Design of LED rotating screen based on ATmega16L chip

With the rapid development of science and technology, LED display screens have become a new type of electronic screen advertising media. Compared with traditional inkjet and photo advertising images that are rigid and inactive billboards and advertising light boxes, LED display screens bring people clear, fresh and lively advertising effects. At the same time, LED display screens can be controlled by computers throughout the process, and screen resources can be reused, which improves the economic benefits of many companies.

At present, most of the LED display screens on the market are large-area flat panel display screens composed of dot matrix modules or pixel units composed of light-emitting diodes. The screen uses fast row scanning or column scanning to form text or patterns. One prominent feature of scanning display is that only one column of LEDs is lit at any time. From the left, the position selection end of each column is controlled, and the display is displayed from the first column to the last column. Based on this, only one column of LEDs can be used to form an equivalent dot matrix by rotation, and the position of the LED can be changed by software program to simulate the column scanning of the dot matrix, that is, the so-called LED rotating screen. Especially in the current context of low-carbon energy saving and environmental protection, how to design a display screen that can achieve the same effect with less materials has certain exploratory significance.

1 System Hardware Design

The system block diagram of the LED rotating screen display system is shown in Figure 1. The AVR main control chip is the core of the entire display system. Since the system needs to use the infrared control unit to update the display content, and the system's later scalability, after comprehensive consideration of application requirements and costs, the high-performance, low-power AVR microprocessor ATmega16L is selected as the core chip of the main control unit. The operating voltage of ATme-gal6L is 2.7~5.5 V, so a 3.7 V lithium battery is used to independently power the AVR chip and the column LED display unit, which solves the power supply problem in the design process of the rotating screen.

1.1 AVR basic peripheral circuit design

The basic hardware circuit of AVR microcontroller includes reset circuit, crystal oscillator circuit, A/D conversion filter circuit, ISP download interface and other parts.

1.1.1 Reset Circuit

The design of the reset circuit is shown in area ① in Figure 2. ATmega16L has built-in power-on reset design, and the extra time during reset can be controlled in the fuse position, so the power-on reset circuit outside the AVR can be designed very simply: just connect a 10 kΩ resistor (R17) to Vcc. For reliability, add a 0.1μF capacitor (C1) to eliminate interference and clutter. In the figure, D17 (IN4148) has two functions: one is to clamp the highest voltage of the reset input to about Vcc+0.5 V; the other is to short-circuit the R17 (10 kΩ) resistor when the system is powered off, so that C1 can be discharged quickly, so that an effective reset can be generated when the next call comes. When the AVR is in working state, press switch S1, the reset pin RESET becomes low level, triggering the AVR chip reset.

1.1.2 Clock Circuit

The design of the clock circuit is shown in area ② in Figure 2. The RC oscillator circuit is built in and can generate oscillation frequencies of 1 MHz, 2 MHz, 4 MHz, and 8 MHz. However, when a more accurate baud rate is required, it is recommended to implement it through an external circuit, such as the circuit shown in area ② in Figure 2. In the figure, 22 pF capacitors are connected to both ends of the crystal oscillator. When ATmega16L is actually used, these two small capacitors can work normally without being connected. However, it is still recommended to connect them for the standardization of the circuit.

The clock unit in FIG1 includes not only the clock circuit of ATmega16L, but also the circuit for realizing the real-time time display on the rotating screen, that is, a clock chip such as DS1302 is connected to the D interface of the ATmega16L main control chip, so that the rotating screen can display the time values ​​such as year, month and day before 2100 in real time.

1.1.3 ISP Download Interface

The ISP download interface is shown in area ③ of Figure 2. This interface does not require other devices. A double-row 2×5 socket is used here to access the interface. The use of the socket form also provides convenience for upgrading the software in the AVR in the future. Since there are no peripheral devices, the PB5 (MOSI), PB6 (MISO), PB7 (SCK), and RESET pins can still be used normally without interference from the ISP.

