Implementation of a Wimedia UWB Media Access Controller

Transcription

1 Implementation of a Wimedia UWB Media Access Controller Hans-Joachim Gelke Institute of Embedded Systems Zurich University of Applied Sciences Technikumstrasse 20/22 CH-8401,Winterthur, Switzerland hans.gelke@zhaw.ch ABSTRACT Wireless Ultra-Wideband Technology intends to replace the wide spread USB cable. Thanks to high data rates above 300 Mbit/s it is also suitable to transmit uncompressed video streams point to point. A digital Endoscope, with which physicians examine bodily cavities, avoids the constraining cables through wireless transmission. The prototype, which is based on a micro controller with mask programmable transistor array, transmits in the lab through a 30 cm brick wall. Keywords UWB, Ultra-Wideband, WiMedia, Endoscope, ECMA INTRODUCTION 1.1 Background Ultra Wide Band (UWB), a wireless transmission technology for high bandwidth ( Mb/s), short range (10-50m), slowly finds its way into mainstream PC. The semiconductor market focuses mainly on replacing the USB cable with UWB technology, which implies that most commercially available UWB chips are geared toward Wireless-USB hubs or PCI bus hosts. However, there are many other niche applications, for which this technology is ideally suited for. e.g. uncompressed video streaming, wireless automation or medical instrumentation. This paper describes a WiMedia compliant UWB Media Access Controller (MAC) realized with an Atmel Customizable Application Processor (CAP). This micro controller consists, besides the well known standard ARM9 peripherals like USB and A/D converters, of a mask programmable transistor array, which can be used to implement the UWB MAC. The customer programming area of this particular Atmel micro controller makes it equivalent to a mask programmable ASIC. UWB protocol handling is completely controlled by software and such the hardware design could be kept as simple as possible. Software also controls the MAC configuration as host or device (receiver or transmitter); this means that the same mask programmable SoC can be used in the receiver and the transmitter. The MAC interfaces to a UWBtransceiver module according to the ECMA-368 MAC-PHY interface standard. Data rates of up to 355 Mb/s, utilizing isochronous data transfers, can be achieved. 1.2 Endoscope for Gastroenterology Figure 1: Endoscope for Gastroenterology In order to examine body cavities, physicians use so called endoscopes, which are available in flexible and rigid versions (see picture 1). Flexible Endoscopes do have a long and thin tube, which will be introduced to the body of the patient. The tube contains a light conductor for the light source and an optical pipe for the image. Through an ocular at the end of the endoscope, the physician is able to view the images. The low efficiency of the light conductor makes it necessary to use a high output light source, which is connected to the hand grip of the endoscope with another light condufctor. New LED light sources and miniature CMOS cameras allow placement right on the tip of the endoscope tube. This way the expensive optical pipe to be replaced with copper wires. Additionally, the image must not be viewed through the ocular anymore, but can be displayed on a Monitor and even recorded electronically. A further advantage is, that the image can now be transmitted wireless to the monitor. This way, the system has no constraints anymore and can be handled more easily by the physician. The prototype of such a wireless system was realized in co-

