DP83901AV datasheet(35/62 Pages) NSC

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120 Loopback Diagnostics (Continued)

3 Verify that the Address Recognition Logic can

a) Recognize address match packets

b) Reject packets that fail to match an address

LOOPBACK OPERATION IN THE SNIC

Loopback is a modified form of transmission using only half

of the FIFO This places certain restrictions on the use of

loopback testing When loopback mode is selected in the

TCR the FIFO is split A packet should be assembled in

memory with programming of TPSR and TBCR0 TBCR1

registers When the transmit command is issued the follow-

ing operations occur

Transmitter Actions

1 Data is transferred from memory by the DMA until the

FIFO is filled For each transfer TBCR0 and TBCR1 are

decremented (Subsequent burst transfers are initiated

when the number of bytes in the FIFO drops below the

programmed threshold)

2 The SNIC generates 56 bits of preamble followed by an

8-bit synch pattern

3 Data transferred from FIFO to serializer

4 If CRC e 1 in TCR no CRC calculated by SNIC the last

byte transmitted is the last byte from the FIFO (Allows

software CRC to be appended) If CRC e 0 SNIC calcu-

lates and appends four bytes of CRC

5 At end of Transmission PTX bit set in ISR

Receiver Actions

1 Wait for synch all preamble stripped

2 Store packet in FIFO increment receive byte count for

each incoming byte

3 If CRC e 1 in TRC receiver checks incoming packet for

CRC errors If CRC e 0 in TCR receiver does not check

CRC errors CRC error bit always set in RSR (for address

matching packets)

4 At end of receive receive byte count written into FIFO

receive status register is updated The PRX bit is typically

set in the RSR even if the address does not match If

CRC errors are forced the packet must match the ad-

dress filters in order for the CRC error bit in the RSR to be

set

EXAMPLES

The following examples show what results can be expected

from a properly operating SNIC during loopback The re-

strictions and results of each type of loopback are listed for

reference The loopback tests are divided into two sets of

tests One to verify the data path CRC generation and byte

count through all three paths The second set of tests uses

internal loopback to verify the receiver’s CRC checking and

address recognition For all of the tests the DCR was pro-

grammed to 40H

Path

TCR

RCR

TSR

RSR

ISR

SNIC Internal

(Note 1)

(Note 2)

(Note 3)

Note 1

Since carrier sense and collision detect are generated in the EN-

DEC module They are blocked during NIC loopback carrier and

CD heartbeat are not seen and the CRS and CDH bits are set

Note 2

CRC errors are always indicated by receiver if CRC is appended by

the transmitter

Note 3

Only the PTX bit in the ISR is set the PRX bit is only set if status is

written to memory In loopback this action does not occur and the

PRX bit remains 0 for all loopback modes

Note 4

All values are hex

Path

TCR

RCR

TSR

RSR

ISR

SNIC Internal

(Note 1)

Note 1

CDH is set CRS is not set since it is generated by the external

encoderdecoder

Path

TCR

RCR

TSR

RSR

ISR

SNIC External

(Note 1)

(Note 2)

Note 1

CDH and CRS should not be set The TSR however could also

contain 01H 03H 07H and a variety of other values depending on

whether collisions were encountered or the packet was deferred

Note 2

Will contain 08H if packet is not transmittable

Note 3

During external loopback the SNIC is now exposed to network traf-

fic it is therefore possible for the contents of both the Receive

portion of the FIFO and the RSR to be corrupted by any other

packet on the network Thus in a live network the contents of the

FIFO and RSR should not be depended on The SNIC will still abide

by the standard CSMACD protocol in external loopback mode

(ie The network will not be disturbed by the loopback packet)

Note 4

All values are hex

CRC AND ADDRESS RECOGNITION

The next three tests exercise the address recognition logic

and CRC These tests should be performed using internal

loopback only so that the SNIC is isolated from interference

from the network These tests also require the capability to

generate CRC in software

The address recognition logic cannot be directly tested The

CRC and FAE bits in the RSR are only set if the address in

the packet matches the address filters If errors are expect-

ed to be set and they are not set the packet has been

rejected on the basis of an address mismatch The following

sequence of packets will test the address recognition logic

The DCR should be set to 40H the TCR should be set to

03H with a software generated CRC

Packet Contents

Results

Test

Address

CRC

RSR

Test A

Matching

Good

01 (Note 1)

Test B

Matching

Bad

02 (Note 2)

Test C

Non-Matching

Bad

Note 1

Status will read 21H if multicast address used

Note 2

Status will read 22H if multicast address used

Note 3

In test A the RSR is set up In test B the address is found to match

since the CRC is flagged as bad Test C proves that the address

recognition logic can distinguish a bad address and does not notify

the RSR of the bad CRC The receiving CRC is proven to work in

test A and test B

Note 4

All values are hex

NETWORK MANAGEMENT FUNCTIONS

Network management capabilities are required for mainte-

nance and planning of a local area network The SNIC sup-

ports the minimum requirement for network management in

hardware the remaining requirements can be met with soft-

ware counts There are three events that software alone

can not track during reception of packets CRC errors

Frame Alignment errors and missed packets

DP83901AV Datasheet(PDF) 35 Page - National Semiconductor (TI)

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