PIC16F639-E/SS【PDF 13/74 页】IC MCU FLASH 2KX14 20SSOP

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Micrel, Inc.

KSZ8841-PMQL

October 2007

M9999-100407-1.5

LinkMD Cable Diagnostics

The KSZ8841-PMQL LinkMD uses time domain reflectometry (TDR) to analyze the cabling plant for common cabling

problems such as open circuits, short circuits, and impedance mismatches.

LinkMD works by sending a pulse of known amplitude and duration down the MDI and MDI-X pairs and then analyzes

the shape of the reflected signal. Timing the pulse duration gives an indication of the distance to the cabling fault with a

maximum distance of 200m and an accuracy of +/–2m.

Access

LinkMD is initiated by accessing register P1VCT, the LinkMD Control/Status register, in conjunction with register

P1CR4, the 100BASE-TX PHY Controller register.

Usage

LinkMD can be run at any time. To use LinkMD, disable HP Auto-MDIX by writing a ‘1’ to P1CR4[10] to enable manual

control over the pair used to transmit the LinkMD pulse. The self-clearing cable diagnostic test enable bit, P1VCT[15], is

set to ‘1’ to start the test on this pair.

When bit P1VCT[15] returns to ‘0’, the test is complete. The test result is returned in bits P1VCT[14-13] and the

distance is returned in bits P1VCT[8-0]. The cable diagnostic test results are as follows:

00 = Valid test, normal condition

01 = Valid test, open circuit in cable

10 = Valid test, short circuit in cable

11 = Invalid test, LinkMD failed

If P1VCT[14-13]=11 case, this indicates an invalid test, occurs when the KSZ8841-PMQL is unable to shut down the

link partner. In this instance, the test is not run, since it would be impossible for the KSZ8841-PMQL to determine if the

detected signal is a reflection of the signal generated or a signal from another source.

Cable distance (in meters) can approximately be determined by the following formula:

Distance = P1VCT[8-0] x 0.4m

This constant may be calibrated for different cabling conditions, including cables with a velocity of propagation that

varies significantly from the norm.

Media Access Control (MAC) and other

The KSZ8841-PMQL strictly abides by IEEE 802.3 standards to maximize compatibility.

Inter Packet Gap (IPG)

If a frame is successfully transmitted, the minimum 96-bits time for IPG is between the two consecutive packets. If the

current packet is experiencing collisions, the minimum 96-bits time for IPG is from carrier sense to the next transmit

packet.

Back-Off Algorithm

The KSZ8841-PMQL implements the IEEE standard 802.3 binary exponential back-off algorithm in half-duplex mode.

After 16 collisions, the packet is dropped.

Late Collision

If a transmit packet experiences collisions after 512 bit times of the transmission, the packet is dropped.

Flow Control

The KSZ8841-PMQL supports standard 802.3x flow control frames on both transmit and receive sides.

On the receive side, if the KSZ8841-PMQL receives a pause control frame, the KSZ8841-PMQL will not transmit the

next normal frame until the timer, specified in the pause control frame, expires. If another pause frame is received

before the current timer expires, the timer will be updated with the new value in the second pause frame. During this

period (while it is flow controlled), only flow control packets from the KSZ8841-PMQL are transmitted.

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