Summary of design scheme based on simple digital frequency meter of single chip microcomputer

**Design Scheme Based on a Simple Digital Frequency Meter Using a Single-Chip Microcontroller (1)** The system block diagram is shown in Figure 1. It mainly consists of the AT89C52 microcontroller, an XOR device, an LCD 1602 display, a power supply, and other components.

**Principle of Frequency Measurement** The timer/counter operates in mode 1. Each time a timer 0 interrupt occurs, it counts 65536 pulses. These pulses come from the oscillator's 12-divided signal, with a period of 1 μs. By counting how many times the timer 0 interrupt occurs when the external interrupt 0 is triggered, and using the value X in the timer/counter 0 count register at that moment, the period T of the square wave under test can be calculated as: T = (65536 × μ + X) μs. The frequency of the square wave can then be found by taking the reciprocal of T, with two decimal places retained, resulting in a precision of 0.1 Hz. **Phase Difference Measurement Principle** The ratio of the pulse width t obtained by combining two square waves of different phase signals through a phase detector (XOR) to the period T of the square wave gives the phase difference between the two signals. The formula for this is:

The method for measuring the pulse width is similar to measuring the square wave period. A schematic of the phase difference measurement is shown below:

In this design, the P0 port (pins 32–39) is defined as the N1 function control port, connected to the corresponding pins of N1. When the microcontroller is operating normally, it requires a clock circuit and a reset circuit. In this case, an internal clock mode and a button-level reset circuit are used to form the minimal microcontroller system, as shown in Figure 4.

Figure 4: MCU Minimal System

The overall circuit diagram of the frequency and phase meter is shown in Figure 5:

Figure 5: Overall Circuit Diagram of the Frequency Phase Meter

**Design Scheme Based on a Simple Digital Frequency Meter Using a Single-Chip Microcontroller (2)** This paper uses the prescaler SAB6456A and high-speed digital divider 74HC390, combined with the new MSP430F449 microcontroller, to propose a novel, fully automatic digital display RF frequency measurement design.

Figure 1: Signal Front-End Processing and Frequency Dividing Circuit

**Main Device Introduction** **MSP430F449 Microcontroller** The MSP430F449 features a 16-bit RISC architecture with extensive on-chip peripherals and high-capacity working registers and memory, offering a high price-to-performance ratio. Its key features include: - Ultra-low power consumption: Operates from 1.8V to 3.6V, with five low-power modes (LPM). In low-power mode, the CPU can be awakened by interrupts within less than 6μs. - Strong computing power: 16-bit RISC structure, rich addressing modes, 16 interrupt sources, and up to 125ns instruction cycle at 8MHz. Includes an internal hardware multiplier, 64KB Flash, and 2KB RAM. - Rich on-chip peripherals: Watchdog timer, basic timer, comparator, 16-bit timers (TA, TB), serial ports, LCD driver, 6×8-bit I/O ports, and 12-bit ADC (up to 200kHz sampling rate). - Convenient development environment: Flash-based device with JTAG interface for program download and debugging without an emulator or programmer. **Prescaler SAB6456A** The SAB6456A is a prescaler designed for UHF/VHF frequencies. It can divide input frequencies from 70MHz to 1GHz by 64 or 256, depending on the MC pin status. High sensitivity and strong harmonic suppression make it ideal for RF applications.

Figure 2: MCU Peripheral Circuit

**Working Principle** The design consists of two main parts: frequency division and counting. First, the input signal is clipped and divided by the SAB6456A. Then, the 256-divided signal is further divided by two 74HC390 high-speed dividers, converting the analog signal into a low-frequency digital signal below 10kHz. This signal is then fed into the MSP430F449 MCU, which uses its 16-bit timer A for timing and counting. The timer has several functions, including a counter section, capture/compare registers, and an output unit. Using the continuous counting mode and rising edge capture mode, the time of N pulse signals is measured in the timer interrupt, and the frequency is calculated by dividing N. **Hardware Design** Figure 1 shows the front-end processing and crossover design of the signal. The output signal is divided by two SN74HC390s, each capable of dividing up to 50MHz. Connecting them in series allows a total division of 1000, reducing the signal frequency to about 4kHz or lower, suitable for direct counting by the microcontroller. Figure 2 shows the peripheral circuit of the microcontroller, including the reset circuit, power supply, and crystal oscillator necessary for operation. The crystal oscillator provides 8MHz and 32.768kHz signals, with 8MHz used as the timer A input clock and 32.768kHz for the digital tube display. The 74LS373 is a D-type latch that can directly drive common cathode digital tubes due to its high output current. **Software Design** The divided output terminal OUT is connected to the frequency input of the MCU. After a delay to stabilize the signal, the program opens the capture interrupt and timer A, counts N pulses in the timer A interrupt, and calculates the frequency by dividing N. The result is multiplied by the frequency division coefficient to get the actual frequency, which is then displayed. After a short delay, the process repeats. When measuring low-frequency signals, the LED display is turned off to ensure accuracy. For such cases, fewer pulses are measured or the delay is reduced to minimize average measurement time. Dynamic scanning display works by sending scanning signals to each digit one by one via P4, while P3 inputs the segment data to drive the digits. This ensures that each digit is displayed sequentially, with a display time of more than 1ms, preventing visible flicker. **Main Measurement Procedure:** ```c void frequency_measure(void) { float tmp, tmp1; Key_flag = 0; // Clear button flag P1OUT |= BIT0; Delay(1000); // Wait for signal stabilization while (1) { IE2 &= ~0x80; // Disable BT, turn off LED Firstflag = 1; // Start measuring first pulse TACTL |= TAIE; // Enable capture CCTL1 |= CCIE; // Enable Timer A while (f_ok_flag == 0); // Wait for measurement end f_ok_flag = 0; if (aa1 != aa2) { overflow--; } tmp = aa2 - aa1; tmp1 = 40.0 / (overflow * 0.008191875 + (tmp / 8000000.0)); result = tmp1 * 0.256; IE2 |= 0x80; // Enable BT, turn on LED Yanshi(2, 2); // Adjust delay here CCTL1 &= ~CCIE; // Disable capture TACTL &= ~TAIE; // Disable Timer A return; } } ``` The flowchart is shown in Figure 3.

