Simple Digital Voltmeter and Ammeter Based on CW32

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.
Package Selection
As can be seen from the datasheet, compared with the SOT23 package's 280℃/W, the SOT89 package's 165℃/W has better heat dissipation capabilities (Thermal Resistance, Junction-to-Ambient), unit: degrees Celsius/watt.
LDO Model Selection
Since the main control core board of the project is powered by 5V, according to the ordering information in the table below, the SE8550K2-HF is selected, with an output of 5V. K indicates the SOT89 package, and ±2% accuracy is sufficient, saving costs. HF or LF is related to environmental protection requirements and needs to be considered in actual product design. This project does not require attention.
The design manual for the capacitor
provides the application schematic of this LDO:
the principle of almost all LDO devices is basically the same when designing circuits. Therefore, the reference significance of this diagram is not very high. The more common considerations are how C1 and C2 should be designed. If your LDO has EN (enable), you also need to consider the design of the enable pin to ensure that the chip works properly. In addition, some LDOs have adjustable outputs, which have a dedicated pin (adj) that requires the configuration of corresponding external resistors to achieve the specified output voltage.
The most crucial part of LDO peripheral circuit design is the design of the capacitors.
The capacitors around the LDO play a key role in the circuit, and their functions mainly include the following aspects:

Filtering: The capacitors around the LDO, especially the input capacitor, can effectively filter out the ripple interference from the previous power supply.
Improving load transient response: The output capacitor plays an important role in improving the load transient response. When the load current changes rapidly, the LDO has an adjustment time. At this time, the output capacitor acts as a temporary power supply to provide the required current to the circuit and prevent the output voltage from being pulled too low. A larger output capacitor value can further improve the transient response of an LDO to large load current changes.
Phase compensation: For adjustable output LDOs, the capacitor connected in parallel with the upper resistor (R1) (called the feedback capacitor CFB) provides leading phase compensation, increasing oscillation margin and improving load transient response. Zeroing both CFB and R1 contributes to loop stability.
Oscillation prevention: Proper capacitor configuration can help prevent oscillations in the LDO circuit, ensuring stable circuit operation.
Ripple suppression: Capacitors in LDOs also help improve the power supply rejection ratio (PSRR), reducing the impact of power supply noise on circuit performance.
Startup inrush current control: The input capacitor acts as a temporary power source for startup inrush current during LDO startup, preventing the input voltage from being pulled low and affecting the stability of the preceding power supply.

Summary: The capacitors surrounding the LDO play an important role in filtering, improving load transient response, phase compensation, oscillation prevention, ripple suppression, and startup inrush current control. By properly configuring these capacitors, the stability and performance of the LDO circuit can be ensured.
If you carefully review the datasheet, you'll find that the official documentation provides detailed suggestions and references, including layout design recommendations. This avoids engineers having to perform a lot of complex theoretical calculations.
It's worth noting that in my power supply design, I used electrolytic capacitors and ceramic capacitors connected in parallel, one after the other. When electrolytic and ceramic capacitors are connected in parallel, they can be "matched in high and low frequencies," resulting in good filtering in both high and low frequency ranges. Electrolytic capacitors filter out low-frequency interference, while ceramic capacitors filter out high-frequency interference; their combination achieves better filtering.
In practical electronic products, the main function of electrolytic capacitors connected in parallel with ceramic capacitors is to achieve high and low frequency filtering and coupling through their complementary characteristics, thereby improving circuit performance, stability, and anti-interference capabilities. This parallel connection method is widely used in electronic devices. For
reverse connection protection,
considering that high-voltage reverse connection can cause irreversible damage to the module, the voltmeter power supply circuit uses a series diode scheme.
Note: This project uses a series diode for reverse connection protection because the power supply voltage of this device is usually higher than 5V; the 0.7V voltage drop of the diode will not affect the power supply. When the supply voltage is low, the overall power consumption of the project is low, and the measured supply current is low (20mA). Due to the unique structure of the Schottky diode D4 (1N5819), its VF is lower than that of general-purpose switching diodes, as shown in the figure below, with a voltage drop of approximately 0.2V or less.
In conventional circuit design, using a reverse-parallel diode + series fuse can also achieve the purpose of reverse connection protection and circuit protection.
The diode voltage-current curve and
the role of the series small resistor (10Ω) are also discussed.
This project additionally uses a series small resistor (10Ω) for voltage division. This serves two purposes: firstly, it reduces the problem of excessive heat generation in the LDO due to large voltage differences under high voltage conditions; secondly, it utilizes the principle that the series 10-ohm low-power resistor has low overcurrent, acting as a low-resistance fuse to provide overcurrent or short-circuit protection. (The reason the resistor acts as a fuse is that, under overcurrent and heating conditions, it is 99% likely to be an open circuit, rarely short-circuiting. Its fault analysis dictates that it will primarily be an open circuit—that is, it will burn out rather than short-circuit.)
The series small resistor (10Ω) also reduces the peak value of the power-on surge, preventing excessive surges from damaging the LDO.
If electrolytic capacitors were not used, the series small resistor (10Ω) also prevents resonance between the wire inductance and the ceramic capacitor during hot-plugging. Because ceramic capacitors have very low ESR, the damping in the LC network is minimal, resulting in high gain at the resonant point. Adding an external resistor to provide damping suppresses this gain.
Circuit Design Essentials and Standards:

