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MOTOROLA Chapter 6. Programming Model 6-21
PRELIMINARYÑSUBJECT TO CHANGE WITHOUT NOTICE
Table 6-22 describes R_HASH Þelds.
6.2.22 Transmit Control Register (X_CNTRL)
The transmit control register (X_CNTRL), shown in Figure 6-22, is written by the user to
conÞgure the transmit block.
Table 6-24 describes X_CNTRL Þelds.
Table 6-23. R_HASH Field Descriptions
Bits Name Description
0Ð7 Ñ Reserved for internal use. When read, these bits are unpredictable.
8Ð20 Ñ Reserved. These bits are read as zeros.
21Ð31 MAX_FRAME_LENGTH User read/write Þeld. Resets to decimal 1518. Length is measured starting at DA
and includes the CRC at the end of the frame. Transmit frames longer than
MAX_FRAME_LENGTH cause an BABT interrupt. Receive frames longer than
MAX_FRAME_LENGTH cause a BABR interrupt and set the LG bit in the
end-of-frame BD. The recommended value to be programmed by the user is 1518
or 1522 (if VLAN tags are supported).
Bits 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Field Ñ
Reset 0000_0000_0000_0000
R/W Read/write
Addr 0xF84
Bits 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Field Ñ FDEN HBC GTS
Reset 0000_0000_0000_0000
R/W Read/write
Addr 0xF86
Figure 6-22. X_CNTRL Register
Table 6-24. X_CNTRL Field Descriptions
Bits Name Description
0Ð28 Ñ Reserved. These bits read as zero.
29 FDEN Full-duplex enable. If set, frames are transmitted independently of carrier sense and collision
inputs. This bit should be modiÞed only when ECNTRL[ETHER_EN] is cleared.
30 HBC Heartbeat control. If HBC = 1 and FDEN = 0, the heartbeat check is performed after transmission
and TxBD[HB] and IEVENT[HBERR] are set, if the collision input does not assert within the
heartbeat window. HBC should be modiÞed only when ECNTRL[ETHER_EN] is cleared.
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