Figure 3: Main Program Flow

**Conclusion** The hardware and software described in this paper have been tested in practice, and the results show high measurement accuracy. The instrument is cost-effective and has a simple structure, making it an economical frequency tester. **Design Scheme Based on a Simple Digital Frequency Meter Using a Single-Chip Microcontroller (3)** The digital frequency meter measures frequency using timing and counting, and dynamically displays 6 digits on a 1602A LCD. The measurement range includes sine, square, and triangle waves from 1 Hz to 10 kHz, with a time base width of 1 μs, 10 μs, 100 μs, and 1 ms. The microcontroller enables automatic measurement functionality. **Design Idea and Frequency Calculation of the Frequency Measuring Instrument** The design involves dividing the signal, measuring the number of cycles of a known standard frequency signal over one or several measured signal periods, and then calculating the frequency. The principle is illustrated in Figure 1 on the right.

If the period of the measured signal is Tx, and the frequency division number is m1, then the period of the divided signal is T = m1 × Tx. As shown in the figure, T = N × To (where To is the period of the standard signal). Thus, the frequency f of the measured signal can be calculated. Since the standard frequency is stable and the quantization error is small, the measurement accuracy depends mainly on the value of N. The larger N is, the smaller the error and the higher the accuracy. **Basic Design Principle** The basic principle of the frequency meter is to directly display the frequency of the measured signal in decimal form. It automatically measures the frequency of sine, square, and triangle waves during a measurement cycle. **Frequency Definition** Frequency refers to the number of times a periodic signal changes within a unit time (1 second). If the number of repetitions N of the periodic signal is measured within a certain time interval T, the frequency can be expressed as f = N/T. The pulse shaping circuit converts the measured signal into a pulse signal with a repetition frequency equal to the measured frequency fx. The time reference signal generator provides a standard time pulse signal. When the gate circuit is opened for exactly 1 second, the measured pulse signal passes through the gate to the counting and decoding display circuit. At the end of the second, the gate closes, and the counter stops counting. The number of pulses counted in one second is N, so the measured frequency fx = N Hz. **Hardware Structure Design of the Digital Frequency Meter (Low Frequency)** **System Hardware Composition** The main component of the data acquisition system is the AT89C51 microcontroller, which handles frequency counting and display. It also includes external devices like a frequency divider and display. The system is divided into modules such as amplification shaping, second pulse generation, shift analog conversion, microcontroller system, and LCD display. The relationship between modules is shown in Figure 2.

**System Working Principle Diagram** The general working principle of the system is shown in Figure 3:

Figure 3: Digital Frequency Meter System Working Principle Diagram

**Signal Conditioning and Amplification Shaping Module** The amplification shaping system includes an attenuator, follower, amplifier, and Schmitt trigger. It converts a sinusoidal input signal Vx into a square wave Vo of the same frequency. The measured signal is divided and amplified to avoid waveform distortion. An operational amplifier-based emitter follower increases the input impedance. The non-inverting op amp has a gain of (R1 + R2)/R1, adjustable by changing R1. The shaping circuit uses a Schmitt trigger, and the shaped square wave is sent to the gate for counting. Since the input signal amplitude is uncertain, it may be too large or too small, causing damage or failure to detect. Therefore, the signal conditioning circuit performs impedance transformation, amplification, limiting, and shaping. The circuit schematic and parameters are shown in Figure 4:

**Time Base Signal Generation Principle** This circuit uses a 32768Hz crystal oscillator and the CD4060 chip to generate a 2Hz signal after 14-stage frequency division. After passing through a CD4013 dual D flip-flop, a 0.5Hz square wave is produced, serving as the second pulse signal for the microcontroller to count.

Figure 7: Second Pulse Generation Circuit Schematic

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