LCSC GeoStar 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. The digital voltmeter and ammeter project encompasses multiple aspects, including the design and implementation of microcontroller circuits, the design of signal acquisition and processing circuits, the development and optimization of the user interface, and the design of the product appearance. It integrates knowledge from multiple fields such as electronic technology, microcontroller programming, circuit design, and industrial design. This project has several highlights:

it adopts a core board plus expansion board design concept and uses plug-in components;
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 everyday instrument.

II. Hardware Design

1. Power Supply Circuit
LDO (Low Dropout Linear Regulator) Selection This project uses an LDO as the power supply, selecting the SE8550K2 with a maximum input voltage of up to 40V. 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; a 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 level of international standards (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.

The main characteristics of the CW32 ADC: This project requires a focus 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 to be 220K+10K, therefore the voltage division ratio is 22:1 (ADC_IN11).
The voltage divider resistor selection

is designed to measure the maximum 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 learning the common ground between the low-side of the sampling circuit and the development board's meter interface, please do not solder R0!!!
The design analysis
for this project involves a sampling current of 3A, with a selected sampling resistor (R0) of 100mΩ. The selection of the sampling resistor mainly considers the following aspects:

the maximum value of the pre-designed measurement current;
the voltage drop 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 metal-wound resistor package was chosen
. The voltage amplification factor across the current sensing resistor is also important. Since no operational amplifier is used in this project, the amplification factor is 1. The

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

Furthermore, 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 will result in a larger voltage drop and higher power dissipation, an unlimited selection of a larger resistor is not advisable.
This project uses a 1W packaged resistor, corresponding to a temperature rise power 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.
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 have better visibility in complex environments. Depending on the actual operating environment, smaller current-limiting resistors can be used to achieve higher brightness. 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 makes it easier to purposefully learn about electronic measurement principles and related development.
In this project, after actual testing, the current-limiting resistors (R1~R6) of the digital tube were configured to 300Ω. The corresponding brightness, whether for red or blue digital tubes, showed good recognizability and was soft and not glaring.
Strictly speaking, the current-limiting resistors should be added to the segments; adding them to the positions would affect the display effect. Our actual design placed them in the positions to save a few resistors, but the impact on the display was not significant. Therefore, we added them to the positions for convenience. The driving principle
of the digital tube
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:

Basic structure of a digital tube:

A digital tube is usually composed 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 have two types: common cathode and common anode. The difference lies in whether the common terminal COM of the LEDs (i.e., the end connecting all LEDs) is connected to the negative or positive terminal of the power supply.


Driving Methods:

Segment Selection: The desired numbers or characters are 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, the display can be switched between multiple digital tubes.


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, the digital tube can also be driven by software. By programming and generating control signals suitable for the digital tube, 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 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.


Encoded display:

In order for the digital tube to display the corresponding number or character, the segment data port must output the corresponding character code. For example, to display the number "0", the character code for a common anode digital tube is 11000000B (i.e., C0H), while the character code for a common cathode digital tube is 00111111B (i.e., 3FH). The specific code 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 a 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 segment of the digital tube one by one in turn, achieving simultaneous visual display through rapid switching.



In summary, the driving principle of a digital tube is to control the switching state of each segment to display numbers, letters, or symbols, and to achieve display switching between multiple digital tubes through segment selection and digit selection. Simultaneously, the driving of the digital tubes can be implemented through hardware circuits or software control, and common cathode or common anode digital tubes can be selected as needed.
This project actually uses dynamic scanning display to drive the digital tubes.
Calculating the required current for the digital tubes
: Since this project uses dynamic scanning display to drive the digital tubes, at any given time, only a maximum of eight segments of the digital tubes (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-pull/sinking capability.
 
6. LED Indicator Lights:
This project includes an additional power indicator and an I/O operation indicator.

LD_PWR is the power indicator .

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 LED current consumption, some LED brightness is sacrificed, and the number of component parameters is reduced; the LED current-limiting resistor is chosen to be 10K.
7. Button Circuit Design:
There are various 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, external button control circuits do 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 additional 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 CW32's inherent ADC performance advantages, this circuit is not necessary. This circuit is designed on the development board for learning related application principles.
The TL431 is a relatively "old" device, a classic with wide applications, and still found in many electronic products.
Many beginners may be encountering this device for the first time, so we'll briefly explain its principles to help you better utilize the TL431.
TI defines it as a "Precision Programmable Reference," and we can focus on several key characteristics on the first page of the references.
Precision: This indicates its output voltage is extremely accurate. The TL431 I used has ±0.5% accuracy, and at room temperature, it measured 2.495V on the board. This is a world of difference in accuracy compared to common Zener diodes. 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 Vref.
Sinking Current Capability: This refers to how much current the output voltage pin can provide. This is greatly influenced by 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, as this will cause unnecessary power consumption.