PNX1500E NXP Semiconductors, PNX1500E Datasheet - Page 723

• Manufacturer Part Number: PNX1500E
• Manufacturer: NXP Semiconductors

Specifications of PNX1500E

• Lead Free Status / Rohs Status: Not Compliant

NXP Semiconductors
Volume 1 of 1
PNX15XX_PNX952X_SER_N_4
Product data sheet
5.8.2 Real-time/non-real-time transmission mode
Each transmit data-path has it’s own associated descriptor and status array in
memory and it’s own DMA manager for transferring data to and from memory. An
arbiter internal to the LAN100 decides which of the transmit DMA managers should
be allowed to transmit a packet, based on the mode of operation, the time-stamp and
the priority.
The modes can be selected by the device driver by programming the QoSEnable bit
in the Command register. Setting the bit to 0 selects real-time/non-real-time mode;
setting the bit to 1 selects QoS mode. The device driver should configure one of the
modes during initialization of the LAN100.
On a simple network stack, QoS mode may be used by creating devices named
“eth0” and “eth1”, one of which maps to the high priority Transmit Datapath while the
other maps to the low priority Transmit Datapath.
In order to use the real-time/non-real-time mode, the network stack should have
some support for time-stamps.
In real-time/non-real-time mode, that is, when the QoSEnable bit of the Command
register is set to 0, the Tx Transmit Datapath transmits non-real-time packets while
the TxRt datapath transmits real-time packets.
Transmit time-stamps allow software to specify the time when the packet should be
transmitted for each packet in the TxRt descriptor array. A transmit time-stamp value
is included by software in the time-stamp field of real-time transmit packet
descriptors. These fields are ignored in the non-real-time descriptors or in QoS mode.
There are two transmit descriptors arrays, one for real-time traffic with transmit
time-stamps and one for non-real-time traffic without transmit time-stamps. Each
descriptor array is handled in sequential order by the associated DMA manager (Tx
and TxRt). Packets in the real-time descriptor array have priority over packets in the
non-real-time array as soon as the time-stamp has supplied. Packets in the real-time
array must be ordered by software in ascending time-stamp value order. The transmit
arbiter will inspect the time-stamp of a real-time packet and pass it for transmission as
soon as the GlobalTimeStamp counter register exceeds the time-stamp located in the
time-stamp field in the descriptor of the packet’s first fragment. While the packet in the
TxRt DMA is waiting for the GlobalTimeStamp to exceed the time-stamp in the
current packet, the arbiter will permit the Tx DMA to transmit packets.
Care must be taken that the scheduling of real-time packets does not completely lock
out non-real-time packets from gaining access to the Ethernet connection. Also, when
multiple real-time packets are scheduled, there must be enough difference between
their time-stamp values to have time to actually complete the packet transmissions.
When a packet misses its time-stamp moment, that is, when the current value of the
time-stamp generator is larger than the time-stamp value at the moment when a
descriptor is read by the Tx DMA manager, then the packet will be sent as soon as
possible after that moment. However, since time-stamps will eventually be reused
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Quality of Service (QoS) mode: In this mode, the TxRt transmission datapath
handles low priority transmission, while the Tx datapath handles high priority
transmissions.
Rev. 4.0 — 03 December 2007
Chapter 23: LAN100 — Ethernet Media Access Controller
PNX15xx/952x Series
© NXP B.V. 2007. All rights reserved.
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