User’s Manual
C166S V1 SubSystem
The External Bus Interface
User’s Manual
8-31
V 1.6, 2001-08
Due to timing constraints, address and write data of an XBUS cycle are reflected on the
external bus interface (see
Table 8-7
below). The “address” mentioned above includes
Port 4, BHE, and ALE (which also pulses for an XBUS cycle). The external CS signals
are driven inactive (high) because the EBC switches to an internal XCS signal.
The external control signals (RD and WR or WRL/WRH if enabled) remain inactive
(high).
8.6
External Bus Arbitration
In high-performance systems, it may be efficient to share external resources such as
memory banks or peripheral devices among more than one controller. The C166S
supports this approach with the possibility to arbitrate the access to its external bus, i.e.,
to the external devices.
This bus arbitration allows an external master to request the C166S’s bus via the HOLD
input. The C166S acknowledges this request via the HLDA output and will float its bus
lines in this case. The CS outputs provide internal pull-up devices. The new master may
now access the peripheral devices or memory banks via the same interface lines as the
C166S. During this time, the C166S can keep on executing, as long as it does not need
Table 8-7
Status Of The External Bus Interface During EBC Idle State
Pins
Internal accesses only
XBUS accesses
PORT0
Tristated (floating)
Tristated (floating) for read
accesses
XBUS write data for write accesses
PORT1
Last used external address
(if used for the bus interface)
Last used XBUS address
(if used for the bus interface)
Port 4
Last used external segment address
(on selected pins)
Last used XBUS segment address
(on selected pins)
Active external CS signal
corresponding to last used address
Inactive (high) for selected CS
signals
BHE
Level corresponding to last external
access
Level corresponding to last XBUS
access
ALE
Inactive (low)
Pulses as defined for X-Peripheral
RD
Inactive (high)
Inactive (high)
WR/WRL
Inactive (high)
Inactive (high)
WRH
Inactive (high)
Inactive (high)
Содержание C166S V1 SubSystem
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Страница 452: ...User s Manual C166S V1 SubSystem General Purpose Timer Unit User s Manual 12 44 V 1 6 2001 08...
Страница 454: ...User s Manual C166S V1 SubSystem Instruction Index User s Manual 13 2 V 1 6 2001 08...
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