ENC28J60-CSLASHSP datasheet(46/96 Pages) MICROCHIP

46 / 96 page

ENC28J60

DS39662B-page 44

Preliminary

TABLE 7-3:

RECEIVE STATUS VECTORS

7.2.3

READING RECEIVED PACKETS

To process the packet, the host controller will normally

use the RBM SPI command and start reading from the

beginning of the next Packet Pointer. The host controller

will save the next Packet Pointer, any necessary bytes

from the receive status vector and then proceed to read

the actual packet contents. If ECON2.AUTOINC is set, it

will be able to sequentially read the entire packet without

ever modifying the ERDPT registers. The Read Pointer

would automatically wrap at the end of the circular

receive buffer to the beginning.

In the event that the application needed to do random

access to the packet, it would be necessary to manu-

ally calculate the proper ERDPT, taking care to not

exceed the end of the receive buffer if the packet spans

the ERXND-to-ERXST buffer boundary. In other words,

given the packet start address and a desired offset, the

application

should

the

logic

shown

Example 7-1.

EXAMPLE 7-1:

RANDOM ACCESS ADDRESS CALCULATION

Bit

Field

Description

Zero

Receive VLAN Type Detected

Current frame was recognized as a VLAN tagged frame.

Receive Unknown Opcode

Current frame was recognized as a control frame but it contained an

unknown opcode.

Receive Pause Control Frame

Current frame was recognized as a control frame containing a valid pause

frame opcode and a valid destination address.

Receive Control Frame

Current frame was recognized as a control frame for having a valid

type/length designating it as a control frame.

Dribble Nibble

Indicates that after the end of this packet, an additional 1 to 7 bits were

received. The extra bits were thrown away.

Receive Broadcast Packet

Indicates packet received had a valid broadcast address.

Receive Multicast Packet

Indicates packet received had a valid multicast address.

Received Ok

Indicates that at the packet had a valid CRC and no symbol errors.

Length Out of Range

Indicates that frame type/length field was larger than 1500 bytes (type field).

Length Check Error

Indicates that frame length field value in the packet does not match the

actual data byte length and specifies a valid length.

CRC Error

Indicates that frame CRC field value does not match the CRC calculated

by the MAC.

Reserved

Carrier Event Previously Seen

Indicates that at some time since the last receive, a carrier event was

detected. The carrier event is not associated with this packet. A carrier

event is activity on the receive channel that does not result in a packet

receive attempt being made.

Reserved

Long Event/Drop Event

Indicates a packet over 50,000 bit times occurred or that a packet was

dropped since the last receive.

15-0

Received Byte Count

Indicates length of the received frame. This includes the destination

address, source address, type/length, data, padding and CRC fields. This

field is stored in little-endian format.

if Packet Start Address + Offset > ERXND, then

ERDPT = Packet Start Address + Offset – (ERXND – ERXST + 1)

else

ERDPT = Packet Start Address + Offset

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