROM code onto the bus. ROM1 and ROM4 will place a 0 whereas ROM2 and ROM3 will place a 1. As these devices are all ‘WIRE ANDed’ the result will be a logic 0. IC1 now reads another bit. Seeing that this is the ROM SEARCH command being executed, the DS- 1820s will now place on the bus the complement of the ROM code bit that was previously sent. ROM1 and ROM4 will place a 1 whereas ROM2 and ROM3 will place a 0. The result, again, will be logic 0. Each subsequent ‘dual read’ will result in one of the following: 00 There are still DS1820s attached which have conflicting bits in this position. 01 All DS1820s still coupled have a 0 bit in this bit position. 10 All DS1820s still coupled have a This photo shows the board removed and the rectangular cutout in the case for the DB25 socket. 84  Silicon Chip 1 bit in this position. 11 There are no DS1820s attached to the bus. So far, IC1 has determined that some DS1820s have a 0 as the first bit of the ROM code whereas the rest have a 1 in this position. You are probably thinking, how can this be of any use! Well, IC1 will now write a 0 back to the DS1820s. This will cause all the DS1820s with 1 as the first bit of the ROM code to switch off, which in this example are ROMs 2 and 3. IC1 could write back a 1, which would cause ROM1 and ROM4 to switch off. Step 3 is repeated again. This time the ‘dual read’ will result in 01, which means that all DS1820s still connected to the bus have a 0 bit in this position. You can see that this is the case with ROM1 and ROM4. IC1 writes back a 0, which keeps ROM1 and ROM4 connected. Step 3 is repeated again. This time the ‘dual read’ will result in 00, which means that this ROM code position has a conflicting bit; ie, either ROM1 has a 0 and ROM4 has a 1 (or vice versa). In this case, ROM4 has a 0. IC1 writes back a 0. This causes ROM1 to switch off, leaving only ROM4 still connect­ed. Subsequent ‘dual reads’ will result in either 01 or 10 be­cause ROM4 is the only device left on the bus. Once 64 bits have been read, the eight received bytes are passed through a CRC calculator, which if correct, will yield a zero. The whole process is repeated for the other DS1820s. To prevent reading the same path over and over again, the ‘pathway’ has to be marked in much the same way as if you were exploring a maze. Each time you come to a fork (dual read = 00, meaning 0 or 1 in a bit position), mark it, so that next time you encounter this fork, take the other path (ie, write back a 1). This path is also marked, so that next time you encounter a fork where both paths are marked, back track to the previous fork, where there is still an unmarked path. As noted earlier, it is tricky! The ROM codes of all the detected DS1820s are stored in the internal RAM The PC board is mounted upside down in the case with the displays facing upwards, as shown in this photograph. The 25-pin D socket connects via a standard RS232 cable to the serial port of your computer. of the Z8 controller. A maximum of 64 bytes is set aside for this task, which is enough for eight DS1820s. Once all connected DS1820s have been detected, the number of devices found is displayed on the lefthand digit of the display. After this, all DS1820s are RESET and the MATCH ROM code is sent to all DS1820s. This causes all the DS1820s to ‘sniff’ the bus for their own unique ROM code. The ROM code of the first device is read from RAM and sent out on the bus (note: all data is least significant bit first). After this, MEMORY COMMANDS are sent to the addressed device. In this case, the CONVERT T command tells the DS1820 to read the temperature and place it into its onboard scratchpad RAM. This takes about 500ms. Next, the READ SCRATCHPAD PARTS LIST 1 plastic box, 120 x 65 x 39mm 1 PC board, 113 x 63mm 1 DB25 socket with rightangle mounting 1 3.5mm stereo socket 1 3.5mm stereo jack plug 1 8MHz crystal Semiconductors 1 Z86E08 programmed microprocessor (IC1) 1 4511 BCD to 7-segment decoder (IC2) 1 MAX232 RS232 transmitter (IC3) 1 LM358 dual op amp (IC4) 1 7805 5V 3-terminal regulator (IC5) 1 to 8 DS1820 digital thermometers 6 BC548 NPN transistors (Q1-Q6) 4 FND500 common cathode 7-segment displays (H1 - H4) 1 red rectangular LED (H5) 1 1N4004 silicon diode (D1) Capacitors 1 1000µF 16VW electrolytic 1 10µF 25VW tantalum electrolytic 4 10µF 16VW electrolytic 1 0.1µF monolithic 2 22pF ceramic Resistors 9 10kΩ 1 4.7kΩ 1 470Ω 8 180Ω 1 150Ω Miscellaneous Heatshrink tubing, IC sockets, solder. command is sent to the DS1820. This causes the DS1820 to send the contents to IC1. Note that all nine bytes are sent, even if they are not necessarily wanted. The ninth byte is a CRC of the previous eight bytes sent. If a CRC error is detected, then the number of the offending sensor is flashed in the LHS display. After five seconds, IC1 will RESET and start again. The DS1820 has its own inbuilt CRC generator. This really cuts down on the ambiguity of any data read from the sensors – if the CRC doesn’t match, don’t display it. Simple! The same circuit is implemented in software in the Z8 and the Qbasic program. Data is fed in, LSB first. The last byte sent by the DS1820 is the CRC. The result, once passed through the CRC routine, will be zero if all bits are received correctly. This unit is designed to work unattended, therefore the Z8 watchdog instruction (WDT) is used. Any spikes that upset the Z86E08 will cause it to RESET. The WDT instruction, once enabled, has to be ‘refreshed’ every 10ms or so. It is set up in such a way, that the micro will RESET if it is caught in any loop longer than required. Many programmers misuse the WDT instruction, simply putting it in the timer loop, where it will be ‘refreshed’, regardless of whether the main loop has crashed or not. A method I have found that seems to work OK is to January 1997  85 Fig.6: use this template to cut the rectangular hole for the DB25 socket and for the display window in the lid of the case. put the WDT instruction in a timer routine, but within a loop that always executes, but only if a variable in the main loop is loaded with $FF at every reasonable opportunity. The variable is decremented towards zero in the timer routine, and at the same