am79c972b Advanced Micro Devices, am79c972b Datasheet - Page 37

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

Figure 15 shows a typical burst write access. The
Am79C972 controller arbitrates for the bus, is granted
access, and writes four 32-bit words (DWords) to the
system memory and then releases the bus. In this ex-
ample, the memory system extends the data phase of
the first access by one wait state. The following three
data phases take one clock cycle each, which is deter-
mined by the timing of TRDY. The example assumes
that EXTREQ (BCR18, bit 8) is set to 1, therefore, REQ
is not deasserted until the next to last data phase is fin-
ished.
Target Initiated Termination
When the Am79C972 controller is a bus master, the cy-
cles it produces on the PCI bus may be terminated by
the target in one of three different ways: disconnect
DEVSEL
FRAME
TRDY
C/BE
IRDY
REQ
GNT
PAR
CLK
AD
1
DEVSEL is sampled
Figure 15. Burst Write Transfer (EXTREQ = 1)
2
ADDR
0111
3
PAR
Am79C972
4
DATA
5
with data transfer, disconnect without data transfer, and
target abort.
Disconnect With Data Transfer
Figure 16 shows a disconnection in which one last data
transfer occurs after the target asserted STOP. STOP
is asserted on clock 4 to start the termination se-
quence. Data is still transferred during this cycle, since
both IRDY and TRDY are asserted. The Am79C972
controller terminates the current transfer with the deas-
sertion of FRAME on clock 5 and of IRDY one clock
later. It finally releases the bus on clock 7. The
Am79C972 controller will again request the bus after
two clock cycles, if it wants to transfer more data. The
starting address of the new transfer will be the address
of the next non-transferred data.
PAR
DATA
BE
6
DATA
PAR
7
DATA
PAR
8
PAR
9
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