CW32 Digital Voltage and Current Meter

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 adopts a core board plus expansion board design concept and uses plug-in components, 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 is highly comprehensive and practical, and 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
This project uses the LCSC CW32F030C8T6 development board (core board) as the main controller. The correct selection of the main controller is very important, as it relates to the overall advantages of the project.
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: 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.

3. Voltage sampling circuit : The voltage
divider resistor in this project is designed as 220K+10K, therefore the voltage division ratio is 22:1 (ADC_IN11).
The voltage divider resistor selection

design measures the maximum value of the measured voltage. For safety reasons, this project uses 30V (the actual maximum display value can be 99.9V or 100V).
The ADC reference voltage is 1.5V in this project, and this reference voltage can be configured through the program.
To reduce the power consumption of the sampling circuit, the low-side resistor (R7) is usually chosen as 10K based on experience.

Then, the high-side resistance of the voltage divider resistor can be calculated using the above parameters.

The required voltage division ratio is calculated, i.e., the ADC reference voltage. The input voltage is designed; using known parameters, 1.5V/30V = 0.05 can be calculated.
The high-side resistance is calculated as the low-side resistance/voltage division ratio; using known parameters, 10K/0.05 = 200K can be calculated.
A standard resistor is selected: a resistor slightly higher than the calculated value of 200K is chosen. We usually choose E24 series resistors; therefore, in this project, 220K, which is greater than 200K and closest to the calculated value, is selected.

If, in actual 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. The following example illustrates this:

Assuming the measured voltage is no higher than 24V and other parameters remain unchanged,
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 actual use, the voltage to be measured is higher than the module's 99V design voltage, a different resistor can be selected. To achieve a wider voltage measurement range, one can choose to replace the voltage divider resistor or modify the reference voltage. The following example illustrates this:

Assuming the measured voltage is 160V, the solution is to increase the voltage reference to expand the range.
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 range (increasing the range will reduce accuracy).

Considering the potential fluctuations in the measured power supply, a 10nF filter capacitor is connected in parallel with the low-side voltage divider resistor to improve measurement stability.
Range switching:
In this project, an additional voltage sampling circuit was added. Therefore, we can discuss the significance of range switching for improving measurement accuracy. Multimeters often have multiple range settings for 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 achieve 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, use the ADC_IN9 channel for measurement. In this case, the measurement accuracy is greatly improved due to the reduced voltage division ratio.
4. Current Sampling Circuit : This project uses a low-side current sampling circuit for current detection. When
the low-side of the sampling circuit shares a common ground with the development board's meter interface , please do not solder R0!!! Design Analysis: The sampling current designed for this project is 3A, and the selected sampling resistor (R0) is 100mΩ. The following aspects should be considered when selecting the sampling resistor: the maximum value of the pre-designed measurement current; the voltage difference caused by the 3A current sensing resistor in this project; generally, it is not recommended to exceed 0.5V.







The power consumption of the current sensing resistor should be selected based on the appropriate package. Considering the power consumption (temperature) under high current conditions, a 1W metal wire-wound resistor was chosen for this project
. The voltage amplification factor of the current sensing resistor is 1 since no operational amplifier is used in this project.

The current sensing resistance value can then be calculated using the above parameters.

Since no amplifier circuit is used in this project, a larger sampling resistor is needed to obtain a higher measured voltage for measurement.
However, 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 possible.
A 1W package resistor was chosen for this project, corresponding to a power rise of 1W

. Based on the above data, a 100mΩ current sensing resistor was selected. According to the formula, 3A * 100mΩ = 300mV, 900mW.
To handle different operating environments, especially high current scenarios, the R0 resistor 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 digital tubes as the display unit.
Two 0.28-inch three-digit common-cathode digital tubes are used as display devices. Compared to displays, digital tubes offer better visibility in complex environments. The brightness of the digital tubes can be increased by using smaller current-limiting resistors depending on the actual usage environment. Furthermore, digital tubes have better mechanical properties and are not as easily damaged by external forces as displays. They are widely used in industrial applications requiring stability and reliability. From a development board learning perspective, this facilitates targeted learning of electronic measurement principles. In this project,
actual testing showed that the current-limiting resistors (R1~R6) for the digital tubes were configured to 300Ω. The corresponding brightness for both red and blue digital tubes was good and the brightness was soft and not glaring.
Strictly speaking, the current-limiting resistors should be added to the segments; adding them to the digits would affect the display effect. Our actual 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 a digital tube
mainly involves controlling the switching state of each segment of the LED display to show numbers, letters, or symbols. The following is a detailed explanation of the driving principle:

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.


Driving Methods:
Segment Selection: The desired number or character is displayed by controlling the switching state of each segment of the LED display. 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 the bit lines of the digital tube. 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, the display switching between multiple digital tubes can be achieved.


Driving Circuit:
The digital tube driving circuit can be implemented through hardware circuits, such as using 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, thereby displaying the desired numbers or letters.


Software Control:
In addition to hardware driving circuits, digital tubes can also be driven through software control. By programming to generate control signals suitable for the digital tubes, more flexible and complex display effects can be achieved, such as scrolling or alternating display of numbers.


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 certain 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.


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.


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 switching 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. Furthermore, the driving of digital tubes can be implemented through hardware circuits or software control, and common cathode or common anode digital tubes can be selected for driving as needed.
This project actually uses dynamic scanning to drive the digital tube display.
Calculating the required current for the digital tube:
Since this project uses dynamic scanning to drive the digital tube display, 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 digit can be lit. According to the design, the required driving current is approximately 11mA (IO port high-level voltage 3.3V ÷ 300Ω).
At this point, it is important to ensure that the selected MCU has sufficient current sinking/pulling capability.
Analysis of the datasheet shows that the CW32 is suitable.
6. LED Indicator Lights:
This project additionally designed a power indicator light and an IO operation indicator light.

LED1 is the power indicator light

. Since chip I/O often has a greater current sinking capability than 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 number of component parameters is reduced, and the current-limiting resistor for the LED is chosen to be 10K.
7. Button Circuit Design:
There are multiple design methods for button control circuits. Thanks to the CW32's internal I/O ports which can be configured with pull-up and pull-down resistors, the button control circuit on the outside of the chip does not require configuration. 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. This 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 unnecessary. This circuit was designed on the development board to learn the relevant application principles.
The TL431 is a relatively "old" device, a classic, widely used, and still found in many electronic products.
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 ±0.5% accuracy TL431, which measured 2.495V on the board at room temperature. Compared to common Zener diodes, the accuracy is vastly different. In the application circuit diagram, the TL431 is represented by a Zener diode symbol.
Adjustable Output Voltage: The adjustable output voltage is between Vref and 36V. In this 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 2.5V in practice.
Sinking current capability: This refers to how much current the output voltage pin can provide, which is greatly related to the resistance value (R13) 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.