2 operation with Brütsch Electronics ( and the Institute of Embedded Systems at Zurich University of Applied Sciences in Switzerland. 1.3 Camera Architecture NTSC Camera Microcontroller User- Light controll Digital Video I2C PWM SDRAM SDRAM Controller UWB MAC A/D PWM UWB PHY Battery Charger Figure 2: Block diagram of UWB camera For reasons of picture quality and avoidance of distortions, it has been decided, that the transmission should be digital. For the comfort of the physician, movements of the object should appear on the screen with little delay, which implies that the video signal may not pass through compression circuits or extensive layer stacks. Due to the high bandwidth requirements (166Mbit/s for a picture in NTSC quality) and the operating range of the near field (1-10m), UWB is the suitable transmission media for this instrument. The video sensor is a CMOS type geared toward medical applications in regards to picture quality and physical size. In order to compensate for data rate fluctuations on the transmit path, the image is captured in a frame store before being sent to the UWB MAC. A tightly coupled micro controller bus (Advanced Host Bus) between the UWB MAC and the micro controller allows most UWB functions, like 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 2 hours on one battery charge. The battery charging system is controlled by the micro controller as well. 1.4 Main Principle of the UWB Streaming MAC Memory Firmware UWB MAC Fast User Data Path Scheduler Figure 3: Data flow of UWB MAC UWB PHY User data (mainly video content) and the tables required for UWB network management are contained in memory. This memory could be internal to the MAC ASIC, or bulk memory outside of the MAC device. To provide a continuous data flow from the data source (camera) to the destination (display), the UWB MAC utilizes isochronous data streaming. To reach this goal, a concept consisting of a fast user data path and a scheduler was developed. (Figure 3 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-Transceiver according to the UWB MAC timing requirements. This way, the non time critical functions like 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 the micro controller. 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 point to point transfers like opening a connection with the display device and specifying the desired bandwidth. 1.5 Firmware Beacon Processing UWB transmission is organized in super frames (see Figure 4). A super frame is divided in Beacon Period (BP) and payload. Beacons and payload occupy 256 Medium Access Slots of the super frame. Starting at the end of the beacon period, the firmware processes the received beacons and calculates its own beacon for the next super frame. Radio BP own beacon A: Calculates Answer target beacon Superframe A B [n-1] A B time Superframe [n] B: Interprets Figure 5: Device responds to a Beacon Request Figure 5 shows the Beacon Processing of two devices A and B during two super frames. Both devices send Beacons during the beacon period (BP) of each super frame[4]. During the time the payload of super frame[n-1] is transmitted, the firmware of device-a processes the beacon information element sent by device-b. At the time super frame[n-1] has finished transmitting, the firmware completed the information for device-b, which appears in the Beacon Information Element (IE) for device-a in super frame [n]. Figure 6 (see top of next page) shows a communication cycle between device-a and -B. The Application of device-a makes a request to device B. Within SF[n-1] the firmware of device-a generates the Beacon IE request. The request will be 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.

3 Figure 4: UWB Superframe with 256 Media Access Slots BP own beacon target beacon time Radio A B Superframe [n-1] A B Superframe [n] A B Superframe [n+1] Application Request A: Application Request B: Calculating Answer A: Interpret Answer Calculate Beacon For Superframe n Other Device Reacts Process Answer to Request Figure 6: Device A requests data from device B

