AM79C984AKCW AMD [Advanced Micro Devices], AM79C984AKCW Datasheet - Page 20

• Manufacturer Part Number: AM79C984AKCW
• 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
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 in-
puts are LOW. In Boolean notation, the formula for COL
is:
COL = (Active port & (SELI
where
20
SELI_0
RST
Figure 4. Expansion Bus Mode Selection
SELI_1
X
X
& represents the Boolean AND operation
+ represents the Boolean OR operation
0-1
(SELI
, as illustrated in Figure 4.
1
& SELI
SELI_0
Mode Selection
1
0
0-1
1
0
)
+ SELI
are arbitration inputs and
0
Internal
IMR+
Arbitration
))+
Mode
P R E L I M I N A R Y
20650B-9
Am79C984A
ACK and COL are mutually exclusive. If an eIMR driv-
ing 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 con-
trol 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 ar-
biter. The arbiter should drive ACK LOW when exactly
one eIMR device is active. It should drive COL when
more than one eIMR device is active. SELO is an out-
put from the eIMR device. It indicates that the eIMR de-
vice has an active port and is requesting access 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 Applications section
(Figure 13) for the configuration of IMR+ mode of
operation.
Note: The IMR+ mode is recommended when arbitrating
between multiple boards.
.