When drawing power supply circuits, whether in schematics or PCB layouts, several points should be noted:


Schematic Standards: GND should face down, power supply should face up; avoid ground facing upwards.


Capacitor Design: In both schematics and PCB layouts, electrolytic capacitors should come first, followed by ceramic capacitors. Grounding


Design: Single-point grounding; the ground of the current power supply should be connected to the GND of the main electrolytic capacitor of the current power supply, and the main electrolytic capacitors of each stage of the power supply should be connected to the GND of the main electrolytic capacitor of the preceding stage.

2. MCU Selection Analysis:
To reduce the learning cost, this project uses the LCSC CW32F030C8T6 development board (core board) as the main controller, but this does not mean we will discuss this section less. 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 voltage and current meters, the author used STM32/CW32 and some other 32-bit microcontrollers for 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 needs: 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: 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. (Flash0 pending).
A detailed explanation of these advantages will be provided in the ADC sampling section and extended sections.


Key features of CW32's ADC:
This project requires a focus on 4 reference voltage sources. Content from the "CW32x030 User Manual".
3. Voltage Sampling Circuit :
This project uses a voltage divider circuit to achieve high voltage acquisition, designed to acquire voltages up to 100V; the current configuration acquires voltages from 0-30V.
The voltage divider resistors in this project are designed to be 220K+10K, therefore the voltage division ratio is 22:1 (ADC_IN11).
The voltage divider resistor selection



is based on the maximum measured voltage; for safety reasons, this project uses 30V (the actual maximum display value can be 99.9V or 100V).




The ADC reference voltage in this project is 1.5V, which can be configured through the program.




To reduce power consumption in the sampling circuit, the low-side resistor (R7) is usually selected as 10K based on experience.



The high-side resistance of the voltage divider resistors can then be calculated using the above parameters.


The required voltage division ratio is calculated as follows: ADC reference voltage: Design input voltage, calculated using known parameters as 1.5V/30V=0.05.


The high-side resistance is calculated as: Low-side resistance/voltage division ratio, calculated using known parameters as 10K/0.05=200K.


A standard resistor is selected: A resistor slightly higher than the calculated value is chosen; the calculated value is 200K. We usually choose E24 series resistors, therefore in this project, 220K is selected, which is greater than 200K and closest to the calculated value.


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 expand the voltage measurement range, you can choose to replace the voltage divider resistor or modify the reference voltage. Here's a case study:


Assuming the measured voltage is 160V, we can choose 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.
Diode clamping ensures MCU safety
. In designing this project, I added an additional 1N4148 (D1, etc.) as a clamping diode to the sampling circuit. This helps avoid damage to the chip pins due to incorrect voltage input during learning and debugging. Diode clamping is an important electronic circuit design technique; its main function is to protect the circuit by limiting the voltage amplitude, preventing damage or malfunction caused by excessively large or small signals.
In circuit design, clamping refers to limiting voltage. Diode clamping specifically refers to the technique of using a diode to limit the potential at a point in a circuit.
Diode clamping primarily utilizes the unidirectional conductivity of a diode. When the voltage across the positive terminal of a diode is greater than the voltage across its negative terminal and the diode is turned on, the voltage across the diode is limited to its voltage drop across the diode. Typically, the voltage drop across a silicon diode is about 0.7V.
The clamping process involves forcibly pulling the clamped potential towards the reference terminal through the clamping action of the diode, thereby limiting the potential. Clamping does not change the waveform of the original signal; it only raises or lowers the reference potential of the signal.
Depending on the diode connection method, clamping circuits can be divided into positive clamping circuits and negative clamping circuits. This project only designs positive clamping.

