CW32-Voltmeter and Current Meter Training Camp

I. Design Background
An ADC (Analog-to-Digital Converter) is an indispensable key component in electronic systems. It converts continuous analog signals into digital signals, enabling digital processing and analysis. ADCs play a crucial role in signal conversion, measurement and data acquisition, control system input, and communication and signal processing. Their widespread application promotes the intelligent and precise control of electronic equipment across various industries, and is one of the key factors driving modern technological progress.
Digital voltmeters and ammeters combine ADC technology with circuit measurement principles, accurately converting analog voltage and current signals into digital displays for easy reading and analysis by electronic engineers. This device not only improves the accuracy and efficiency of circuit measurements but also helps engineers better understand circuit behavior, serving as a powerful assistant in electronic design and troubleshooting, and playing a vital supporting role in the work of electronic engineers. In product applications, digital voltmeters ensure the accuracy and safety of circuit design, while also providing strong support for product quality control and subsequent maintenance. Learning to design and build a digital voltmeter and ammeter
using a benchtop digital multimeter (Agilent 34401A)
is highly beneficial for improving one's professional skills. This digital voltmeter and ammeter project covers multiple aspects, including microcontroller circuit design and implementation, signal acquisition and processing circuit design, user interface development and optimization, and product appearance design. It integrates knowledge from multiple fields such as electronics, microcontroller programming, circuit design, and industrial design. Considering the learning pace and knowledge absorption capacity of beginners, we have specially launched this introductory-level digital voltmeter and ammeter project, which is very suitable for beginners in electronics and those who want to learn more about microcontroller applications. This project has the following highlights:
• It uses a core board plus expansion board design concept, employing plug-in device design, making learning simpler and exploration more in-depth;
• The core board uses the domestic Wuhan Xinyuan Semiconductor CW32 as the main controller, while also being compatible with other similar development boards; however, the CW32 has advantages;
• The project has a high degree of integration and strong practicality; after completion, it can be used as a desktop instrument;
• The project has abundant learning materials, including circuit design tutorials, PCB design, code programming learning, and training for engineers' debugging skills.
II. Hardware Design
1. Power Supply Circuit
LDO (Low Dropout Linear Regulator) Selection
This project uses an LDO as the power supply. Considering that most voltmeter products are used in industrial scenarios with 24V or 36V power supplies, the SE8550K2 with a maximum input voltage of up to 40V was selected as the power supply. The main reason for not using a DC-DC step-down circuit to handle 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
To reduce the learning cost for everyone, this project uses the LCSC CW32F030C8Tx development board (core board) as the main controller, but this does not mean that we will talk less about this section. From the perspective of training engineers, the correct selection of the main controller is very important, as it relates to the overall advantage of the project.
Regarding the voltmeter and current meter, the author used STM32/CW32 and some other 32-bit microcontrollers for some debugging and testing. This comparison is only with the STM32F103C8T6 as a reference for device selection, primarily aimed at providing ideas and improving understanding.
Avoid blind selection.
When selecting an MCU (Microcontroller Unit) for this project, multiple aspects need to be considered to ensure the chosen MCU meets project requirements.
• Clearly define your project requirements: Understand the required computing power, including clock speed, processor core type, and whether a floating-point unit is needed.
• Identify the required I/O ports and important peripherals, such as ADC peripherals. Since this is a development board project, primarily for debugging and learning, there are no strict limitations on the number of I/O ports: i.e., the associated costs are not considered.
Key advantages of CW32 in this project
: • Wide operating temperature range: -40~105℃
• Wide operating voltage range: 1.65V~5.5V (STM32 only supports 3.3V systems)
• Strong anti-interference capability: 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:
The voltage divider resistors in this project are designed as 220K + 10K, therefore the voltage division ratio is 22:1 (ADC_IN11).
Voltage Divider Resistor Selection
: 1. The maximum value of the design measurement voltage: For safety reasons, this project uses 30V (the actual maximum display value can be 99.9V or 100V);
2. ADC reference voltage: In this project, it is 1.5V, which can be configured through the program;
3. Power consumption: To reduce the power consumption of the sampling circuit, the low-side resistor (R14) is usually selected as 10K based on experience;
then the high-side resistance of the voltage divider resistor can be calculated using the above parameters:
1. Calculate the required voltage division ratio: i.e., ADC reference voltage: design input voltage. Using known parameters, 1.5V/30V = 0.05
2. Calculate the high-side resistance: i.e., the low-side resistance / voltage divider ratio. Using the known parameters, we can calculate 10K / 0.05 = 200K.
3. Select a standard resistor: Select a resistor with a value slightly higher than the calculated value. The calculated value is 200K. We usually choose E24 series resistors. Therefore, in this project, we choose 220K, which is greater than 200K and the closest.
If, in practical use, the voltage to be measured is lower than 2/3 of the module's design voltage (66V), the voltage divider resistor can be replaced and the program modified to improve measurement accuracy. An example is provided below:
1. Assume the measured voltage is no higher than 24V, and other parameters remain unchanged
. 2. Calculations show 1.5V/24V = 0.0625, 10K/0.0625 = 160K. 160K is a standard E24 resistor and can be directly selected, or a higher value 180K can be chosen with some redundancy.
