barcode reader in asp.net DESIGN DESCRIPTION OF AN AVR-PROCESSOR-BASED FREQUENCY COUNTER in Software

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DESIGN DESCRIPTION OF AN AVR-PROCESSOR-BASED FREQUENCY COUNTER
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FIGURE 17.7 Circuit schematic for the frequency counter with an RS-232 interface.
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Output RS-232 data from the Frequency Counter to the PC
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Stop Bit Input Frequency to the Frequency Counter
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FIGURE 17.8 Logic analyzer trace of the data transmitted by the frequency counter to the PC and the input frequency to the frequency counter.
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Pin PB2 of the processor is programmed as an input pin, and the external signal whose frequency is to be measured is applied to this pin. The Timer0 is used for two purposes in this project. When the processor needs to communicate with the PC, the Timer0 is used to generate the time ticks for the serial data transmission bit times. When the frequency counter needs to measure the frequency of the incoming signal, Timer0 is used to generate the gate period. To begin a measurement cycle, the Timer0 is set up to increment either using the system clock CK frequency (which is 3.58 MHz) in the .1-s and 1-s gating period case, or CK/8 when using the 10-s gating period mode.
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298 AVR PROJECT 8: A PULSE FREQUENCY COUNTER WITH AN RS-232 INTERFACE
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Data Received on the PB1 pin of the processor, after converting the incoming RS-232 signal to a unipolar voltage clamped to the +Vcc supply of the processor.
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Data Transmitted by the PC RS-232 signal TxD signal pin. Signal is legal RS-232 level.
FIGURE 17.9 Logic analyzer trace of the data transmitted by the PC RS-232 port on the TxD pin and rectified and clamped to convert to a unipolar, TTL-level signal on the PB1 pin of the frequency counter.
After the measurement cycle is over, the frequency counter transmits the frequency of the incoming wave to the PC, and for this, it programs the timer to generate ticks for the bit interval times. The frequency counter communicates with the PC at 9600 bits/s, 8 data bits, no parity, and 1 stop bit (9600, 8, N, 1). The software-driven UART inverts (or complements) the incoming RS-232 signal to account for the signal inversion on an RS-232 line, and before transmitting any value on the line, it complements the number and then transmits the bits.
17.6 Usage
This frequency counter was designed specifically for interfacing to a pulse output astronomical photometer. However, it can be used easily with any sensor that has a frequency output.
17.7 Fabrication
This small circuit was fabricated on a general-purpose PCB measuring 3 cm by 3 cm. A 9-pin D type, female connector was mounted on the PCB for mating to an RS-232 cable.
17.8 Design Code
The design for the project is available in the code directory in the file avpulse.asm. The design code is split up in a few subroutines. These subroutines are as follows:
TESTING 299
1. Reset Initialization: initializes the pins, the timer, the stack pointer, and other variables. 2. Main Program Loop: Prints a welcome message on the PC screen and waits for a
3. 4. 5. 6. 7. 8. 9.
user selection to select one of the three gate periods, .1 s, 1 s, and 10 s. It then initializes the timer accordingly aad goes on to count the number of pulses for the duration of the selected time. The accumulated count is converted to BCD, the timer is reinitialized to provide serial port bit time intervals, and the result is displayed on the screen. The timer is then reinitialized to provide the gate period, and so on. Get Sample Number: This gets the user response for the gate period value. Collect Data: Counts the pulses on the signal input pin and stores the count. Print Data: Prints the BCD value of the count on the serial port output pin. Get Byte: Receives a byte from the PC serial port at 9600, 8, N, 1. Send Byte: Sends out a byte to the serial port at 9600, 8, N, 1. Tx Msg: Transmits the initial welcome message. Timer0 ISR: This is the Timer0 Interrupt Subroutine that occurs during the pulse count measurement cycle. The ISR determines if the gate period is complete or not.
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