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NXP Semiconductors
PNX15xx/952x Series
Volume 1 of 1
Chapter 23: LAN100 — Ethernet Media Access Controller
PNX15XX_PNX952X_SER_N_4
NXP B.V. 2007. All rights reserved.
Product data sheet
Rev. 4.0 — 03 December 2007
23-724
(because the GlobalTimeStamp counter wraps around 0), packets with time-stamps
that are late are only sent out when the difference between the time-stamp of the
packet and the local time-stamp of the time-stamp generator is less than
half the total
time-stamp range, that is, less than 231. If the difference is more than 231, then the
packet will not be transmitted, and the packet and all other packets in the real-time
descriptor list must wait until the value of the time-stamp generator wraps around to
its time-stamp value, or until software removes such a “stuck” packet by soft resetting
the Transmit Datapath.
The register BlockZone can be used to specify a time period before a transmit
time-stamp value during which no new transmission of non-real-time packets can be
started. This can help to free up the Ethernet wire for the impending real-time packet.
The unit of time of the BlockZone register is equal to the unit of time of the time-stamp
generator.
In pseudo-code, the real-time/non-real-time arbitration proceeds as follows:
diff[31:0] = GlobalTimeStamp[31:0]
- TxRtDescriptor.timeStamp[31:0];
bs[31:0] = GlobalTimeStamp[31:0] + BlockZone[31:0]
- TxRtDescriptor.timeStamp[31:0]
if (!diff[31]) // is GlobalTimeStamp > descriptor time-stamp?
// True
// If possible, issue TxRt packet
else if (!bs)
// We are in the BlockZone, so do not issue
else
// If possible, issue Tx packet
If a non-real-time packet is still occupying the transmit logic when the time-stamp
moment of a real-time packet is reached, then the packet will be transmitted as soon
as the non-real-time packet has nished. When the value in BlockZone is 0, the
BlockZone mechanism is disabled.
Apart from using time-stamps for real-time transmission, the LAN100 also reports
back to software the actual moment that packets are received or transmitted, as