DP83936AVF datasheet(20/104 Pages) NSC

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30 Functional Description (Continued)

TLF12597 – 12

FIGURE 3-9 Transmit FIFO

36 STATUS AND CONFIGURATION REGISTERS

The SONIC-T contains a set of statuscontrol registers for

conveying status and control information tofrom the host

system The SONIC-T uses these registers for loading com-

mands generated from the system indicating transmit and

receive status buffering data tofrom memory and provid-

ing interrupt control Each register is 16 bits in length See

Section 60 for a description of the registers

37 BUS INTERFACE

The system interface

(Figure 3-10) consists of the pins nec-

essary for interfacing to a variety of buses It includes the

IO drivers for the data and address lines bus access con-

trol for standard microprocessors ready logic for synchro-

nous or asynchronous systems slave access control inter-

rupt control and shared-memory access control The func-

tional signal groups are shown in

Figure 3-10 See Section

70 for a complete description of the SONIC-T bus interface

38 LOOPBACK AND DIAGNOSTICS

The SONIC-T provides three loopback modes for self-test-

ing from the controller interface to the transceiver interface

The loopback function is provided to allow self-testing of the

chip’s internal transmit and receive operations During loop-

back transmitted packets are routed back to the receive

section of the SONIC-T where they are filtered by the ad-

dress recognition logic and buffered to memory if accepted

Transmit and receive status and interrupts remain active

during loopback This means that when using loopback it is

as if the packet was transmitted and received by two sepa-

rate chips that are connected to the same bus and memory

MAC Loopback

Transmitted data is looped back at the

MAC Data is not sent from the MAC to either the internal

ENDEC or an external ENDEC (the external ENDEC inter-

face pins will not be driven) hence data is not transmitted

from the chip Even though the ENDEC is not used in MAC

loopback the ENDEC clock (an oscillator or crystal for the

internal ENDEC or TXC for an external ENDEC) must be

driven Network activity such as a collision does not affect

MAC loopback and the CSMACD MAC protocol is not

completely followed

ENDEC Loopback

Transmitted data is looped back at the

ENDEC If the internal ENDEC is used data is switched

from the transmit section of the ENDEC to the receive sec-

tion

(Figure 3-5) Data is not transmitted from the chip and

the collision lines CDg are ignored hence network activi-

ty does not affect ENDEC loopback The LBK signal from

the MAC tells the internal ENDEC to go into loopback mode

If an external ENDEC is used it should operate in loopback

mode when the LBK signal is asserted CSMACD MAC

protocol is followed even though data is not transmitted

from the chip

Transceiver Loopback

Transmitted data is looped back at

the external transceiver (which is always the case regard-

less of the SONIC-T’s loopback mode) CSMACD MAC

protocol is followed since data will be transmitted from the

chip This means that transceiver loopback is affected by

network activity The basic difference between Transceiver

Loopback Mode and the other loopback modes is that the

SONIC-T loads the receive FIFO and buffers the packet to

memory In normal operations the SONIC-T only monitors

the packet that is looped back by the transceiver but does

not fill the receive FIFO and buffer the packet

381 Loopback Procedure

The following procedure describes the loopback operation

1 Initialize the Transmit and Receive Area as described in

Sections 54 and 55

2 Load one of the CAM address registers (see Section 61)

with the Destination Address of the packet if you are veri-

fying the SONIC-T’s address recognition capability

3 Load one of the CAM address registers with the Source

Address of the packet if it is different than the Destination

Address to avoid getting a Packet Monitored Bad (PMB)

error in the Transmit Status (see Section 644)

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