Disconnect When Busy
The Am79C978 controller cannot service any slave ac-
cess while it is reading the contents of the EEPROM.
Simultaneous access is not allowed in order to avoid
conflicts, since the EEPROM is used to initialize some
of the PCI configuration space locations and most of
the BCRs and CSR116. The EEPROM read operation
will always happen automatically after the deassertion
of the RST pin. In addition, the host can start the read
operation by setting the PREAD bit (BCR19, bit 14).
While the EEPROM read is on-going, the Am79C978
controller will disconnect any slave access where it is
the target by asserting STOP together with DEVSEL,
while driving TRDY high. STOP will stay asserted until
the end of the cycle.
CLK
1
2
3
4
5
FRAME
DATA
ADDR
CMD
AD
C/BE
PAR
BE
PAR
PAR
IRDY
TRDY
A second situation where the Am79C978 controller will
generate a PCI disconnect/retry cycle is when the host
tries to access any of the I/O resources right after hav-
ing read the Reset register. Since the access gener-
ates an internal reset pulse of about 1 ms in length, all
further slave accesses will be deferred until the internal
reset operation is completed. See Figure 8.
DEVSEL
STOP
Disconnect Of Burst Transfer
The Am79C978 controller does not support burst ac-
cess to the configuration space, the I/O resources, or
to the Expansion Bus. The host indicates a burst trans-
action by keeping FRAME asserted during the data
phase. When the Am79C978 controller sees FRAME
and IRDY asserted in the clock cycle before it wants to
assert TRDY, it also asserts STOP at the same time.
The transfer of the first data phase is still successful,
since IRDY and TRDY are both asserted. See Figure 9.
22206B-11
Figure 8. Disconnect of Slave Cycle When Busy
CLK
1
2
3
4
5
FRAME
If the host is not yet ready when the Am79C978 control-
ler asserts TRDY, the device will wait for the host to as-
sert IRDY. When the host asserts IRDY and FRAME is
still asserted, the Am79C978 controller will finish the
first data phase by deasserting TRDY one clock later.
At the same time, it will assert STOP to signal a discon-
nect to the host. STOP will stay asserted until the host
removes FRAME. See Figure 10.
AD
1st DATA
DATA
C/BE
PAR
BE
BE
PAR
PAR
IRDY
TRDY
DEVSEL
STOP
22206B-12
Figure 9. Disconnect of Slave Burst Transfer - No
Host Wait States
40
Am79C978
AMD [ AMD ]
