Design of electronic brush based on ARM and acceleration sensor

In this design, the ADuC7022 ADC works in single-ended mode. The ADC0~ADC2 of the ADC module are connected to the MMA7260Q three-axis acceleration output pins. ADC2 is connected to the positive electrode of the battery to monitor the input battery voltage. When the battery voltage drops to close to the minimum input voltage of the LDO, the LED is lit to remind the user to replace the battery. The P0.0 and P0.1 pins of the microcontroller are connected to the SEL1 and SEL2 pins of the MMA7260Q as the control signals of the acceleration sensitivity.

The serial interface of ADuC7022 provides SPI, UART, and I2C interfaces. The I/O port of ADuC7022 is a multiplexed interface. Users can choose between GPIO, UART, UART/SPI/I2C, and programmable logic array by setting the control register of the SPM module. The hardware in this article uses an SPI module working in Master mode and connected to the wireless USB module. The schematic diagram of the microcontroller is shown in Figure 1.

3 Accelerometer

MMA7260Q is a single-chip three-axis low-level acceleration sensor launched by Freescale Semiconductor. It can accurately measure low-level descent, tilt, displacement, positioning, impact and vibration errors in the three directions of X, Y and Z. By selecting the sensitivity of MMA7260Q, it can be designed according to the gravity acceleration sensitivity of different levels (g) of 1.5g, 2g, 4g and 6g. MA7260Q is manufactured using MEMS technology. It integrates acceleration sensors and signal conditioning circuits such as low-pass filtering and temperature compensation in a volume of 6mm×6mm×1.45mm, and presets full-scale 0g bias. Its package size is very small and only requires a small board space. In addition, MMA7260Q can run at a low voltage of 2.2V~3.6V, consumes only 500?Zi A current when working, and is equipped with a 3μA sleep mode and a 1.0ms fast power response. It also provides fast startup and sleep modes. These features greatly extend the battery life of the electronic pen and reserve enough space for appearance design.

SEL1 and SEL2 are sensitivity selection input pins, and the corresponding sensitivity truth table is shown in Table 1. The output voltage VOUT of the acceleration sensor is: [page]

The wireless USB interface circuit is shown in Figure 3. The acceleration data collected by the microcontroller is sent to CYRF6934 through the SPI interface after preprocessing. CYRF6934 modulates the data and sends it to the wireless USB bridge on the PC through the printed antenna on the PCB. A 0.1uF capacitor is used for decoupling all VCCs of the chip and a 10uF capacitor is used as a charging and discharging capacitor.

5 Software Design

As the underlying hardware driver, the software needs to complete system initialization, data acquisition, processing and transmission. System initialization mainly includes the initialization of the microprocessor stack, queue and internal control registers, the selection of MMA7260Q acceleration sensitivity and the initialization of the CYRF6934 control registers. The assembly language completes the initialization of the microprocessor stack and queue and guides the system to execute the main() function in the C language. After the C language program completes the remaining initialization tasks, it starts the cycle of collecting data, processing data and sending data.

In addition to using the accelerometer solution of the MMA7269Q sensor, the software design can also use various components provided in the MMA7260Q reference design development tool to develop according to the needs, so as to improve the efficiency of software and hardware and minimize the factors of system instability caused by software conflicts.

At the same time, the hardware environment of KIT3109MMA7260Q is used to complete the development and testing of MMA7269Q and conduct a comprehensive evaluation to speed up product development. The software flow chart is shown in Figure 4.

After the driver design is completed, the application program needs to use some special API functions of the Windows operating system when accessing the driver. Since these function parameters are relatively large, a dynamic link library can be developed so that users can develop applications without involving the operation of the underlying driver device and can operate the hardware like ordinary API functions. The structure of the entire system is shown in Figure 5.

By using this layered structure, user programs can read and write devices through dll. The interface functions provided by dll to the application include initializing the device, closing the device, reading and writing ports, etc. In this way, after adding your own dynamic link library to the application, you can use inPortb and out-portb to operate the port.

Since the application and market prospects of electronic pens are very broad, some major related manufacturers in the world are now competing to develop and launch new products with their own characteristics. This design scheme organically combines the advantages of wireless communication with the traditional USB interface, which not only provides a higher data transmission rate, but also improves the data access method. At the same time, the MMA7260Q three-axis low-level acceleration sensor and SoC CYRF6934 wireless USB network transceiver can meet the needs of non-network terminals in the wireless field with their perfect performance and low cost, and make the transmission system more convenient and reliable.

Without changing the traditional way of pen-paper communication, this electronic pen can realize natural handwriting input on ordinary paper (or any medium) beyond keyboard, just like providing people with a golden key to open a new door to the information age. It combines the acceleration structure with the wireless network, and has many advantages such as simple structure, reliable operation, convenient data transmission and plug-and-play. The promotion and application of this electronic pen will surely become a super assistant for people to promote social informatization and a tool to promote office automation. In a certain sense, it can realize the writing of computers and mobile phones and occupy a place in the market.

References

[1] Analog Semiconductor. Precision analog microcontroller 12-bit analog I/O, ARM7TDMI MCU Rev A. 2006.

[2] Freescale Semiconductor. ±1.5g-6g Three axis low-g micromachined accelerometer. 2006, 2.

[3] Cypress Semiconductor. WirelessUSBTM LS 1-way hid networks. 2003, 8.

[4] Cypress Semiconductor. WirelessUSB LR 2.4GHz DSSS radio soc. 2005, 3.

[5] Cypress Semiconductor. WirelessUSBTM dual antenna design layout guidelines. 2005, 3.