If, in practical use, the voltage to be measured is higher than the module's 99V design voltage, the voltage divider resistor can be replaced, or the reference voltage can be modified to achieve a larger voltage measurement range. An example is provided below:
1. Assuming the measured voltage is 160V, the solution is to increase the voltage reference to expand the measurement range.
2. Given that the voltage division ratio of the selected resistor is 0.0145, we can calculate 160V * 0.0145 = 2.32V using the formula. Therefore, we can choose a 2.5V voltage reference to expand the measurement range (expanding the measurement range will reduce accuracy).
Considering the potential fluctuations in the measured power supply, a 10nF filter capacitor was connected in parallel with the low-side voltage divider resistor in the circuit design to improve measurement stability.
Range switching is
an additional voltage sampling circuit added to this project; therefore, we can discuss the significance of range switching for improving measurement accuracy. Multimeters often have multiple range settings to achieve more accurate measurements. By adjusting different ranges, the optimal measurement accuracy of the measured point within the corresponding range can be obtained.
This project requires a combination of hardware and software to implement this function. When we first use the ADC_IN11 channel mentioned earlier to measure voltages below 30V, if the measured voltage is within 0~3V, then we use the ADC_IN9 channel for measurement. At this time, due to the reduced voltage division ratio, the measurement accuracy is greatly improved.
There are many ways to implement range switching; the development board design provides more design possibilities.
4. Current Sampling Circuit
This project uses a low-side current sampling circuit for current detection. When learning the low-side of the sampling circuit and the development board's meter interface
, please do not solder U3!!!
The design analysis
for this project involves a sampling current of 3A, and the selected sampling resistor (U3) is 100mΩ.
The following aspects should be considered when selecting the sampling resistor:
1. The maximum value of the pre-designed measurement current, which is 3A in this project
. 2. The voltage difference introduced by the current sensing resistor; it is generally not recommended to exceed 0.5V.
3. The power dissipation of the current sensing resistor; a suitable package should be selected based on this parameter. Considering the power dissipation (temperature) issue under high current, a 1W metal wire-wound resistor was selected in this project.
4. The voltage amplification factor across the current sensing resistor: No operational amplifier is used in this project to build an amplification circuit, so the factor is 1.
The current sensing resistor value can then be calculated using the above parameters.
1. Since this project does not use an amplifier circuit, a larger sampling resistor is needed to obtain a higher measured voltage for measurement.
2. Considering that a larger resistor would result in a larger voltage drop and higher power consumption, an unlimited selection of a larger resistor is not advisable .
3. This project uses a 1W package resistor, corresponding to a power consumption of 1W. Based on the
above data, a 100mΩ current-sensing resistor was chosen. According to the formula, 3A * 100mΩ = 300mV, 900mW.
To handle different operating environments, especially high-current scenarios, resistor U3 can be replaced with constantan wire or a shunt. The appropriate alternative can be chosen based on the specific application scenario. For safety and educational purposes, this project will not discuss measurements exceeding 3A, but the principle remains the same.
5. Digital Tube Display:
This project uses a digital tube as the display unit.
This project uses two 0.28-inch three-digit common-cathode LED displays as the display device. Compared to a display screen, LED displays offer better visibility in complex environments. The brightness of the LED displays can be increased by using smaller current-limiting resistors, depending on the specific needs of the application environment. Furthermore, LED displays have better mechanical properties and are not as easily damaged by external forces as display screens. They are widely used in industrial applications where stability and reliability are crucial. From a development board learning perspective, this makes it easier to learn electronic measurement principles and related development in a targeted manner.
In this project, actual testing showed that configuring the current-limiting resistors (R1, R8~R12) of the LED displays at 300Ω resulted in good visibility for both red and blue LED displays, with a soft and non-glaring brightness.
Strictly speaking, the current-limiting resistors should be added to the segments; adding them to the digits would affect the display effect. Our design places them in the digits to save a few resistors, but the impact on the display is not significant. Therefore, we add them to the digits for convenience.
The driving principle of LED displays mainly involves controlling the switching state of each segment to display numbers, letters, or symbols. The following is a detailed explanation of the driving principle:
1. Basic Structure of a Digital Tube:
• A digital tube typically consists of seven or eight LED segments (eight segments in this project). Each segment represents a part of the digital tube and can display numbers 0-9, letters AF, etc.
• Digital tubes come in two types: common cathode and common anode. The difference lies in whether the common terminal COM (the end connecting all LEDs) is connected to the negative or positive terminal of the power supply.
2. Driving Methods:
• Segment Selection: The desired number or character is displayed by controlling the on/off state of each segment of the digital tube. Each segment corresponds to a control signal; when the control signal is on, the segment lights up, and vice versa. (a, b, c, d, e, f, g, dp)
• Bit Selection: The digital tube to be displayed is selected by controlling its bit lines. Bit line control sets the bit line of the desired digital tube to a high level and the bit lines of other digital tubes to a low level. By continuously switching the state of the bit lines, display switching between multiple digital tubes can be achieved.