4 2. IMPLEMENTATION 2.1 Overview Since the device is battery operated, power consumption becomes an issue. As described in Section 1.3, the architecture of the MAC requires a tight coupling between scheduler and micro controller. The CMOS sensor is controlled by a I2C bus, and the battery management requires A/D converters and a Pulse Width Modulator (PWM). Parts count should be reduced to a minimum, since everything has to fit into the handle of the endoscope. Therefore a standard cell ASIC would be the obvious choice; however predicted volumes don t justify the development costs. For these reasons, the Atmel CAP, an ARM based micro controller with mask programmable custom logic, combining high integration, low NRE, low parts cost and low power, is a feasible solution for this application. The following chapter describes the implementation of the UWB MAC fitted to the architecture of the Atmel CAP. Figure 7 shows the Block Diagram of the AT91CAP9. The UWB MAC is implemented in the user metal programmable block, which is connected to the ARM9 through a 6- layer matrix. To take advantage of cheap bulk memory, the RAM for Data and UWB management (see also section 1.4) is located external to the micro controller and connected to the EBI interface. Micro controller External Bus s are designed to allow the connection of many different Memory Types like DDRAM, Mobile SDRAM, and so on. On the other hand, memory that must be accessed without delay, like the data buffer in the RX/TX logic, the tightly coupled SRAM or dual ported RAM on the Micro controller can be used. Master RXn Endpoint RX0 Endpoint TXn Endpoint TX0 Endpoint RX_STREAM Acknowledge TX_STREAM RX/TX Logic or memory area requires an endpoint. The endpoints can be directed to any address in the memory area. The MAC PHY () is the interface to the UWB physical layer; it complies with the ECMA-369 standard, such that any UWB transceiver, which complies to this standard, may be connected. [2]. The scheduler is responsible for the timing according to ECMA-368. It controlls the flow of the data between Memory and transceiver interface, as well as the timing of the super frames. It receives the information, as of when the network can be accessed, by the Network Allocation vector (NAV), an instruction table a timing calculator and a timer. The timing calculator calculates frame -size and -duration. To offload the, scheduler access to registers and tables is done via the Advanced Peripheral Bus (APB). 2.2 TX Data Stream The TX data stream (see Fig. 9)is a high speed data path for all frames sent from the application to the UWB-transceiver. The TX- Master acts like a DMA between the and the RX/TX Logic. Data transfers are initiated from the scheduler. When the bus is granted, the Master begins a sequential burst transfer and the TX master begins reading from the memory buffer and transfers the data to the buffer of the RX/TX Logic [3]. Burst size, start address, direction and wrap boundaries are provided by the TX endpoint. When transfers are pending on both, the RX Master and the TX master, the TX master has the priority. Header and status information is supplied by the Status/Header RAM, which is part of the tightly coupled memory blocks of the Micro controller. The TX-Endpoint updates the MAC Header Fields according to its own configuration and the information provided by the fragmentation logic(not shown). The RX/TX logic is responsible for updating the PHY header fields according to the scheduler (For information contained in the MAC and PHY header fields see [1]). APB APB Slave Timing Calculation Scheduler Instruction Table Network Allocation Vector Timer Synchronisation Beacon Interpretation to Arbiter TX Master Master Config. Status / Header RAM TXn Endpoint RX/TX Logic Figure 8: Block Diagram des UWB MAC Figure 8 shows a conceptual overview of the UWBMAC architecture. The top half of the picture shows the fast data path between the MAC-PHY () and the Advanced Host Bus () of the micro controller. The lower half of Figure 8 shows the synchronizing of the frames and control of the -Timing. External Memory lookups are performed via the RX- and TX-Endpoints. Each endpoint contains an address pointer, which determines the start address and end address of the memory location addressed by the master. Each table Scheduler/ Fragmentation Figure 9: TX Data Stream with Endpoint 2.3 RX Data Stream Figure 10 shows the RX data stream is a high speed data path for all s received by the UWB-transceiver and sent to the application. When the RX/TX Logic receives a from the UWB-transceiver, it will be stored in an internal buffer(tightly coupled RAM of Micro controller) until it is determined that the frame is addressed to this device and the CRC32 matches. If this is the case,

5 Figure 7: Blockdiagramm of CAP9

6 the RX/TX Logic signals the scheduler a newly available frame. The RX control gets frame information like UWB-address, endpoint status and configuration from the header of the newly received frame and hands it over to the RX end point. The Filter/Configuration RAM of the end point converts this into destination address and burst size. After wards the RX end point configures the RX master, which transfers the data in the external memory of the Micro controller. to Arbiter RX Master Master Config. Filter / Config RAM RXn Endpoint Rx/Tx Logic Figure 10: RX Data Stream with Endpoints 3. CONCLUSION The prototype is equipped with the RTU7010 UWB PHY evaluation module from Realtek, which covers UWB band 1. At a brutto data rate of 320Mbits and at a distance of 3m, bit error rates of % could be achieved. The transmission is also able to pass through a 30cm brick wall without degradation. Humans which stand inbetween the sigth of the antennas hardly increase the failure rate. The current design uses the Realtek/Wionics RTU7012 Dual- Band UWB PHY The UWB-MAC HDL code and Software can be licensed from Institute of Embedded Systems. We also help you implementing your UWB Application. For more information contact: Hans-Joachim Gelke Institute of Embedded Systems Technikumstr. 20 CH-8401 Winterthur Switzerland hans.gelke@zhaw.ch Phone: REFERENCES [1] ECMA-368, High Rate Ultra Wideband PHY and MAC Standard. [2] ECMA-369, MAC-PHY for ECMA-368. [3] D. Alberti. UWB MAC Architecture. Institute of Embedded Systems (InES), 1 edition, [4] D. Alberti. UWB MAC Documentation. Institute of Embedded Systems (InES), 1 edition, 2008.