Adapter Internal Registers
A-37
Register Definitions
A.3.14 Maximum Rx Frame Size Register – MaxRx @ 0x44 (DIO) (Bytes 2 +3)
Byte 3
Byte 2
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
Maximum Rx frame size (in units of 8 bits)
This register is used to set the maximum size of received network frames.
Frames larger than this size are not copied and are counted as Rx-overrun er-
ror frames. Setting this parameter prevents an oversize frame from overrun-
ning the buffer space allocated in its receive list and, thereby, causing an
adapter check.
If you chose to pass the four-byte CRC field along with your data by asserting
the RxCRC in the NetConfig register, MaxRx should be large enough to ac-
commodate this additional field. In this case, the CRC is treated exactly as the
data, and failure to accommodate this field could result in frames not being co-
pied and an Rx overrun.
The register holds the maximum frame size in bytes. The value in the register
indicates the maximum frame size that can be accommodated in the host buff-
ers. Frames larger than this value are discarded, except in PEF mode. A value
of 0 in this register (default) is equivalent to a maximum frame size of 64K by-
tes; the adapter attempts to receive all frames.
In pass-errored-frame mode, oversize frames are written to the host. The
frame size reported in the list CSTAT field is the size of the buffer, not the size
of the frame received. The last eight bytes written are corrupt and should be
ignored.
For example:
If a 133-byte frame is received in PEF mode with MaxRx set to 128, then:
-
The CSTAT frame length indicates 128 bytes
-
The first 120 bytes in the buffer contain the first 120 bytes of the frame
-
The contents of the last eight bytes is not important
It should be noted that frames that exceed MaxRx are always counted as Rx
overrun errors, regardless of PEF mode.
Summary of Contents for ThunderLAN TNETE100A
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