The wireless system is based on a high-rate radio frequency chip video communication system, and has an instant shooting function; in video (continuous) mode, the video data is transmitted at a resolution of QQVGA, which is 160 & TImes; 120 (@13fps), which basically achieves real-time video. The day of the day. In practical applications, the user can view the scene near the transmitting end (far away) in the LCD of the receiving end. When you see the scene of interest, press the button for a while to get an image of 1.3M pixels, which is convenient and practical. In order to simplify the design, the system only uses 8-bit color depth and RGB data format, and does not use CCD camera chip, nor does it use FPGA chip for logic control, which saves cost.
The following focuses on the wireless video communication system with TMS320VC5402 DSP as the control core. It describes the interface design of DSP and camera chip, DSP and RF chip in detail, analyzes the key points in the design, and finally gives some DSP assembly code.
1 The composition and working principle of wireless real-time video system1.1 Introduction to OV9640 Camera Chip
OV9640 is a high-performance CMOS image sensor chip introduced by OmniVision. Support 1.3 megapixel image capture and a variety of resolutions, including l280 & TImes; 960, VGA, QQVGA, CIF, QCIF and many other data output formats, such as Raw RGB, YUV (4:2:2), YcbCr (4: 2:2), etc.; support 8-bit or 16-bit data output; program it through SC-CB interface, can realize various basic functions of image processing, such as exposure control, white balance, color saturation, gamma control, etc.; Low voltage requirements for embedded mobile devices.
1.2 Introduction to nRF24L01 RF Chip
The nRF24L01 is a single-chip wireless GFSK transceiver chip from Nordic, Norway. It operates in the 2.4-2.5 GHz ISM band and has a wireless transmission rate of up to 2 Mbps. The MCU uses an SPI interface for control and data transmission. Compared to its predecessor, the nRF2401, the nRF24LOl offers superior performance and lower power consumption. It can support up to 6 data channels, and each channel supports Enhanced ShockBurst (ESB) technology, with automatic bad (AACK) and automatic retransmission (ART) functions, reducing the burden on the MCU and reducing the loss of wireless data. The packet rate improves the efficiency of two-way transmission. When the ESB is turned on, the nRF24L01 will automatically switch to the receiving mode after waiting for the packet to wait for the response from the other party. Automatic retransmission will be implemented according to the setting of the register.
1.3 system hardware circuit
The hardware circuit consists of two parts: the transmit (acquisition) end and the receive (storage display) end, and the wireless link in the 2.4 GHz band is realized by the high-speed RF chip. The transmitter uses TMS320VC5402 DSP as the control core, OV9610 camera chip as the video (or image) acquisition front end, AT29LV1024 Flash ROM as the DSP bootloader memory chip, K4S161622H IMB capacity SDRAM as the program running space and video data buffer, the video data finally passes The RF chip nRF24LO1 is transmitted; the hardware structure of the receiving end is basically the same as that of the transmitting end, and the front end OV9640 is changed to the LCD display of the back end. The overall structural block diagram of the entire system is shown in Figure 1.
1.4 System working principle and process
1.4.1 The sender works in video streaming mode
The transmitting end is composed of a DSP as a core control chip. The DSP is powered on and initialized. The code in the Flash ROM is loaded into the SDRAM through the BootLoader to realize high-speed operation of the system to speed up the processing speed of the data, and the HPI interface is set to general-purpose I/O. Then, the nRF24L0l is set to the transmission mode through the McBSPO buffer serial port, and the data packet containing the predetermined address is sent out to occupy the detection receiving end, and the nRF24L01 automatically switches to the mode of waiting for the response signal. If there is a correct receiving end (address matching), the nRF24L01 notifies the DSP through the INTO interrupt, causes the DSP to re-set the nRF24L01 to the transmitting mode, and immediately initializes the OV9640, implements the SCCB bus through the McBSP1 buffered serial port, starts the camera and sets For continuous frame mode. The resolution at this time is standard QQVGA, that is, 160 & TImes; 120 (@8bit). Finally, the DSP converts the 8-bit parallel data obtained from D[7:O] into a serial format, and sends it to nRF24L01 through SDRAM buffer and McBSP0. Send video data out. If the correct receiver is not detected (no INTO interrupt occurs), the DSP will wait for INTO to occur or until the user turns off the power.
1.4.2 The sender works in shooting mode
During video streaming, the nRF24LO1 can simultaneously listen to the air signal and automatically answer. If you receive a photo notification from the receiving end (press the button), set the OV9640 to the standard shooting mode with a resolution of l280 & TImes; 960 (@8bit). Then, the DSP sets the nRF24L01 to the transmission mode and transmits the frame data at this time. After the image data transmission is completed and the acknowledgment signal is received, the system will return to the video stream mode. If the reception is unsuccessful, the automatic retransmission function of nRF24LO1 will ensure the integrity of the data transmission.
1.4.3 Workflow at the receiving end
The initialization of the receiving end is basically the same as that of the transmitting end, but the nRF2dL01 (according to the predetermined address) is set to the receiving mode to receive the detection signal. After the matching address is detected, the auto-answer function of the nRF24L0l sends an acknowledgement signal to the sender to confirm the receipt of the signal. At this time, the two handshaking is successful. Next, the DSP is notified by the INTO interrupt so that the DSP re-sets the nRF24LO1 to the receive mode to receive the continuous video stream from the sender, and turns on the LCD module to prepare to display the video. Finally, the DSP buffers the video stream through SDRAM and sends it to the LCD display (if the other back-end modules such as LCD are parallel interfaces, the data needs to be converted into parallel data format). At this point, the system has been able to achieve real-time video data wireless transmission, the resolution of the real-time video stream is QQVGAl60 × 120 (@13fps).
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