barcode reader in asp.net AVR PROJECT 5: RADIO BEACON CONTROLLER in Software

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248 AVR PROJECT 5: RADIO BEACON CONTROLLER
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However, some AVR processors are equipped with some features to distinguish between a power-on reset (or an external reset) and a watchdog reset. Tiny22 is one such processor that allows a program to determine the source of reset in a simple manner. The MCU status register (MCUSR) provides information about the source of reset. The MCUSR contains 2 bits which indicate the source for the reset as per Table 3.4 (in 3). The MCUSR register contains the EXTRF and the PORF flags. When both of these flags are 0, the reset source is the watchdog reset, and when either PORF is 1 or the EXTRF is 1, the source of reset is power-on reset of the external reset (i.e., the reset pin is grounded momentarily). By using this elegant feature of the processor, one could implement a scheme to provide a large (and programmable) time period between generating the beacon signal repeatedly. The other design goal is to be able to store any Morse code sequence in the processor and be able to easily change that when required. We could either program the message as a string in the flash program memory or in the EEPROM, since the Tiny22 has 128 bytes of EEPROM. One advantage of storing the Morse code in the EEPROM is that the message could be changed on the fly without having to program the flash program memory. Therefore we decided to store the Morse code in the EEPROM of the controller rather than in the program memory (Figure 14.2). This beacon controller design is a truly one-chip solution with just a power-on-reset circuit. No crystal is required, as we will be using a Tiny22 part which has an internal RC oscillator for the clock. The output of the controller controls the transmission of a suitable RF oscillator/amplifier. Figure 14.3 illustrates the block diagram of our Tiny22-based beacon controller. One of the outputs is used to generate the audio sidetone of the Morse code signal, and another output is used to switch the RF transmitter on and off (Figure 14.4). Figure 14.5 illustrates the circuit schematic for the beacon controller. A reset switch is provided to reset the system manually, and an LED is a visual indicator when the system starts transmitting the Morse signal. The software could be programmed to transmit every few minutes. We could choose to transmit a five-WPM (words per minute) message from the controller, and this could be repeated every minute or less (say 30 seconds). It is common to transmit carrier for a few seconds followed by the message. Let us choose two seconds of CW, two seconds of
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FIGURE 14.2 Morse output for my callsign VU2NOX.
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DESIGN DESCRIPTION 249
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FIGURE 14.3 Block diagram of a radio beacon controller using the Tiny22 processor.
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silence followed by the message, and this sequence is repeated. The software could be changed to choose any Morse transmission speed between five WPM and thirty WPM. This simply means that one of the I/O pins is set to 1 for transmission of the carrier (by enabling the RF transmitter) and set to 0 for disabling the carrier. The Morse code characters, a dot and a dash, have 1:3 time relation, i.e., a dot is a sound for 1 time unit and a dash is a sound for 3 time units. The interval between a dot and a dash of a single character is 1 time unit, e.g., code for V dit dit dit dash, which is 1 0 1 0 1 0 1 1 1. Here a 1 means that the audio tone is on and 0 means that the audio tone is off. Three consecutive 1 s mean that the sound is on for 3 time units. Space between any two characters of a word is 3 time units of silence and between two words is 5 time units. So a complete transmission of my call sign, VU2NOX will be as illustrated in Figure 14.2. The Tiny22 has a watchdog timer that, if enabled, resets the processor when it expires. The timer has its own oscillator with a nominal frequency of 1 MHz. The watchdog timer can be set so as to expire in 16-K clock cycles to 2-M clock cycles. This gives a watchdog time span ranging from 16-ms minimum to 2-s maximum. I have chosen to use the watchdog timer to implement the wake up from power-down sleep state of the processor. The watchdog timer is programmed to expire (and reset the processor) after 2 s. For this project, the goal is to transmit the message every 30 seconds. To do that, I use an internal RAM location (the internal RAM is not initialized at RESET). If the contents of this location exceed 15, I execute the routine to transmit the message. Else, the RAM contents are incremented by one and the watchdog timer is enabled to reset the system after 2 s. After the watchdog timer is armed and the processor executes the sleep instruction, the processor enters the power-down mode. It wakes up again when the watchdog timer expires and resets the system. This scheme allows system operation at minimum power consumption. Figure 14.4 illustrates the flowchart for the beacon controller system code.
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