Datasheet

ENC28J60
DS39662E-page 44 .  2006-2012 Microchip Technology Inc.
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 follow the logic shown in
Example 7-1.
EXAMPLE 7-1: RANDOM ACCESS ADDRESS CALCULATION
Bit Field Description
31 Zero 0
30 Receive VLAN Type Detected Current frame was recognized as a VLAN tagged frame.
29 Receive Unknown Opcode Current frame was recognized as a control frame but it contained an
unknown opcode.
28 Receive Pause Control Frame Current frame was recognized as a control frame containing a valid pause
frame opcode and a valid destination address.
27 Receive Control Frame Current frame was recognized as a control frame for having a valid
type/length designating it as a control frame.
26 Dribble Nibble Indicates that after the end of this packet, an additional 1 to 7 bits were
received. The extra bits were thrown away.
25 Receive Broadcast Packet Indicates packet received had a valid Broadcast address.
24 Receive Multicast Packet Indicates packet received had a valid Multicast address.
23 Received Ok Indicates that at the packet had a valid CRC and no symbol errors.
22 Length Out of Range Indicates that frame type/length field was larger than 1500 bytes (type field).
21 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.
20 CRC Error Indicates that frame CRC field value does not match the CRC calculated
by the MAC.
19 Reserved
18 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.
17 Reserved
16 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