time executing the WDT instruc­tion. If the variable fails to be loaded with $FF and hits zero, the WDT instruction is bypassed, thereby resetting the processor. The Qbasic program written for this project displays eight boxes on the screen. As the data is received from the Digi-Temp, the box associated with the sensor number is updated. Each box can be given a name (eg, inside, outside, etc) which is saved to disc. With a little effort, a good data logger could easily be developed from this program. A disc containing the full source code (in Z8 assembler) and the Qbasic PC program (Z8temp.BAS and Z8temp.EXE) is avail­able – see details in “Where To Buy The Kit”. Construction All the circuitry for the Digi-Temp 86  Silicon Chip is mounted on a small PC board measuring 113 x 60mm. This mounts the DB25 socket and the 3.5mm stereo socket so there is no wiring except for the three wires which run away to the sensors. The board has corner cutouts so that it becomes a snug fit inside the plastic case which measures 120 x 65 x 39mm. Fig.5 shows the component overlay for the PC board. Note that it is a double-sided board and the four LED displays are mounted on one side, while all the rest of the components are mounted on the other. Another point which should be made is that the PC layout in Fig.5 differs from that of the prototype board shown in the photos. Two rectangular cutouts need to be made in the case, one as a clearance hole for the bracket of the DB25 connector and the other in the lid, for the display window. This is then fitted with a piece of red Perspex. Templates for the two cutouts are shown in Fig.6. Assembly of the PC board is straightforward although there are a few points to be noted. Resistors R1-R7 and R13 are all bunched together so be careful with R13 as it is 470Ω not 180Ω! The cathodes of the rectangular LED and D1 are denoted by the square pad, as are the positive legs of all electrolytic capacitors. The 3-terminal regulator IC5 is bolted to the PC board and the copper pattern provides a degree of heatsinking. Rather than use removable links, just solder wire straps in the LK2 and LK3 positions, and leave LK1 open. The DB25 socket, MAX232 and capacitors C5-C8 can be left off if the unit is not going to be used with a PC. The PC board sits snugly on the ribs halfway down in the case, so there is no need to use any mounting hardware. Actually, as only two wires are used for the RS232 option, the DB25 socket could be omitted and just two wires run directly from the X2terminals on the board to the PC. The LED displays are mounted on lengths of socket strip to bring them closer to the Perspex window. Solder a different coloured wire to each leg of the sensor(s) and cover each connection with a length of suitable heatshrink, then cover the whole with a larger piece. I have found that encasing the DS1820 completely in heatsh­ rink tends to insulate it too much, so it is better to cover the sensor to about halfway up its body, then shrink it. Once this is done, smear some silicone sealant thinly around the protruding part of the sensor to waterproof it. If the unit is to be used outdoors, don’t use the 3.5mm plug and socket; solder the wires direct to the PC board. A hole is drilled for the power\data pair. Temperature sensor setup There is really no setup required when the Digi-Temp is used as a standalone unit. Just connect a 12VDC plugpack and switch on. When it is first switched on, the unit will display the number of sensors found on the lefthand side digit, then the unit will then display the temperature of each one, at one second intervals. If you want the sensors in a particular order, you will have to temporarily connect them all, then switch on the unit. Carefully remove a sensor while the power is on (they are open Drain, so will come to no harm). The display will flash the number of the Silicon Chip BINDERS These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf.  High quality  Hold up to 14 issues The temperature sensors are wired together using a 3-way cable. Two leads are for the power supply rails (+5V and GND), while the third is the data line. one removed. Repeat this process for all the sen­sors. It is best to mark them with the number so that you know the order in which to permanently connect them. PC operation The data format transmitted by the Digi-Temp is set out below (the commas are not in the data but are included here for clarity): (CR),n,(sign),x,x,.,x,C,C . = decimal point ($2E); and C = ASCII HEX No.’s 0 to 9, A to F ($30 to $39, $41 to $46) CC is the CRC of the previous data (but not the CR). It has to be decoded back to its binary equivalent before it can be passed through the CRC routine in the Qbasic program (and the Rain Brain). The reason that this is done is so that an ordinary communications program can read the output of the RBST2. If the straight binary CRC was transmitted, it would cause all sorts of hash to appear on the screen. For instance, a binary $AB is transmitted as ASCII SC ‘AB’ ($41,$42). Where To Buy The Kit Price: $A14.95 (includes postage in Australia). NZ & PNG orders please add $A5 each for postage. Not available elsewhere. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. Use this handy form Enclosed is my cheque/money order for $________ or please debit my  Bankcard    Visa    Mastercard Parts for the Digi-Temp are available as follows: Item Programmed Z86E08 microprocessor PC board DS1820 temperature sensor Z8 source code disc plus Qbasic program Full kit (includes one DS1820) Full kit (less RS232 parts)  SILICON CHIP logo printed in gold-coloured lettering on spine & cover  where CR = $0D; n = sensor number 1 to 8 ($31 to $38); sign = + or - ($2B or $2D); x = ASCII digits 0 to 9 ($30 to $39);  80mm internal width Card No: Price $18 $15 $11 $12 $75 $60 P&P incl. $2 incl. incl. $3 $3 Payment may be made by cheque or money order to Mantis Micro Products, 38 Garnet Street, Niddrie, Vic 3042. Phone/fax 03 9337 1917. ______________________________ Card Expiry Date ____/____ Signature ________________________ Name ___________________________ Address__________________________ __________________ P/code_______ January 1997  87