Positive clamping circuit: When the positive terminal of the diode is grounded, it is a positive clamping circuit. During the positive half-cycle, the diode is cut off; during the negative half-cycle, the diode conducts, and the capacitor is charged to a certain voltage, limiting the output voltage within a certain range.
Negative clamping circuit: When the negative terminal of the diode is grounded, it is a negative clamping circuit. The working principle is the opposite of the positive clamping circuit.

Adding a voltage sampling circuit to achieve 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 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 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.
There are many ways to implement range switching, and the development board design provides more design possibilities. Next to
the circuits used for simulating voltage measurement, measurement calibration, and measurement calibration assistance,
the components labeled T_V and T_GND are the 2mm banana plug interfaces on the development board, used to connect multimeter probes. You can insert the probes of a multimeter or a high-precision benchtop digital multimeter to verify the accuracy of the development board's measurements. You can also insert 2mm banana plug multimeter probes to replace the CH1 port for handheld measurements.
The VP pin is the power supply pin for the development board and should not be connected when using the DC port. When not using DC port power and the measured value is greater than 5V and less than 30V, it can be connected to the power supply under test, or it can be powered independently.
When learning the measurement principles of the corresponding circuits, considering that users may not be able to easily build peripheral circuits for testing and debugging, and adhering to the principle of ease of development of the development board, auxiliary circuits for simulating voltage measurement, measurement calibration, and measurement calibration are specially set up. No external voltage is required for CH1. Use a multi-turn adjustable potentiometer (RP1) to divide the power supply voltage of the development board and connect it to the +V network through the internal circuitry of the development board. Note that JP1 needs to be shorted at this time; a jumper cap is sufficient, and a long-handled jumper cap is recommended. Do not short JP1 if this function is not used.
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 meter interface of the development board
, do not solder R0!!!
Design Analysis:
The sampling current in this project is 3A, and the selected sampling resistor (R0) is 100mΩ.
The following aspects need to 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;
and the power dissipation of the current sensing resistor, which should generally not exceed 0.5V. A suitable package should be selected based on this parameter. Considering the power dissipation (temperature) issue under high current, a 1W packaged metal wire-wound resistor was selected.
The voltage amplification factor across the current sensing resistor: No operational amplifier is used in this project to build the amplification circuit, so the factor is 1.

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

. Since no amplifier circuit is used, a larger sampling resistor is needed to obtain a higher measured voltage for measurement.
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 feasible.
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, the R0 resistor can be replaced with constantan wire or a shunt. The choice of alternative can be 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. When using
auxiliary circuits for simulating current measurement, measurement calibration, and measurement adjustment,
do not solder the R0 sampling resistor. If this function is not used, disconnect JP2.
The essence of current sampling is to collect the voltage drop across the sampling resistor when current flows through it, i.e., the collected voltage value. This circuit uses RP2 to provide a voltage value in the range of 0~0.238V (5V ÷ 210K * 10K), which is connected to the chip's current sampling pin via the I+ network.
In actual use, the voltage at I+ simulates the voltage drop across the unsoldered 100mΩ sampling resistor. Therefore, the simulated measured current value I_measured = this voltage value Vi+ ÷ 100mΩ, which is also exactly equal to the measured voltage value multiplied by 10. That is, it provides a simulated current measurement of 0~2.38A.
Set a multimeter or high-precision benchtop digital multimeter to the voltage measurement port, with a range within 3V. Insert the black negative probe into the T_GND interface next to the voltage measurement terminal, and the red positive probe into the TI+ port for current measurement to measure the actual voltage value of I+. Thus, this circuit can not only complete the above design tasks but also allow for a direct test of the accuracy of the MCU's ADC peripheral. You can write your own program to verify this.
5. Digital Tube Driver:
This project uses digital tubes as the display unit.
In this project, actual testing showed that the current-limiting resistors (R1~R6) for the digital tubes were configured to 300Ω. The corresponding brightness, whether for red or blue digital tubes, provided good visibility and a soft, 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 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.
Analysis of the datasheet shows that CW32 has no problems. (Some chips do not work.)
6. Indicator Lights:
This project additionally designed a power indicator light and an I/O operation indicator light.
Since 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 LEDs, some LED brightness was sacrificed, the number of component parameters was reduced, and the current-limiting resistor for the LEDs was chosen to be 10K.
7. Button Circuit Design:
There are various design methods for the button control circuit. 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 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. This can be used to provide an external voltage reference for calibrating 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 for learning related application principles.