Introduction

The USB ammeter made based on N32G430C8L7+INA199 has an accuracy of 1%. You can check the current when USB is working. The project is divided into two versions, one is an ammeter that supports fast charging, and the other is a 5V meter that does not support fast charging display.

Project Description

This project is used for training camp case teaching projects. It does not support fast charging and can only measure a maximum voltage of 6V. If you need to support fast charging, please modify it yourself!

characteristic

• 0.91-inch OLED screen, clear and compact display;
• The PCB board is suitable for the public version shell;
• Use low-resistance sampling resistors to prevent excessive internal resistance from affecting USB power supply efficiency;
• Maximum voltage measurement support: 6V
• Maximum measurement current: 2.8A
• Support power display

hardware design

master control

The N32G430C8L7 microcontroller product uses a high-performance 32-bit ARM Cortex™-M4F core, integrates a floating-point unit (FPU) and digital signal processing (DSP), and supports parallel computing instructions. The maximum operating frequency is 128MHz, it integrates up to 64KB of on-chip encrypted storage Flash, supports multi-user partition permission management, and supports 16KB of embedded SRAM. Built-in an internal high-speed AHB bus, two low-speed peripheral clock buses APB and bus matrix, supports 40 reusable I/Os, provides rich high-performance analog interfaces, including a 12-bit 4.7Msps ADC, supports 16 external Input channels and 3 internal channels also provide a variety of digital communication interfaces, including 4 U(S)ART, 2 I2C, 2 SPI/I2S, and 1 CAN 2.0B communication interface. The N32G430C8L7 microcontroller product can stably operate in the temperature range of -40°C to +105°C, with a power supply voltage of 2.4V to 3.6V, and provides a variety of power consumption modes.
This case does not use many resources. The internal clock has been open sourced to meet the design requirements, so there is no need for an external clock circuit. It should be noted that each power supply pin of the chip needs to be connected to an additional 100nF external filter capacitor , and pin 1 of the chip needs to be connected to an additional 4.7uf filter capacitor .

Reset circuit, BOOT circuit

Since this design is designed to fit the public version of the casing, the reset circuit and BOOT circuit are not designed with buttons, and two solder joints are used as short circuits.

Programming interface

The chip supports a variety of burning methods, such as ST-Link, USB-TTL, Jlink and other common burners on the market. In the case, the corresponding interface uses a 2.54 pitch pin header for programming.

voltage conversion

Although the MCU used in this case supports a maximum voltage of 5V, considering that the USB voltage or other voltages may have unstable voltage factors, a 6V to 3.3V LDO is used to power the chip. Even if the power supply is not available, Under stable conditions, the MCU can also work normally. It should be noted that the LDO input and output must be connected to 22uF and 100nF capacitors.

Current sampling, USB input/output

USB input/output

The USB input terminal uses a Type-A male connector, and the input voltage is 5V. This interface supports a maximum output of 3A. If it is larger, it will be a bit hot. It is recommended not to exceed the current of 3A. If you need to use a larger current, please replace the Type-A male connector.

The USB output terminal uses a 4P sinking plate female socket. In order to adapt to the public version of the housing, the maximum rated current of this female socket is 1.5A. The actual measured current is 2.5A, but it is not suitable for long-term operation at this current. If necessary If you use a larger current, please replace it yourself.

Current sampling

The current sampling part is the INA199B1DCKR current sense amplifier, (also called a current sense amplifier) ​​which is often used for overcurrent protection, precision current measurement for system optimization, or closed-loop feedback circuits. This family of devices senses the voltage drop across a shunt resistor at a common-mode voltage of –0.3V to 26V, independent of the supply voltage. There are three fixed gains to choose from: 50V/V, 100V/V and 200V/V. This family of devices uses a zero-drift architecture with low offset to maintain the maximum voltage drop across the shunt resistor to a minimum of 10mV full-scale when sensing current. The parameters are as follows:

• Common mode range: –0.3V to 26V
• Offset voltage: ±150μV (maximum)
• Supports 10mV full-scale shunt voltage drop
• Quiescent current: 100μA (max)

Sampling resistor selection

Inserting a low-value sense resistor in series with the current path creates a small voltage drop that is amplified and treated as a signal proportional to the current. However, this technique will pose different challenges to the sense amplifier depending on the application environment and the location of the sense resistor. Generally, the resistance value of the sampling resistor is below 1 ohm, which is a milliohm-level non-inductive resistor. However, some resistors have sampling voltage and other requirements, so a resistor with a large resistance value must be selected, but the resistance base is large and the error is large. In this case, it is necessary to choose a high-precision non-inductive resistor (which can reach 0.01% accuracy, that is, one ten thousandth of an accuracy) to make the sampling data very reliable. SMD ultra-low resistance value resistors (0.0005 ohms, 2 milliohms, 3 milliohms, 10 milliohms, etc.), chip alloy resistors, high-power resistors (20W, 30W, 35W, 50W, 100W) and other products, temperature coefficient It is plus or minus 5PPM.

Sampling method

• This sampling uses a low-side sampling method, that is, the sampling resistor is connected to the GND loop. This design can calculate complete differential, following, amplification, and output when the differential signal is sent to the op amp. If high-side sampling is used, the sampling resistor is placed high between the power supply and the load. Although this placement not only eliminates the ground interference generated in the low-side detection scheme, it can also detect accidental short circuits from the battery to the system ground. , but high-side sensing requires the sense amplifier to handle common-mode voltages close to the supply voltage. This common-mode voltage ranges in value from the levels required to monitor processor core voltages (approximately 1V) to the hundreds of volts common in industrial, automotive and telecommunications applications. Application examples include typical laptop battery voltages (17 to 20V), 12V, 24V or 48V batteries in automotive applications, 48V telecom applications, high voltage motor control applications, current sensing for avalanche diodes and PIN diodes and high voltage LED backlights wait. Therefore, an important advantage of high-side current sensing is the sense amplifier's ability to handle larger common-mode voltages.
   Therefore, the current sampling method of sampling resistor and op amp is best done at the low end. Although low-end sampling will affect the signal ripple due to common ground interference. But compared to high-end solutions, the solution is simple, easy to implement, low in cost, and highly reliable.

Note: If the stock of INA199B1DCKR is insufficient, you can replace the INA199A3DCKT chip. It should be noted that the magnification of INA199B1DCKR is 50. If you use other magnifications, you need to re-modify the program calibration.

voltage sampling

The voltage sampling part consists of a voltage dividing circuit composed of two resistors. The principle is based on the knowledge of voltage dividing by resistors in series. The typical circuit is as follows:

Show part

The display part uses a 0.91-inch 4P white OELD screen module, using IIC communication, and the display effect is clear.

Design display effect

Software description

Compile parameters

• Compiler: ARM Compiler version 5 (-O0)
• MDK version: 5.31
• Debugger: ST-Link V2

Precautions

If the sampled voltage or current is inaccurate, you can modify the sum value main.cin the file based on the actual value . The specific calculation formula is:VOLTAGE_FACTORCURRENT_FACTOR

VOLTAGE_FACTOR = 实际电压 / 显示电压
CURRENT_FACTOR = 实际电流 / 显示电流

Gallery

illustrate

Finally, we would like to thank our open source enthusiasts micespring for their software support.

Source project reference: https://oshwhub.com/micespring/USBmeter

# Update log - August 23, 2022 A new program has been uploaded to fix the overlapping of characters when the display digits exceed 3 digits. ![image.png]