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TECHNOLOGY READER CIRCUITS ured as an input, which also means that an internal pull-up resistor is connected to the pin. The capacitor is charged via this resistor, but the charging time constant is much longer than the processor speed. During the interval when the input is still low, the value of the key is changed in rapid succession by a certain algorithm. This continues until the capacitor is fully charged. The input read as a ‘1’ at this point, at which time the key value stops changing. This results in a key with a random value. The randomness arises from the fact that the charging process is slow relative to the processor speed. The charging time corresponds to approximately 10,000 processor cycles. Tests have shown that the charging time is not entirely constant. A 0.01-percent change in the charging time is sufficient to yield a different result. This also shows that two different transmitters will never generate the same key, especially because most capacitors have a tolerance of around 5%. With a normal start-up process (switching on power), the transmitter first reads the key present in memory, but it cannot be transmitted just like that. A preamble with a duty cycle of exactly 50% is transmitted first to allow the operating point of the data slicer to be adjusted. The transitions in the preamble also serve to synchronise the timing at the receiver end with the timing at the transmitter end. A start bit is transmitted after the preamble to indicate that the key will be transmitted next. The ‘real’ data is only sent after the start bit, after which the transmitter is switched off immediately and the microcontroller enters the power-down mode to minimise battery consumption. A different program is naturally necessary at the receiver end. In contrast to the transmitter, the receiver is always on. In this case the microcontroller listens to see whether anything is being received. If it is, it checks whether the received data stream has the correct format, which includes checking whether the start bit synchronisation is valid. After this it reads the key and compares it with the value of the key stored in its EEPROM memory. The microcontroller has a jumper that enables it to read the key. If the jumper is fitted, the receiver is in ‘learn mode’. If a key is received in this mode, it is stored in the EEPROM. This means that the first time the receiver is switched on, the jumper must be fitted 40 and the code transmitted using the key transmitter so the key can be stored in the receiver’s memory. After the jumper is removed, the receiver should switch its outputs when the key transmitter is activated. It may be necessary to disconnect the receiver antenna for this procedure and activate the transmitter close to the receiver. This will reduce the effect of any interference signals present at 433.92 MHz. The watchdog timer is also enabled in the microcontroller on the receiver end. If the program counter is affected by a noise pulse on the supply line, for instance, which can lead to a ‘hung’ system, the microcontroller will be reset automatically after approximately 1 second and the receiver will execute a normal start-up. Data slicer and Manchester coding The data slicer in the receiver is built around an opamp in the receiver IC that is intended to be used for this purpose. A data slicer is simply a comparator with a reference level that is automatically set when a data signal with a duty cycle of 50% is received. It is thus important that the duty cycle of the data signal is in fact 50%. Manchester coding is used to achieve this result. The circuit of the data slicer is shown in Figure 3. The time constant of R and C is large relative to the data bit rate that is used. As a result, the DC level at the inverting input is equal to the average value of the received signal. With a 50% duty cycle, it will thus be exactly halfway between the minimum and maximum values. However, a certain amount of hysteresis is desirable because the receiver also generates noise when no signal is present or the signal is relatively weak. This is provided by Rl and R2. It is also necessary to have the transmitter send a preamble before the actual data. The preamble consists of a series of clock pulses with a duty cycle of 50% and a period that is much shorter than the RC time constant of the data slicer. This ensures that the operating point of the data slicer is set properly. Manchester coding must be used to achieve the required 50% duty cycle with the signal used here. This essentially amounts to EXORing the data with a clock signal having the same period as the bit interval. The original bit stream is then recovered in the same manner by EXORing RSSI R1 R C the received data with a clock signal. A drawback of Manchester coding is that it increases the bandwidth of the transmitted signal by a factor of 2, but this is not a problem with the low bit rate used here. In theory, the bytes output by the decoding logic when the clock is high are the same as the bytes output by the decoding logic when the clock is low. Both bytes are in fact decoded by the receiver software. The second byte should essentially be a copy of the first one. The correctness of the timing is monitored closely by comparing the two bytes after each set of eight decoded bits. Timing The most important aspect with regard to the