CW32-based voltage and current meters

Related Resources:
CW32 Digital Voltmeter and Ammeter Training Camp Project Tutorial Document | LCSC Development Board Technical Documentation Center
Explanation Video:
Step-by-Step Guide to Building a Voltmeter and Ammeter_bilibili

Physical Diagram
Hardware Design

1. Power Supply Circuit
LDO (Low Dropout Linear Regulator) Selection
This project uses an LDO as the power supply. Considering that most actual voltmeter products are used in industrial scenarios with 24V or 36V power supplies, this project selected the SE8550K2 with a maximum input voltage of up to 40V as the power supply. The main reason for not using a DC-DC step-down circuit to deal with the large voltage drop is to avoid introducing DC-DC ripple interference during the design process, and the secondary reason is to reduce project costs.
2. MCU Selection Analysis:
Key Advantages of CW32 in this Project

: Wide operating temperature range: -40~105℃;
Wide operating voltage: 1.65V~5.5V (STM32 only supports 3.3V systems)
; Strong anti-interference: HBM ESD 8KV; All ESD reliability reaches the highest international standard level (STM32 ESD 2KV);
Project Focus - Better ADC: 12-bit high-speed ADC, achieving ±1.0LSB INL 11.3ENOB; Multiple Vref reference voltages... (STM32 only supports VDD=Vref)
; Stable and reliable eFLASH technology.

A detailed explanation of these advantages will be provided in the chapters on ADC sampling and related extensions.
The main characteristics of the CW32 ADC:
This project focuses on the 4 reference voltage sources.
(Content from the "CW32x030 User Manual")
3. Voltage Sampling Circuit and Calibration Circuit:
The voltage divider resistors in this project are designed to be 220K+10K, therefore the voltage division ratio is 22:1 (ADC_IN11).
The calibration circuit is used to assist in the calibration of the voltage sampling circuit.
4. Current Sampling Circuit:
This project uses a low-side current sampling circuit for current detection. The low-side of the sampling circuit shares a common ground with the development board's meter interface
. Do not solder R17 during learning!
The calibration circuit is used to assist in the calibration of the current sampling circuit.
Do not solder resistor R17 during calibration!
5. Digital Tube Display:
This project uses a digital tube as the display device, specifically a 0.36-inch common cathode digital tube.
6. LED Indicators:
Power indicator,
user LED.
7. Button Circuit Design:
GPIO needs to be configured as pull-up input mode .
8. TL431 Circuit Design for Voltage Measurement and Calibration:
This project adds an extra TL431 circuit to provide a 2.5V reference voltage, which can be used to provide an external voltage reference for the chip to calibrate the AD converter.
Software Design
: 1. Using Systick as the main interrupt:
Based on Systick, an interrupt occurs every 1ms. All places requiring timed interrupts are placed in the Systick interrupt service function .
//cw32f030_systick.c

/**
  * @brief This function handles System tick timer.
  */
void SysTick_Handler(void)
{
    /* USER CODE BEGIN SysTick_IRQn */
    uwTick += uwTickFreq;
    Systick_Use();
    /* USER CODE END SysTick_IRQn */
}

//main.c

/**
 * @brief Called once every 1ms interrupt
 *
 */
void Systick_Use()
{
    if (++Adc_tc >= 1)
    {
        Adc_tc = 0;
        Adc_ref = 1;
    }

    if (++Nixie_ref >= 2)
    {
        Nixie_ref = 0;
        Nixie_Display();
    }

    if (++Key_tc >= 10)
    {
        Key_tc = 0;
        Key_Loop();
    }

    if (++Nixie_cha_tc >= 300)
    {
        Nixie_cha_tc = 0;
        Nixie_cha_ref = 1;
    }
}
2. Button Function

K1: Switching Mode
Mode 0: Displays normal voltage and current values ​​(the upper row of digital tubes displays the voltage value .V or .V automatically switches, the lower row displays the current value _ . ** A)
Mode 1: Voltage 5V calibration value setting. The upper row of digital tubes displays 5.05. The next row displays the current voltage value in .V or .V. In this mode, the multimeter should be set to 5.00V when measuring the measured position. Pressing the K2 key will calibrate the current value to 5V.
Mode 2: 15V voltage calibration setting. The upper row of the digital display shows 5.15. The next row displays the current voltage value in .V or .V. In this mode, the multimeter should be set to 15.0V when measuring the measured position. Pressing the K2 key will calibrate the current value to 15V.
Mode 3: 0.5A current calibration setting. The upper row of the digital display shows A.0.5. The next row displays the current current value in _.**A. Pressing the K2 key will calibrate the current value to 0.5A.
Mode 4: Current calibration value setting of 1.5A. The upper row of the digital tube displays A.1.5. The lower row displays the current current value *.** A. After pressing the K2 key, the current value is calibrated to 1.5A.


K2: Select to confirm the calibration value in non-mode 0 cases. K3
: Directly switch to mode 0.

3. Calibrate
5V
10V.
Because a 12V charger is used as the power source, it is not possible to calibrate 15V
0.5A
1A.