EETimes India - October 8, 2008 - (Page 11)

In Focus | Wireless Networking UWB video streaming in laryngoscopes continued from page 1 extensive layer stacks. Due to the high-bandwidth requirements (166Mbps for a picture in NTSC quality) and the operating range of the near field (1-10m), UWB is suitable for this instrument. Camera architecture The video sensor is a CMOS type. To compensate for data rate fluctuations on the transmission line and to allow for repeated transmission of video frames, the image is captured in a frame store before being sent to the UWB MAC. A tightly coupled MCU bus (Amba Host Bus) between the UWB MAC and the MCU allows most UWB functions, such as beacon generation, to be software supported. The UWB unit consists of a UWB transceiver (UWB PHY) and the UWB streaming media access controller. Since the camera unit is battery powered, care must be taken to keep power consumption to a minimum, such that the system can be operated at least 2hrs on one battery charge. The battery charging system is also controlled by the MCU. Network management User data (mainly video content) and the tables required for UWB network management are contained in memory. This memory can be internal to the MAC ASIC or bulk memory outside the MAC device. To provide a continuous data flow from the data source (camera) to the destination (display), the UWB MAC utilises isochronous data streaming. To reach this goal, a concept consist- Figure 2: Device A requests data from device B. ing of a fast user data path and a scheduler has been developed. Fast User Data Path formats the data stream according to the UWB protocol defined in the ECMA-368 standard. A scheduler controlled by the firmware sends the packets to the UWB PHY according to the UWB MAC timing requirements. This way, non-time-critical functions, such as network management and resource negotiation between the MAC on the camera side and the MAC on the display side, can be implemented in the firmware of an MCU. The Fast User Data Path and the scheduler need to be implemented in hardware. In addition to classic MAC layer functionality, the firmware also provides simple link control functionality for pointto-point transfers such as opening a connection with the display device and specifying the desired bandwidth. Firmware beacon UWB transmission is organised in superframes. A superframe is divided into beacon period (BP) and payload. Beacons and payload occupy 256 Medium Access Slots of the superframe. Starting at the end of the beacon period, the firmware processes the received beacons and calculates its own beacon for the next superframe. Figure 1 shows the beacon processing of two devices A and B during two superframes. The horizontal axis is the timeline from left to right. Both devices send beacons during the BP of each superframe. During the time the payload of superframe[n-1] is transmitted, the firmware of device A processes the beacon information element sent by device B. At the time superframe[n-1] has finished transmitting, the firmware has completed the information for device B, which appears in the Beacon Information Element (BIE) for device A in superframe[n]. As the MAC program can be seen as a real-time thread, a request and response queue is used for communication with the application. Figure 2 shows a communication cycle between device A and B. The application of device A makes a request. Within SF[n-1], the firmware of device A generates the BIE Request. The request is transmitted in the BP of SF[n]. Device B receives this beacon and calculates its answer in the data period SF[n]. The beacon received in SF[n+1] can now be interpreted by device A. MAC architecture Power consumption becomes an issue, since the device is battery operated. The architecture of the MAC requires a tight coupling between scheduler and MCU. The CMOS sensor is controlled by an I2C bus, and the battery management requires ADCs and a pulse width modulator (PWM). Part count should be reduced to a minimum, since everything has to fit into the handle of the laryngoscope. Thus, a standardcell ASIC would be the obvious choice; however volumes don’t justify the development costs. For these reasons, the Atmel CAP, an ARM-based MCU with mask-programmable custom logic, combining high integration, low NRE, low parts cost and low power, is a suitable solution for this application. The UWB MAC in the AT91CAP9 is implemented in the user metal programmable block connected to the ARM9 AHB through a six- layer AHB matrix. Read the full article to learn more about Tx and Rx data streams, and how you can customise MCUs for UWB video streaming MAC. Online | Figure 1: Device responds to a Beacon Request. Wireless HD video: Raising the UWB throughput bar (again) Video-modem approach for wireless video transfer 11 EE Times-India | October 1-15, 2008 | www.eetindia.com http://www.eetindia.co.in/SEARCH/SUMMARY/technical-articles/UWB.HTM?ClickFromNewsletter_081001 http://www.eetindia.co.in/SEARCH/SUMMARY/technical-articles/CMOS.HTM?ClickFromNewsletter_081001 http://www.eetindia.co.in/SEARCH/SUMMARY/technical-articles/ASIC.HTM?ClickFromNewsletter_081001 http://www.eetindia.co.in/ART_8800545801_1800005_TA_ee5474ea.HTM?ClickFromNewsletter_081001 http://www.eetindia.co.in/article/sendInquiry.do?articleId=8800545801&catId=1800005?ClickFromNewsletter_081001 http://www.eetindia.co.in/article/emailToFriend.do?articleId=8800545801&catId=1800005?ClickFromNewsletter_081001 http://www.eetindia.co.in/article/email_friend.php3?article_id=8800515679&type=TA&cat_id=1800000&back_url=%2Farticle%2Farticle_content.php3%3Fin_param%3D8800515679_1800000_TA_427b9233%26 http://www.eetindia.co.in/ART_8800510359_1800002_TA_e8a9f6cc.HTM?ClickFromNewsletter_081001 http://www.eetindia.co.in/ART_8800545314_1800004_TA_18628653.HTM?ClickFromNewsletter_081001 http://www.eetindia.com/STATIC/REDIRECT/Newsletter_081001_EETI02.htm?ClickFromNewsletter_081001

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