algorithms used in the software at both ends (transmitter and receiver) is proper timing. The transmitter must generate a bit stream with a certain frequency, and the receiver must be able to recover it with sufficient accuracy. For this reason, the key transmitter sends the receiver a preamble consisting of clock pulses at the same frequency as the bit rate before it sends the coded 24-bit key. The preamble is followed by a start bit that enables the receiver to deduce that the preamble is finished and the actual data will follow. The ATtiny15L microcontrollers at each end run on their internal RC clocks. A calibration byte for the clock generator is programmed into each microcontroller by the manufacturer. However, this calibration value is intended to be used with a supply voltage of 5 V. As the transmitter operates at 3 V, its clock frequency is lower than it would be at 5 V. For this reason, an extra provision has been made in the software of the receiver. With the receiver in learn mode (as described below), the receiver R2 050364 - 13 DATA OUT Figure 3. Design of the data slicer in the MAX1473. elektor electronics - 12/2006
copies the timing of the transmitter as well as its 24-bit key. Both values are stored in the EEPROM of the microcontroller and used when the receiver is operating in the normal mode. Programming settings The code for the ATtiny microcontrollers in the transmitter and receiver is available free of charge on the Elektor Electronics website under item number 050364-11 (see month of publication). The microcontrollers must be programmed using a suitable programmer before they are fitted to the circuit boards. When you program the microcontroller, don’t forget to read the oscillator calibration byte from the signature memory of the microcontroller and reprogram it in flash memory location 1023 ($3FF). It is also important to tick BOD as enabled and set the BOD level to 4.0 V for the receiver or 2.7 V for the transmitter. If BOD is not programmed, the content of the EEPROM memory (which contains the key code) can be affected when power is switched on or off, with the result that things may no longer work properly. The ‘very quickly rising power’ fuse settings (CKSEL = 11) must also be programmed for the transmitter and the receiver. This is partly related to proper operation of the watchdog timer. Initial use After you have fully assembled the transmitter board, you must first activate it so it can generate and store a code. It’s a good idea to do this a couple of times to be sure that a 24-bit code has been generated and stored. Next, fit J1 on the receiver board. Then switch on the receiver, but do not connect any antenna yet (to avoid picking up interference). Hold the transmitter close to the receiver board and activate it a couple of times. Now remove J1 while the receiver is still on and reset the receiver (by switching it off and on) without J1. The supply voltage must drop to nearly zero during this process, so watch out for the effects of any large capacitors in the power supply. If everything goes well, the LED connected to OUT1 of the receiver will now light up for around 1 second each time you press the transmit button. To be on the safe side, hold the transmit button of the receiver pressed for at least 1 second each time you activate the transmitter. If everything is working the way it should, you can connect the receiver outputs to the device or circuit to be operated. Bear in mind that it may be necessary to connect a buffer stage or semiconductor relay between the receiver and the load. [1] Crystals used in this project: www.hongkongcrystal.com Tx: 9SMI356000E03FAFZOOO Rx: 9SMI322560E03FAFZOOO (050364-1) Coil data L1 (transmitter): 3 turns 0.3 mm dia. silver-plated copper wire, coil diameter 2.5 mm, length 5 mm L1 (receiver): see Figure 2. L2 (receiver): see Figure 2 12/2006 - elektor electronics Advertisement Professional Features – Exceptional Price 34 Channels sampled at 500 MHz Sophisticated Multi-level Triggering Transitional Sampling / Timing and State Connect this indispensable tool to your PC’s USB 1.1 or 2.0 port and watch it pay for itself within hours! Economical 2-day Shipping! 500 MHz Max.Timing Mode Sample Rate (internal clock) 200 MHz Max. Sample Rate State Mode (external clock) Multi-level Triggering on Edge, Pattern, Event Count, Group Magnitude/Range, Duration etc. Real-Time Hardware Sample Compression Qualified (Gated) State Mode Sampling Intronix Test Instruments, Inc. 16421 N.Tatum Blvd. Unit 130 Phoenix,Arizona 85032 USA Tel: (001) 602 493-0674 Fax: (001) 602 493-2258 www.pcTestInstruments.com www.pcTestInstruments.com Interpreters for I2C, SPI and RS232 included Integrated 300 MHz Frequency Counter +6V to -6V Adjustable Logic Threshold supports virtually all logic families Full version of software free to download Mictor adapter available Visit our website for screenshots, specifications and to download the easy-to-use software. * The amount of £210 was calculated at the time this ad was created. The actual price will depend upon the exchange rate at the time of purchase. Price does not include the UK VAT of 17.5% plus a small import fee of typically £5-10. These fees will be collected at the time of delivery (companies can claim a refund for the 17.5% VAT from HMRC). E&OE
Page 1 and 2: free supplement Tune that dial shor
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