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IDT Ethernet Interfaces
Ethernet Medium Access Controller (MAC)
79RC32438 User Reference Manual
11 - 25
November 4, 2002
Notes
Read Value:
Previous value written
Write Effect:
Modify value
APE
Description:
Auto Pad Enable.
When this bit is set to 1 and padding is enabled, the MAC automatically
detects the frame type, either tagged or untagged, by comparing the two bytes following the
source address with 0x1800 (VLAN protocol ID) and pads accordingly. untagged frames are
padded to 60 bytes while tagged frames are padded to 64 bytes. When this bit is set to 0, the
Auto Padding function is disabled. Refer to Table 11.4.
Initial Value:
0x0
Read Value:
Previous value written
Write Effect:
Modify value
PPE
Description:
Pure Preamble Enforcement.
When this bit is set to 1, the MAC will verify the content of the
preamble to ensure it contains 0x55 and is error-free. A frame with an error in the preamble is
discarded. When this bit is cleared, no preamble checking is performed.
Initial Value:
0x0
Read Value:
Previous value written
Write Effect:
Modify value
LPE
Description:
Long Preamble Enforcement.
When this bit is set to 1, the MAC only allows receive frames
which contain preamble fields less than 12 bytes in length. When this bit is cleared to 0, the MAC
allows any length preamble.
Initial Value:
0x0
Read Value:
Previous value written
Write Effect:
Modify value
NB
Description:
No Backoff.
When this bit is set to 1, the MAC will immediately retransmit following a collision
rather than using the Binary Exponential Backoff algorithm. When this bit is set to 0, the MAC will
use the Binary Exponential Backoff algorithm.
Initial Value:
0x0
Read Value:
Previous value written
Write Effect:
Modify value
BP
Description:
Back Pressure / No Backoff.
When this bit is set to 1, after incidentally causing a collision dur-
ing back pressure, the MAC will immediately retransmit without backoff. This reduces the chance
of further collisions and ensures that transmit frames get sent.
Initial Value:
0x0
Read Value:
Previous value written
Write Effect:
Modify value