am79c972b Advanced Micro Devices, am79c972b Datasheet - Page 77

• Manufacturer Part Number: am79c972b
• Description: Pcnet?-fast+ Enhanced 10/100 Mbps Pci Ethernet Controller With Onnow Support
• Manufacturer: Advanced Micro Devices

The EROMCS is driven low for the value ROMTMG +
1. Figure 43 assumes that ROMTMG is set to nine.
EBD[7:0] is sampled with the next rising edge of CLK
ten clock cycles after EBUA_EBA[7:0] was driven with
a new address value. This PCI slave access to the
Flash/EPROM will result in a retry for the very first ac-
cess. Subsequent accesses may give a retry or not, de-
pending on whether or not the data is present and valid.
The access time is dependent on the ROMTMG bits
(BCR18, bits 15-12) and the Flash/EPROM. This ac-
cess mechanism differs from the Expansion ROM ac-
cess mechanism since only one byte is read in this
manner, instead of the 4 bytes in an Expansion ROM
access. The PCI bus will not be held during accesses
through the Expansion Bus Data Port. If the LAAINC
EBUA_EBA [7:0]
Latched Address
EBDA [15:8]
EROMCS
AS_EBO
DEVSEL
FRAME
TRDY
IRDY
EBD
CLK
EBUA_EBA[7:0]
EBDA[15:8]
AS_EBOE
EROMCS
EBD[7:0]
5
Figure 43. Flash Read from Expansion Bus Data Port
CLK
Figure 42. Expansion ROM Bus Read Sequence
10
A[19:16]
15
EBUA[19:16]
1
20
A[7:2], 0, 0
2
Am79C972
25
3
4
30
(BCR29, bit 15) is set, the EBADDRL address will be
incremented and a continuous series of reads from the
Expansion Data Port (EBDATA, BCR30) is possible.
The address incrementor will roll over without warning
and without incrementing the upper address EBAD-
DRU.
The Flash write is almost the same procedure as the
read access, except that the Am79C972 controller will
not drive AS_EBOE low. The EROMCS and EBWE are
driven low for the value ROMTMG again. The write to
the FLASH port is a posted write and will not result in a
retry to the PCI unless the host tries to write a new
value before the previous write is complete, then the
host will experience a retry. See Figure 44.
5
A[7:2], 0, 1
6
35
7
EBDA[15:8]
EBA[7:0]
8
40
A[7:2], 1, 0
9
45
10
11
50
12 13
55
A[7:2], 1, 1
60
21485C-45
21485C-46
66
77