AM79C976 Advanced Micro Devices, AM79C976 Datasheet - Page 40

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

Master Bus Interface Unit
The master Bus Interface Unit (BIU) controls the acqui-
sition of the PCI bus and all accesses to the initializa-
tion block, descriptor rings, and the receive and
transmit buffer memory. Table 3 shows the usage of
PCI commands by the Am79C976 controller in master
mode.
The Am79C976 logic will determine when a DMA
transfer should be initiated. The first step in any
Am79C976 bus master transfer is to acquire ownership
of the bus. This task is handled by synchronous logic
within the BIU. Bus ownership is requested with the
REQ signal and ownership is granted by the arbiter
through the GNT signal.
Figure 10 shows the Am79C976 controller bus acquisi-
tion. REQ is asserted and the arbiter returns GNT while
an othe r bu s m as te r i s tran sfer r i ng dat a. Th e
Am79C976 controller waits until the bus is idle (FRAME
and IRDY deasserted) before it starts driving AD[31:0]
and C/BE[3:0] on clock 5. FRAME is asserted at clock
5 indicating a valid address and command on AD[31:0]
40
DEVSEL
FRAME
PERR
TRDY
C/BE
IRDY
CLK
PAR
AD
1
2
ADDR
CMD
3
P R E L I M I N A R Y
PAR
4
Am79C976
5
DATA
BE
and C/BE[3:0]. REQ is deasserted at the same time
that FRAME is asserted. The Am79C976 controller
does not use address stepping which is reflected by
ADSTEP (bit 7) in the PCI Command register being
hardwired to 0.
There are four primary types of DMA transfers. The
Am79C976 controller uses non-burst as well as burst
cycles for read and write access to the main memory.
The Am79C976 controller uses non-burst cycles to ac-
cess descriptors when SWSTYLE (BCR20, bits 7-0) is
0 or 2. All Am79C976 controller non-burst read ac-
cesses are of the PCI command type Memory Read
(type 6). Note that during a non-burst read operation,
all byte lanes will always be active. The Am79C976
controller will internally discard unneeded bytes.
6
PAR
7
8
9
10
22929B11
8/01/00