NXP Semiconductors LPC24XX UM10237 - Manual (Page 222)

UM10237_4 © NXP B.V. 2009. All rights reserved.
User manual Rev. 04 — 26 August 2009 222 of 792
NXP Semiconductors
UM10237
Chapter 11: LPC24XX Ethernet
3 DELAYED CRC This bit determines the number of bytes, if any, of proprietary header information
that exist on the front of IEEE 802.3 frames. When 1, four bytes of header (ignored
by the CRC function) are added. When 0, there is no proprietary header. 0
4 CRC ENABLE Set this bit to append a CRC to every frame whether padding was required or not.
Must be set if PAD/CRC ENABLE is set. Clear this bit if frames presented to the
MAC contain a CRC. 0
5 PAD / CRC ENABLE Set this bit to have the MAC pad all short frames. Clear this bit if frames presented
to the MAC have a valid length. This bit is used in conjunction with AUTO PAD
ENABLE and VLAN PAD ENABLE. See
Table 11–189 - Pad Operation for details on
the pad function.
0
6 VLAN PAD ENABLE Set this bit to cause the MAC to pad all short frames to 64 bytes and append a valid
CRC. Consult
Table 11–189 - Pad Operation for more information on the various
padding features.
Note: This bit is ignored if PAD / CRC ENABLE is cleared.
0
7 AUTO DETECT PAD
ENABLE Set this bit to cause the MAC to automatically detect the type of frame, either tagged
or un-tagged, by comparing the two octets following the source address with
0x8100 (VLAN Protocol ID) and pad accordingly.
Table 11–189 - Pad Operation
provides a description of the pad function based on the configuration of this register.
Note: This bit is ignored if PAD / CRC ENABLE is cleared.
0
8 PURE PREAMBLE
ENFORCEMENT When enabled (set to ’1’), the MAC will verify the content of the preamble to ensure
it contains 0x55 and is error-free. A packet with an incorrect preamble is discarded.
When disabled, no preamble checking is performed. 0
9 LONG PREAMBLE
ENFORCEMENT When enabled (set to ’1’), the MAC only allows receive packets which contain
preamble fields less than 12 bytes in length. When disabled, the MAC allows any
length preamble as per the Standard. 0
11:10 - Reserved. User software should not write ones to reserved bits. The value read
from a reserved bit is not defined. 0x0
12 NO BACKOFF When enabled (set to ’1’), the MAC will immediately retransmit following a collision
rather than using the Binary Exponential Backoff algorithm as specified in the
Standard. 0
13 BACK PRESSURE /
NO BACKOFF When enabled (set to ’1’), after the MAC incidentally causes a collision during back
pressure, it will immediately retransmit without backoff, reducing the chance of
further collisions and ensuring transmit packets get sent. 0
14 EXCESS DEFER When enabled (set to ’1’) the MAC will defer to carrier indefinitely as per the
Standard. When disabled, the MAC will abort when the excessive deferral limit is
reached. 0
31:15 - Reserved. User software should not write ones to reserved bits. The value read
from a reserved bit is not defined. 0x0
Table 188. MAC Configuration register 2 (MAC2 - address 0xFFE0 0004) bit description
Bit Symbol Function Reset
value
Table 189. Pad operation
Type Auto detect
pad enable
MAC2 [7] VLAN pad
enable
MAC2 [6] Pad/CRC
enable
MAC2 [5] Action
Any x x 0 No pad or CRC check
Any 0 0 1 Pad to 60 bytes, append CRC
Any x 1 1 Pad to 64 bytes, append CRC
Any 1 0 1 If untagged, pad to 60 bytes and append CRC. If VLAN tagged: pad to
64 bytes and append CRC.