AM79C985KCW AMD [Advanced Micro Devices], AM79C985KCW Datasheet - Page 20

• Manufacturer Part Number: AM79C985KCW
• Manufacturer: AMD [Advanced Micro Devices]

Expansion Bus Interface
The eIMR+ device expansion bus allows multiple
eIMR+ devices to be interconnected.
The expansion bus supports two modes of operation:
internal arbitration mode and IMR+ mode. The internal
arbitration mode uses a modified daisy-chain scheme
to eliminate the need for any external arbitration cir-
cuitry. The IMR+ mode maintains the full functionality of
the IMR+ (Am79C981) expansion bus and benefits
from minimum delays. In this mode, the eIMR+ device
requires external circuitry to handle arbitration for con-
trol of the bus.
The eIMR+ arbitration mode is determined at reset.
This occurs on the trailing edge of RST according to the
state of SELI
The eIMR+ device can be connected to a HIMIB device,
as described in the eIMR+/HIMIB Interconnection sec-
tion. The connection to a HIMIB device is not dependent
on the mode of the expansion bus. In other words, the
eIMR+ device can be connected to a HIMIB device
whether the expansion bus is in internal-arbitration
mode or IMR+ mode.
Internal Arbitration Mode
The internal arbitration mode uses a daisy-chain (cas-
cade) configuration. SELI
SELO is the arbitration output. SELO goes LOW when
there is activity on one or more of the eIMR+ ports, or
a SELI input is LOW. The SEL lines are connected as
shown in Figure 5. This technique allows activity indica-
tion to propagate down the chain to the end device. All
unused SELI inputs must be tied to VDD.
ACK and COL are global activity I/O pins. When the
eIMR+ device senses activity, it drives ACK LOW.
An eIMR+ device drives COL LOW when it senses
more than one device is active; that is, if the device has
an active port AND a SELI input is LOW, OR both SELI
inputs are LOW.
20
Figure 4. Expansion Bus Mode Selection
SELI_0
RST
SELI_1
0-1
X
X
, as illustrated in Figure 4.
SELI_0
Mode Selection
0-1
1
0
are arbitration inputs and
Internal
IMR+
Arbitration
Mode
P R E L I M I N A R Y
20651B-9
Am79C985
In Boolean notation, the formula for COL is as follows:
where
ACK and COL are mutually exclusive. If the eIMR+ de-
vice driving ACK senses COL LOW, the device will
deassert ACK.
DAT and JAM are synchronized to CLK. DAT is the rep-
etition of data from any connected port (either TP or
AUI port) encoded in NRZ format. JAM is an internal
collision indicator. If JAM is HIGH, the active eIMR+ de-
vice has detected an internal collision across one or
more of its ports. When this occurs, the DAT signal dis-
tinguishes between single-port collisions and multiport
collisions. DAT = 1 indicates a single port collision;
DAT = 0 indicates a multiport collision.
The drive capabilities of the I/O signals on the expan-
sion bus (DAT, JAM, ACK, and COL) are sufficient to
allow seven eIMR+ devices to be connected together
without the use of external transceivers or buffers.
The maximum number of eIMR+ devices that can be
daisy chained is limited by the propagation delay of the
eIMR+ devices. In practice, the depth of the cascade is
limited to three eIMR+ devices, thus allowing a maxi-
mum of seven eIMR+ devices connected together via
this expansion bus as shown in Figure 5.
The active device will not drive the data line, DAT,
until one bit time (100 ns) after SELO goes LOW. This
is to avoid a situation where two devices drive DAT
simultaneously.
IMR+ Mode
In IMR+ mode, the expansion bus requires an external
arbiter. The arbiter allows only one eIMR+ device to
control the expansion bus. If more than one device at-
tempts to take control, the arbiter terminates all access
and signals a collision condition.
In IMR+ mode, DAT and JAM retain the same function-
ality as in internal arbitration mode, but ACK and COL
are inputs to the eIMR+ device, driven by the external
arbiter. The arbiter should drive ACK LOW when ex-
actly one eIMR+ device is active. It should drive COL
when more than one eIMR+ device is active. SELO is
an output from the eIMR+ device. It indicates that the
eIMR+ device has an active port and is requesting ac-
cess to the bus. When ACK is HIGH, DAT and JAM are
in the high-impedance state. DAT and JAM go active
when ACK goes LOW. Refer to the Systems Applica-
tions section (Fig.14) for the configuration of IMR+
mode of operation.
Note: The IMR+ mode is recommended when arbitrating
between multiple boards.
COL = (Active port & (SELI
& represents the Boolean AND operation
+ represents the Boolean OR operation
(SELI
1
& SELI
0
)
1
+ SELI
0
))+