AM79C976 Advanced Micro Devices, AM79C976 Datasheet - Page 135

• Manufacturer Part Number: AM79C976
• Description: PCnet-PRO 10/100 Mbps PCI Ethernet Controller
• Manufacturer: Advanced Micro Devices

CMD3: Command3
Offset 054h
8/01/00
Bit
31
30
29
28
27
26
25
24
23
22
21
20
DIS_WRITE_WAIT
DIS_READ_WAIT
PREFETCH_DIS
DISABLE_MWI
RST_PHY
INIT_MIB
JUMBO
VSIZE
Name
APEP
VAL3
VAL2
RES
Value bit for byte 3. The value of this bit is written to any bits in the CMD3 register that correspond
to bits in the CMD3[30:24] bit map field that are set to 1.
Disable Prefetchability. This bit, which can be loaded from EEPROM, is the inverse of the value
reported in the PREFETCH bit in the PCI Memory-Mapped I/O Base Address Register, which is
read-only.
Setting PREFETCH_DIS to 1 indicates that the memory space claimed by this device can not be
prefetched.
Because of the side effects of reading the Reset Register at offset 14h or 18h (depending on the
state of DWIO (CMD2, bit 28)), locations at offsets less than 20h cannot be prefetched. The
Am79C976 device will disconnect any attempted burst transfer at offsets less than 20h.
If a logical 1 is written to this bit position, the corresponding bit in the register will be loaded with
the contents of the VAL3 bit. If a logical 0 is written to this bit position, the corresponding bit in the
register will not be altered.
Disable Read Wait. When this bit is set to 1, the controller will not insert IRDY wait states in burst
read transfers.
Disable Write Wait. When this bit is set to 1, the controller will not insert IRDY wait states in burst
write transfers.
Disable MWI. When this bit is set to 1, the controller will not generate MWI PCI bus commands.
Reset PHY. When this bit is set to 1, the controller will assert the PHY_RST signal. The signal
will remain asserted for as long as this bit remains set.
Initialize Management Information Base Counters. Setting this bit will cause all of the MIB
counters to be reset to 0. Resetting these counters takes about 55 ERCLK cycles. This bit is
cleared automatically after the counters have all been reset to 0. This bit must not be set to 1 by
the EEPROM logic.
If a logical 1 is written to this bit position, the corresponding bit in the register will be loaded with
the contents of the VAL3 bit. If a logical 0 is written to this bit position, the corresponding bit in the
register will not be altered.
MII Auto-Poll External PHY (APEP) When set to 1, the Am79C976 controller will poll the MII
status register in the external PHY. This feature allows the software driver or upper layers to see
any changes in the status of the external PHY. An interrupt, when enabled, is generated when
the contents of the new status is different from the previous status.
This bit is an alias of BCR32, bit 11.
Value bit for byte 2. The value of this bit is written to any bits in the CMD3 register that correspond
to bits in the CMD3[22:16] bit map field that are set to 1.
Reserved. Written as 1 and read as undefined.
Accept Jumbo Frames. This bit affects the way the MIB counters count long frames. If JUMBO is
0, only frames that are between 64 and 1518 bytes (or 1522 bytes if VLAN is set to 1) are counted
as valid frames. When JUMBO is 1, any frame longer than 63 bytes with a valid FCS field is
counted as a valid frame.
VLAN Frame Size. This bit determines the maximum frame size used for determining when to
increment the XmtPkts1024to1518Octets, XmtExcessiveDefer, RcvPkts1024to1518Octets, and
RcvOversizePkts MIB counters and when to assert the Excessive Deferral Interrupt.
When this bit is set to 1 the maximum frame size is 1522 bytes. When it is cleared to 0, the
maximum frame size is 1518 bytes.
Table 44. CMD3: Command3 Register
P R E L I M I N A R Y
Am79C976
CMD3 is a command-style register. All bits in this reg-
ister are cleared to 0 when the RST pin is asserted, be-
fore the serial EEPROM is read, and after a serial
EEPROM read error.
Description
135