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Packet Size Check
When a TD is loaded, the packet size must be guaranteed to complete within the remaining frame
time. The sections below discuss FS and LS transaction frame requirements.
1) Full speed Check
The full speed packet size check is accomplished by comparing the value of a data transactions
data packet size (in bits) with the current value of the Largest Data Packet Counter. The data
packet size is provided by the List Processor and in the form of the MaximumPacketSize field
of the Endpoint Descriptor. If the data packet size is greater than the current value of the
Largest Data Packet Counter the transaction may not be started. If the data packet size is less
than or equal to the current value of the Largest Data Packet Counter the transaction may be
started.
Largest Data Packet Counter Operation
The Largest Data Packet Counter is used to determine whether or not a full speed
transaction with a data packet of a given size may be completed in the current frame. The
value accounts for bit stuffing requirements and overhead of the token, data, and handshake
packets.
At each frame boundary, coincident with the loading of FrameRemaining, the Largest
Data Packet Counter is loaded with the value of the FSLargestDataPacket field of the
HcFmInterval register. The counter value is decremented 6 of every 7 bit times (too
simulate the bit stuffing)
2) Low Speed Check
The low speed Packet Size Check is accomplished by comparing the current value of
FrameRemaining with the value of LSThreshold. If FrameRemaining is less than
LSThreshold, the low speed transaction cannot be started.
Transaction Requests
Transaction requests to the SIE are initiated by either the List Processor or the Frame Management
block. All of the requests are delivered to the SIE via the Frame Management block. The List
Processor requests must be qualified with the Packet Size Check. This check happens continually
from the time the Frame Management block receives the request from the List Processor until a
request acknowledge is received from the SIE indicating the start of the transaction. If the Packet
Check fails prior to the SIE acknowledge, a Packet Check failure signal is sent to the List
Processor by the Frame Management block.
In addition to qualifying List Processor transaction requests, the Frame Management block will
issue the SOF Token request. This request is issued when the value of FrameRemaining is 0001h.
The early request is to account for the SIE output delay between the request and a change on the
bus such that the SOP of the SOF token coincides with the reloading of FrameInterval in
FrameRemaining
Rev. 3.0, 03/01, page 288 of 390
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