#LCSC EDA Instrumentation Training Camp# CW32 Point Crystal Multifunctional Test Pen

Following the LCSC training camp's replica of the PointChip CW32 multi-functional test pen
 
: I. Main Controller: CW32F030C8T6.
Vcc, GND, DIO, CLK, and REST are brought out separately for easy programming. A reset button is also included; this button doesn't necessarily need to be soldered on, but can be reset by shorting REST and GND with tweezers.
 
II. Signal Output Circuit:
        The signal output circuit can output DC signals or directly output PWM signals. Essentially, it directly outputs signals from the microcontroller. The DC output level is achieved by adjusting the duty cycle of the PWM (the set signal frequency is 20kHz; for other frequencies, the DC calibration value needs to be modified), and then converting it into a "DC" signal through two stages of low-pass filtering.
        However, since the output voltage of the PWM-to-DC converter is lower than the high level of the PWM, it can only output voltages from 0 to (+3.3VX), which does not meet the 0-5V target I proposed in the design. Therefore, an operational amplifier was added to double the voltage output, ultimately allowing an output voltage from 0 to 6V, meeting the target requirements. The amplified output voltage, after passing through a low-pass filter, yields a more ideal DC signal. (This uses a positive proportional amplifier circuit, amplifying the signal transmitted from the front end by a factor of 2.)
 
III. Signal Input Circuit
The diagram above (black box) shows the most basic signal input circuit. First, a bidirectional TVS diode protects the subsequent circuitry. Then, analog switch 3 (COM3 and NC3, NO3) allows selection between signal input and output modes (default output mode). Analog switch 2 allows selection of whether the input signal is attenuated. The analog front-end design references an oscilloscope, with an input impedance of 1MΩ. Like an oscilloscope, it allows selection of X1 and X10 ranges, with X10 selected by default. This design maximizes the safety of the subsequent circuitry, similar to how, when storing a multimeter with a separate power button, the range should be set to "AC voltage, maximum range."
 
IV. BLE Bluetooth Transmission
To facilitate connection between the test pen and a computer or mobile phone, I chose to use Bluetooth BLE technology for wireless data transmission.
The main reasons for not using the more common CH340 serial-to-USB direct connection are as follows:
① The data cable has a certain weight and rigidity, making it less flexible to operate while wireless;
② It is not recommended to use the device with the data cable plugged in, as this will cause the floating ground of the test probe to become grounded, potentially causing a short circuit during testing;
③ Most importantly: safety! If a high voltage is input to the test probe due to operational error, and the protection circuit fails, causing the microcontroller to burn out, this high voltage could potentially be directly input to the phone or computer along the data cable, causing serious damage!
 
During PCB design, the area around the antenna should be kept clear to prevent interference from other signals and noise, which could lead to signal transmission bugs.
 
V. Screen Display
This project uses a 0.96-inch TFT display screen with a resolution of 80*160, employing an ST7735 display driver chip. It is connected to the PCB via FPC soldering and communicates through an SPI interface. R18 is a backlight current-limiting resistor, which can control the display backlight via the BLK network controlled by the microcontroller, or it can be controlled by PWM dimming via a program.
 
VI. Five-way Remote Control Switch (Complete Operation Input)
The user operation section of this project uses a five-way joystick switch, which can be understood as equivalent to five ordinary button switches. Programming it is also done as ordinary buttons.
The functions corresponding to the joystick switch in different directions will be explained in the program section.
 
VII. Overall Physical Effect Image (
Back and
Front) Note!!! After soldering, before use, be sure to connect the COM terminal and the analog ground terminal with a jumper wire!!! After downloading the program, calibration is mandatory for the first use! Otherwise, the measured data or output signal will be inaccurate! If used without calibration, reset the microcontroller using the reset button and then restart for calibration!