DP83261AVF datasheet(100/132 Pages) NSC

100 / 132 page

70 Signal Descriptions (Continued)

If RQRCLS remained asserted the token would be held as

long as possible and multiple frames could be transmitted

In this case the

uTXRDY x uRQSEND x v TXRDY

x vRQSEND handshake for the beginning of each

frame remains identical

Single Frame Transmission without Prestaging

Figure 7-14 prestaging is not used Multiple requests are

present at the interface of which only the highest priority

request is presented to the interface RQRCLS is changing

because higher priority requests become ready to be serv-

iced The scheduling decision is made until a Service Op-

portunity occurs Once TXRDY is asserted RQRDY is as-

serted and the r6 request is serviced

When the data associated with r6 is ready to be transmitted

RQSEND is asserted This in turn causes TXRDY to be

deasserted when transmission begins (entrance to MR2)

The deassertion of TXRDY causes RQSEND to be deas-

serted

During the first frame of the request the end of Service

Opportunity condition becomes true as a result of

THT reaching the THT priority threshold if the request

was an asynchronous priority request

THT expiration if the request was an asynchronous re-

quest or

TRT expiration if the request was a synchronous request

TXPASS is asserted to indicate that this Service Opportunity

is complete

If RQRCLS remains greater than 0 the next usable token

will be captured and servicing of the request will continue If

RQRCLS remained asserted the token would be held as

long as possible and multiple frames could be transmitted

this

case

the

uTXRDYxuRQSENDx

vTXRDYxvRQSEND handshake for the beginning of

each frame remains identical

Aborted Frame Transmission

A transmission as in

Figure 7-14 is started During the trans-

mission an interface error occurs (for example) and RQABT

is asserted to cause the current frame to be aborted (see

Figure 7-15 ) TXACK is then deasserted and TXABORT is

asserted to indicate that the frame was aborted as a result

of a FIFO underrun or an equivalent reason This is signaled

with RQABT After the frame is aborted TXRDY is asserted

to indicate that another frame may be transmitted Since no

frames are ready to be transmitted a Void fill frame is trans-

mitted During the Void frame transmission the interface

then sets RQRCLS to zero to indicate that the Token should

be issued TXPASS is then asserted once the Ending Delim-

iter of the Void frame is transmitted

In this scenario the transmitted Void frame serves two pur-

poses It is transmitted because the interface was stalling

waiting for another frame and also in response to the abort-

ed frame A Void frame is transmitted every time a transmis-

sion is aborted

MAC Reset

Figure 7-16 a MAC reset occurs during a frame transmis-

sion This causes the current frame to be aborted and the

Ring Operational Flag (TXRINGOP) to be deasserted

TXPASS is asserted with TXABORT after the frame is abort-

ed Since the ring is not operational no Void frames are

transmitted

Figure 7-16 the MAC Reset occurs while the Ending Del-

imiter is being transmitted In

Figure 7-17 the boundary case

is shown where the MAC Reset occurs during the Frame

Status Note that the Ending Delimiter of the frame is trans-

mitted with the frame status TXRDY is asserted for one

cycle followed by TXPASS with TXABORT

Synchronous Request followed by Asynchronous

Request

Figure 7-18 frames from two requests are serviced on

the same Service Opportunity Once the synchronous frame

is being transmitted the RQRCLS is changed to that for the

asynchronous frame At the end of the synchronous frame

TXRDY is asserted since the token is still usable for the

asynchronous request RQRDY is then asserted and the

Asynchronous frame is then transmitted

Notice that the value of THTDIS changes after the Frame

Status for the synchronous frame is transmitted THT is dis-

abled for synchronous transmission and enabled for normal

asynchronous transmission

Restricted Begin

Figure 7-19 a restricted dialogue is begun A non-restrict-

ed Token is captured a single frame is transmitted and a

Restricted Token is issued

An Rbeg Request is a request to capture a Non-restricted

Token and issue a Restricted Token Since there is only one

frame in this example RQFINAL is asserted during the first

frame In the example RQFINAL is asserted one byte time

after RQSEND is deasserted while RQRDY is still asserted

but it may be asserted anytime while RQRDY is asserted

Notice that TXCLASS changes to restricted after RQFINAL

is asserted

Immediate Claim

Figure 7-20 an immediate Claim frame is transmitted

from the Claim state

A Lower

Claim frame is received from an upstream station

causing this station to enter its Claim state and deassert

TXRINGOPRQRCLS is set to immediate and RQCLM is as-

serted

An internally generated Claim frame is first transmitted (at

least one internally generated Claim or Beacon frame is al-

ways transmitted upon entry to the Claim or Beacon state)

After the internally generated Claim frame is transmitted

TXRDY is asserted since the transmitter is still in the Claim

state (the ring can hold more than one Claim frame) The

frame is then transmitted following the normal handshake

Similar timing applies for externally generated Beacon

frames

Remember that for Immediate Requests from the Claim and

Beacon States RQSEND must be asserted no later than

8 byte times after TXRDY is asserted This guarantees that

a minimum size preamble will be generated

After the frame is transmitted TXRDY is asserted again

since the transmitter is still in the Claim state

If this station wins the Claim Process TXPASS is asserted

without TXABORT If another station causes this station to

backoff (this station receives a Higher

Claim) TXPASS is

asserted with TXABORT

100

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