3. Driving Circuit:
The driving circuit for a digital tube can be implemented using hardware circuits, such as integrated circuits like digital signal processors (DSPs), microcontrollers (MCUs), or shift registers to generate control signals suitable for the LEDs.
These control signals can be in the form of pulse width modulation (PWM) signals, serial data signals, etc. By controlling the frequency, width, and amplitude of these signals, the brightness of the digital tube can be controlled, thus displaying the desired numbers or letters.
4. Software Control:
Besides hardware driving circuits, digital tubes can also be driven by software. By programming to generate control signals suitable for the digital tube, more flexible and complex display effects can be achieved, such as scrolling or alternating display of numbers.
5. Driving Common Cathode and Common Anode Digital Tubes:
For common cathode digital tubes, the common cathode pin is connected to the negative terminal of the power supply, and the control pin is connected to the output pin of the control chip. When a number needs to be displayed, the control chip outputs the corresponding encoded signal to the control pin, causing the corresponding LED segment to light up.
For common anode digital tubes, the working principle is similar to that of common cathode digital tubes, except that the common anode pin is connected to the positive terminal of the power supply, and the control pin is connected to the output pin of the control chip.
6. Encoding Display:
To display the corresponding numbers or characters on the digital tube, the segment data port must output the corresponding character encoding. For example, to display the number "0", the character encoding for a common anode digital tube is 11000000B (i.e., C0H), while the character encoding for a common cathode digital tube is 00111111B (i.e., 3FH). The specific encoding depends on the actual digital tube.
7. Dynamic and Static Display:
Digital tubes can use either static or dynamic display methods. In static display, each of the eight segments of each digital tube is connected to an 8-bit I/O port address. As long as the I/O port outputs a segment code, the corresponding character is displayed and remains unchanged. Dynamic display, on the other hand, lights up each digit of the digital tube one by one, achieving simultaneous visual display through rapid switching.
In summary, the driving principle of digital tubes is to control the on/off state of each segment of the digital tube to display numbers, letters, or symbols, and to achieve display switching between multiple digital tubes through segment selection and digit selection. Simultaneously, the digital tube can be driven through hardware circuits or software control, and either common cathode or common anode digital tubes can be selected for driving as needed.
This project actually uses dynamic scanning display to drive the digital tube.
Let's estimate the current required for the digital tube.
This project actually uses dynamic scanning to drive the digital tube, so at any given time, only a maximum of 8 segments of the digital tube (or LEDs) can be lit, or in other words, only one segment can be lit. According to the design, the required driving current is approximately 11mA, which is the high-level voltage of the IO port 3.3V ÷ 300Ω.
At this point, it is important to ensure that the selected MCU has sufficient current sinking/pull-up capability.
Analysis of the datasheet shows that the CW32 has no problem. (Some chips are not suitable.)
6. LED Indicator
This project additionally designed a power indicator and an IO working indicator.
LED_PWR is the power working indicator .
Since the chip I/O often has a greater current sinking capability than a current pulling capability, LED1 is designed to be active low (on).
To reduce the current consumption of the LED, some LED brightness is sacrificed, the types of component parameters are reduced, and the current limiting resistor for the LED is selected as 10K.
7. Button Circuit Design
There are various ways to design the button control circuit. Thanks to the fact that the CW32's I/O port can be configured with pull-up and pull-down resistors internally, the button control circuit on the outside of the chip does not need to be configured. One end of the button is connected to the MCU's I/O, and the other end is grounded. When the button is pressed, the I/O is pulled low.
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. From a product design perspective, due to the inherent ADC performance advantages of the CW32, this circuit is not necessary. This circuit is designed on the development board to learn the relevant application principles.
The TL431 is a relatively "old" device, very classic, and widely used; it is still found in many electronic products.
Many beginners may be encountering this device for the first time, so we will briefly explain the principles of this product to help everyone better apply the TL431.
TI defines it as a precision programmable reference. On the first page of the references, we can focus on several key characteristics.
Precision: Precision indicates that its output voltage is very accurate. I used a TL431 with ±0.5% accuracy, and the measured voltage on the board at room temperature was 2.495V. Compared to common Zener diodes, the accuracy is vastly different. In application circuit diagrams, the TL431 is internally represented by a Zener diode symbol.
Adjustable output voltage: The adjustable output voltage is between Vref and 36V. In our project, we use the output Vref voltage, which is approximately 2.5V. Therefore, we use 2.5V in the description, which is approximately equal to the actual voltage.
Sinking current capability: This refers to how much current the output voltage pin can provide, which is greatly related to the resistance value (R4) in the application circuit. It should not be less than 1mA. If there is no need for sinking current, do not design the current to be too high, causing unnecessary power consumption.
III. Demonstration Video
: The video only demonstrates voltage measurement; current was not measured because a matching resistor was unavailable, and I was too lazy to select another one. However, it should be fine...
https://www.bilibili.com/video/BV1VYpQeQERe/?vd_source=120bf933e5f157500508230f7580f9e3
IV. Program Code:
The program code follows the official tutorial, which provides detailed tutorials and examples. I won't elaborate further here...
V. Physical Demonstration