1.1.4 A/D conversion filter circuit

The A/D conversion filter circuit is shown in Figure 3. To reduce the power supply interference of A/D conversion, the ATmega16L chip has an independent A/D power supply. A 10μH inductor L1 is connected in series with Vcc, and then a 0.1μF capacitor C5 is connected to GND. The 2.56 V standard reference voltage built into the ATmega16L is used, and the reference voltage can also be input from an external circuit, such as the TL431 reference voltage source can be connected externally, and a 0.1μF capacitor C3 is connected to GND at the AREF pin.

1.2 Infrared receiving/transmitting control unit

When the LED rotating screen is rotating, the displayed content and images can be updated at any time with the help of infrared sensing technology. The infrared transmitting module is a circuit device similar to a remote control and independent of the rotating screen. The infrared receiving module (irDA) is fixed on the top of the cylindrical rotating screen.

1.3 Hall sensor unit

In order to make the displayed content smooth and flicker-free during the rotation of the LED rotating screen, the key issue to be addressed is to make the starting point of the displayed content the same for each rotation of the rotating screen, which is the so-called synchronization. In actual operation, it is necessary to use the induction signal to identify the starting position of the rotating screen after one rotation, and then trigger the corresponding program to keep the displayed content stable. This design uses the Hall sensor to obtain the induction signal, thereby realizing synchronous processing.

2 System Software Design

The software program design of the LED rotating screen mainly includes the multi-content and multi-form pattern display realized by two interrupts. After the system is powered on, the infrared signal is detected first. If the infrared signal is received, the content to be displayed is first determined, and then the Hall element installed at the bottom of the system detects the switch signal to determine the starting position of the column LED when rotating. After obtaining the corresponding signal, the AVR chip processes the data and sends the data of each column to the rotating column LED in turn, and then displays the corresponding content in the form of a dot matrix screen. The main flow chart of the system is shown in Figure 4.

3 Key issues to be addressed during the design process

3.1 Power Issues

Based on the working voltage of ATmega16L, a 3.7 V lithium battery is used to independently power the main control system. The lithium battery is fixed on the rotating base to coordinate the entire control system and rotate with the motor rotor. The reserved interface can be used to charge the battery at any time.

3.2 Balance Issues

The display system based on SMD devices is smaller in size than the system composed of DIP devices, but the mechanical structure of the whole machine is still a problem that cannot be underestimated. In the design, the symmetrical structure is used to balance the base design and device placement to minimize the negative impact caused by mechanical vibration. Figure 5 (a) is a structural diagram of the LED rotating screen in a static state; Figure 5 (b) is a screenshot of the rotating screen in working state.

3.3 Synchronization Issues

To make the rotating screen display stable and clear images, the speed of the DC motor is the biggest key. When a fixed voltage source is used to power the motor, the motor can be controlled to reach the optimal speed through a voltage divider resistor. During the debugging stage, this design uses an adjustable voltage-stabilized power supply, which is relatively easier to operate.

3.4 Latency Issue

The system mainly showed two display problems during the debugging process. One is that there is a string of garbled characters after a round of display content, and the other is that the display content has horizontal smearing, making the text unclear. For the former, the delay sub-function can be used to give a suitable delay time. For the latter, one or more columns of inverse level can be added during the column scanning process to increase the time gap between characters.

4 Conclusion

The LED rotating screen designed based on AVR chip technology and SMD technology is a row of SMD LEDs installed on a bracket. It rotates under the drive of a DC motor and uses people's visual persistence effect to display complete text or patterns. Because the screen is a rotating display, the display content can be viewed from a 360° full range. At the same time, the rotating screen uses a small number of light-emitting diodes to achieve a display screen that requires a large number of light-emitting diodes in traditional methods, and the design of SMD devices also makes the system more compact in size. After completing the corresponding software and hardware debugging, the results show that the design can achieve low-cost and high-quality display and publicity effects, and has certain practical value.