ammeter and voltmeter

I. Hardware Design
1.1 Microcontroller-Controlled
Current and Voltage Meter The current and voltage meter uses the Divinstar CW32F030C8T6 development board as the main controller. The main controller chip has a maximum clock frequency of 64MHz, 64KB FLASH, and 8KB RAM. This project utilizes a total of 26 I/O ports: 14 pins for digital tube display, 3 pins for button control, and 4 pins for ADC conversion.
1.2 Power Supply Circuit

This circuit uses an LDO as the power supply, specifically the SE8550K2 with a maximum input voltage of 40V.
Electrolytic capacitors are placed before the power supply to filter low-frequency noise and provide a stable DC voltage; ceramic capacitors are placed after the load to handle high-frequency noise and improve the circuit's high-frequency performance.
An additional 10Ω series resistor is used in the circuit for voltage division, which can act as a low-resistance fuse and also reduce the peak power-on surge.

1.3 Sampling Circuit
1.3.1 Voltage Sampling
A voltage divider circuit is used to acquire voltage. The design allows for the acquisition of 100V, but the current configuration allows for the acquisition of voltages from 0-30V.

Selection of Divider Resistor: The maximum voltage to be measured is 30V, and the ADC reference voltage is 1.5V. A 10K resistor is chosen for the low-side to reduce power consumption. The calculated voltage division ratio is 1.5V/30V = 0.05, resulting in a calculated high-side resistance of 10K/0.05 = 200K. A standard resistor of 220K is chosen that is closest to but greater than the calculated result.
Application of Clamping Diode: To protect the MCU from damage, an additional 1N4148 diode is added as a clamping diode. The forward conduction characteristic of the diode limits the voltage in the circuit to within the forward voltage drop (typically 0.7V), achieving a protective function.
Implementation of Measurement Range Switching: An additional voltage sampling circuit is introduced, making voltage range switching possible. The voltage sampling circuit uses the ADC_IN11 channel to measure voltages up to 30V. If the measured voltage is within the 0~3V range, the system will switch to the ADC_IN9 channel for measurement. This switching significantly improves measurement accuracy due to the reduced voltage division ratio.

1.3.2 Current Sampling
: A low-side current sampling circuit is used for current detection. The low side of the sampling circuit shares a common ground with the development board's meter interface. When selecting the sampling resistor, the following factors need to be considered:

the expected maximum measured current is 3A;
it is not recommended that the voltage drop introduced by the current sensing resistor exceed 0.5V;
the power consumption of the current sensing resistor should be considered, and a suitable package should be selected; in this case, a 1W metal wire-wound resistor is selected;
there is no amplification factor, the factor is 1;
calculations show that a 100mΩ current sensing resistor introduces a voltage of 300mV and a power consumption of 900mW at 3A current.

In summary, selecting a 100mΩ current sensing resistor is the optimal choice based on the given conditions to ensure measurement accuracy, power consumption, and voltage drop are within acceptable ranges.
1.4 Calibration
Circuit: An additional TL431 circuit is added to provide a 2.5V reference voltage. The core of the TL431 circuit is an operational amplifier used as a comparator. In the circuit, Vref (approximately 2.5V) is applied to the inverting input of the comparator. The non-inverting input of the comparator receives a voltage input to the REF pin. When this voltage is greater than Vref, the comparator outputs a high level, turning on the transistor and connecting it to the cathode and anode of the TL431. When REF and the cathode are at the same potential, the potential of the REF pin is pulled low. When the REF potential is lower than Vref, the comparator outputs a low level, causing the transistor to turn off and the REF potential to rise again. This comparison and control process is repeated cyclically. Due to the very fast hardware response speed, the REF potential is almost equal to Vref.
The display circuit
uses two 0.36-inch three-digit common-cathode LED displays as the display device. Compared to a display screen, LED displays have better visibility and are easier to identify in complex environments. LED displays have better mechanical properties and are less susceptible to damage from external forces. The brightness of the LED displays can be increased by adjusting the current-limiting resistor according to different environmental requirements. In this project, the current-limiting resistors (R8~R15) of the digital tube are configured to 300Ω, exhibiting good visibility and soft brightness.
II. Software Design
2.1 Main Function
The main function of the project is shown in the figure. It is mainly used to initialize various peripherals and implement the current and voltage meter functions. The following is a brief introduction to the `main` function:
1. `board_init()`: This function is used to initialize the development board hardware peripherals and system clock settings.
2. `tim1_Init()`: Initializes Timer 1 and simultaneously enables timer interrupts.
3. `LED_Init()`: LED initialization function. 4. `
Seg_Init()`: Digital tube initialization function, used to initialize the pin configuration of the digital tube, etc.
5. `Key_Init()`: Key initialization function, configuring and initializing the key pins and related parameters.
6. `ADC_init()`: ADC initialization function, used to initialize the analog-to-digital converter.
7. `read_vol_cur_calibration()`: Reads the calibration values ​​of voltage and current, used to calibrate the accuracy of measured and displayed values.
8. `ComputeK()`: Calculates the calibration coefficient K, used to establish an accurate relationship between the measured value and the actual value.
2.2 Details of the digital tube display
code can be found in `Seg_Display.c`. The following is a brief description of using timer interrupts to implement dynamic refreshing of the digital tube:

