User Manual
BCM1250/BCM1125/BCM1125H
10/21/02
B r o a d c o m C o r p o r a t i o n
Document
1250_1125-UM100CB-R
Section 9: Ethernet MACs Page
301
MAC R
EGISTERS
Note
All of the MAC registers need to be initialized by software before it will function. The BCM1125/H has
MACs 0 and 1. Accesses made to the address range allocated to MAC 2 may cause all MACs to exhibit
UNPREDICTABLE behavior.
Table 176: MAC Configuration Registers
mac_cfg_0 -
00_1006_4100
mac_cfg_1 -
00_1006_5100
mac_cfg_2 -
00_1006_6100
This register is used in both Ethernet and Packet FIFO modes
Bits
Name
Default
Description
0
ss_tmode
1'b0
This bit must always be zero for normal operation (Broadcom Use Only test bit).
1
tx_hold_sop_en
1'b0
When this bit is set the interface will retain the first few bytes of each transmitted
packet in the transmit FIFO. This enables the interface to automatically retry any
packet that encounters an error during the initial part of its transmission. The
tx_rl_thrsh parameter (in the mac_threshold register) sets the number of 64 bit
FIFO entries that are retained, once this number of entries have been
successfully transmitted the FIFO is unlocked and no data is retained.
For example if tx_hold_sop_en is set and tx_rl_thrsh is set to 8, then if the MAC
detects a collision before 8 entries (64 bytes) are read from the transmit FIFO,
the FIFO can be restored the start of the packet and the packet can be
automatically retried.
Automatic retry can be triggered by any error that happens (collision, underrun
error, excessive collision error, or late collision) while the FIFO is in locked state.
(i.e. less than the number of entries set by tx_rl_thrsh have been read from the
transmit FIFO for a given packet.). The retry_en, ret_drprep_en and ret_ufl_en
bits enable automatic retry selectively for each of the error types.
If the tx_hold_sop_en bit is clear or the number of entries set by tx_rl_thrsh have
been read from the FIFO then the FIFO will be unlocked and automatic retry is
not possible. In this case the packet with the error will be dropped.
Used in Ethernet and Packet FIFO modes.
2
retry_en
1'b0
If this bit is set then automatic retry is enabled when collisions are detected (but
not late or excessive collisions). The tx_hold_sop_en bit (bit [1]) must be set to
allow automatic retry, see the description above for more details.
Used in Ethernet and Packet FIFO modes.
3
ret_drpreq_en
1'b0
If this bit is set then automatic retry is enabled when late collisions or excessive
collisions are detected. The tx_hold_sop_en bit (bit [1]) must be set to allow
automatic retry, see the description above for more details.
Used in Ethernet and Packet FIFO modes.
4
ret_ufl_en
1'b0
If this bit is set then automatic retry is enabled when a transmit FIFO underflow
is detected. The tx_hold_sop_en bit (bit [1]) must be set to allow automatic retry,
see the description above for more details.
Used in Ethernet and Packet FIFO modes.
5
burst_en
1'b0
This bit should be set to enable burst operation when the MAC is running in 1000
Mbp/s mode. Burst mode enables transmission of multiple packets without
releasing the network between them.
Used in Ethernet mode only.