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DP83266 Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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DP83266 Datasheet(HTML) 8 Page - National Semiconductor (TI) |
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8 / 152 page  30 Architectural Description (Continued) During frame transmission the Transmitter performs the fol- lowing functions Captures a token to gain the right to transmit Transmits one or more frames Generates the Frame Check Sequence and appends it at the end of the frame Generates the Frame Status field that is transmitted at the end of the frame Issues the token at the end of frame transmission During frame repeating the Transmitter performs the follow- ing functions Repeats the received frame and modifies the Frame Status field at the end of the frame as specified by the standard Whether transmitting or repeating frames the Transmitter also performs the following functions Strips the frame(s) that are transmitted by this station Generates Idle symbols between frames Data is presented from the Transmitter to the PLAYERa device in byte stream format (PH Request) 323 MAC Parameter RAM The MAC Parameter RAM is a dual port RAM that contains MAC parameters such as the station’s short and long ad- dresses These parameters are initialized via the Control Interface Both the Receiver and Transmitter Blocks access the RAM The Receiver uses these parameters to compare addresses in incoming frames with the individual and group addresses stored in the Parameter RAM The Transmitter uses the Parameter RAM for generating the Source Address for all frames (except when Source Ad- dress Transparency is enabled) and for the Destination Ad- dress and Information fields on Claim and Beacon frames 324 CountersTimers The CounterTimer Block maintains all of the counters and timers required by the ANSI X3T95 MAC standard Events which occur too rapidly for software to count such as the various Frame Counts are included in the Event Counters The size of the wrap around counters has been chosen to require minimal software intervention even under marginal operating conditions Most of the counters incre- ment in response to events detected by the Receiver The counters are readable via the Control Interface The Token Rotation and Token Holding Timers which are used to implement the Timed Token Protocol are contained within the Timer Block 33 DATA STRUCTURES 331 Data Types The architecture of the MACSI device defines two basic types of objects Data Units and Descriptors A Data Unit is a group of contiguous bytes which forms all or part of a frame A Descriptor is a two-word (64-bit) control object that provides addressing information and controlstatus informa- tion about MACSI device operations Data and Descriptor objects may consist of one or more parts where each part is contiguous and wholly contained within a memory page Descriptor pages are selectable as all 1 kBytes or all 4 kBytes Data Units are described by Descriptors with a pointer and a count A single Data Unit may not cross a 4k boundary All Descriptors may be marked as First Middle Last or Only Thus multiple De- scriptors may be combined to describe a single entity (ie one frame) A single-part object consists of one Only Part a multiple-part object consists of one First Part zero or more Middle Parts and one Last Part In Descriptor names the object part is denoted in a suffix preceded by a dot Thus an Input Data Unit Descriptor (IDUD) which describes the last Data Unit of a frame received from the ring is called an IDUDLast A Data Unit is stored in contiguous locations within a single 4 kByte page in memory Multiple-part Data Units are stored in separate and not necessarily contiguous 4 kByte pages Descriptors are stored in contiguous locations in Queues and Lists where each Queue occupies a single 1 kByte or 4 kByte memory page aligned on the queue-size boundary For Queues an access to the next location after the end of a page will automatically wrap-around and access the first location in the page Data Units are transferred between the MACSI’s Service Engine and Ring Engine via five simplex Channels three used for Indicate (receive) data and two for Request (trans- mit) data Parts of frames received from the ring and copied to memory are called Input Data Units (IDUs) parts of frames read from memory to be transmitted to the ring are called Output Data Units (ODUs) Descriptors are transferred between the MACSI device and Host via the ABus whose operation is for the most part transparent to the user There are five Descriptor types rec- ognized by the MACSI device Input Data Unit Descriptors (IDUDs) Output Data Unit Descriptors (ODUDs) Pool Space Descriptors (PSPs) Request Descriptors (REQs) and Confirmation Message Descriptors (CNFs) Input and Output Data Unit Descriptors describe a single Data Unit part ie its address (page number and offset) its size in bytes and its part (Only First Middle or Last) Frames consisting of a single part are described by a Des- criptorOnly frames consisting of multiple parts are de- scribed by a single DescriptorFirst zero or more Descrip- torMiddles and a single DescriptorLast Every Output Data Unit part is described by an ODUD Out- put Data Unit Descriptors are fetched from memory so that frame parts can be assembled for transmission Every Input Data Unit part is described by an Input Data Unit Descriptor (IDUD) Input Data Unit Descriptors are generat- ed on Indicate Channels to describe where the MACSI de- vice wrote each frame part and to report status for the frame Request Descriptors (REQs) are written by the user to spec- ify the operational parameters for the MACSI device Re- quest operations Request Descriptors also contain the start address of part of a stream of ODUDs and the number of frames represented by the ODUD stream part (ie the num- ber of ODUDLast descriptors) Typically the user will define a single Request Object consisting of multiple frames of the same request and service class frame control and expect- ed status 8 |
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