First, the timer needs to be initialized to generate interrupts at a certain frequency for dynamic display. Initialize the digit selection and segment selection pins, set them to output mode, and connect them to the corresponding digital tube pins and control chip pins.
Store the numbers or characters to be displayed on each digital tube in an array, such as `Seg_Table[]`. Write a loop to sequentially control the digit selection line of each digital tube to be high and the others to be low, while simultaneously setting the segment selection pin state to display the corresponding number. Continuously switch the state of the digit selection line in the timer interrupt to achieve rapid switching between digital tubes, thus realizing a dynamic display effect.
In the timer interrupt service function, record the current digit selection position, and set the corresponding digital tube's digit selection line to high and the others to low based on that position. Simultaneously, based on the current digit selection position, the number or character to be displayed is read from the Seg_Table[] array, and the state of the segment selection pin is set to display the corresponding content. The display switches to the next digit selection position in the next timer interrupt, and so on, displaying in a loop.

2.3 Key Detection
This project implements the switching of digital tube display modes and parameter calibration functions through key operations. A total of 5 working modes are defined, each used to display different types of voltage and current values ​​and perform calibration.
Key Operation Functions:
K1 key: Used to switch display modes.
K2 key: Used to set the parameter value in the corresponding mode and save the value to FLASH.
K3 key: Used to return to mode 0.
Introduction to each mode:
Mode 0: Displays normal voltage and current values; the upper row of digital tubes displays the voltage value, and the lower row displays the current value.
Mode 1: 5V voltage calibration setting, displays calibration number 5.05. After adjusting the measured bit to 5.00V, press the K2 key to calibrate the current value to 5V.
Mode 2: 15V voltage calibration setting, displays calibration number 5.15. After adjusting the measured bit to 15.0V, press the K2 key to calibrate the current value to 15V.
Mode 3: 0.5A current calibration setting, displays calibration number A.0.5. Press the K2 key to calibrate the current value to 0.5A.
Mode 4: 1.5A current calibration setting, displays calibration number A.1.5. Press the K2 key to calibrate the current value to 1.5A.
2.4 Timer Interrupt
The above code is a timer interrupt handler function BTIM1_IRQHandler(), used to handle the overflow interrupt of Timer 1. The following is a brief description of the timer interrupt handling function:
Each time a Timer 1 overflow interrupt occurs, first check if the overflow interrupt flag is set to ensure that the interrupt handling function is triggered by a Timer 1 overflow interrupt.
Perform the following main operations:
Obtain the ADC value using the Get_ADC_Value() function.
Control the LED to blink; after a certain time interval (ledcount greater than or equal to 1000), toggle the PC13 pin to achieve the LED blinking effect.
Update the runtime counter timecount.
Call the Dis_Refresh() function for scanning and displaying the digital tube.
Handle button operations:
Monitor button K1 (connected to pin 12 of GPIOB) to switch button modes.
Monitor button K2 (connected to pin 13 of GPIOB) and perform different processing based on the current mode:
Mode 1: Calibration voltage 5V.
Mode 2: Calibration voltage 15V.
Mode 3: Calibration current 0.5A.
Mode 4: Calibration current 1.5A.
The monitoring button K3 (connected to pin 13 of GPIOB) is used to return to mode 0, clear calibration data, and update the digital tube display.
This timer interrupt handling function implements the response to button operations, LED blinking control, digital tube display refresh, and voltage and current value calibration functions for different modes.
2.5 ADC Conversion
The above code is used to initialize the ADC. The following is a brief description of this function:

The ADC clock divider is set to 128, the reference voltage is selected as 1.5V, and the sampling time is set to 10 ADC clock cycles.
Channel ADC_SqrCh11, as the first channel of the ADC sequence (ADC_Sqr0Chmux), is connected to pin PB1.
Channel ADC_SqrCh9, as the second channel of the ADC sequence (ADC_Sqr1Chmux), is connected to pin PB0.
Channel ADC_SqrCh8, as the third channel of the ADC sequence (ADC_Sqr2Chmux), is connected to pin PB10.
Channel ADC_SqrCh12, as the fourth channel in the ADC sequence (ADC_Sqr3Chmux), is connected to pin PB11.

This configuration indicates that these four ADC channels (PB01, PB00, PB10, PB11) will be added to the ADC conversion sequence in a specific order and sampled in continuous conversion mode. This configuration allows the system to simultaneously and continuously acquire and convert the analog signals from these four channels to meet the system's needs for multiple analog signals.
III. Physical Demonstration
3.1 PCB
3.2 Panel and Housing
3.3 Overall Demonstration