User’s Manual
C166S V1 SubSystem
The External Bus Interface
User’s Manual
8-33
V 1.6, 2001-08
C166S that has surrendered its bus interface to regain control of it in case it must access
the shared external resources. This glue logic is required if the other bus master does
not automatically remove its hold request after having used the shared resources.
When two C166Ss are to be connected in this way, the external glue logic can be left
out. One of the controllers must be operated in its Master Mode (default after reset,
DP6.7=0) while the other one must be operated in its Slave Mode (selected with
DP6.7=1).
In Slave Mode, the C166S inverts the direction of its HLDA pin and uses it as an input,
while the master’s HLDA pin remains an output. This approach does not require any
additional glue logic for the bus arbitration (see
Figure 8-11
below).
Figure 8-11
Sharing External Resources Using Slave Mode
When the bus arbitration is enabled (HLDEN=1), the three corresponding pins are
controlled automatically by the EBC. Normally, the respective port direction register bits
retain their reset value, which is 0. This selects Master Mode. Slave Mode is enabled by
intentionally switching pin BREQ to output (DP6.7=1).
Entering the Hold State
Access to the C166S’s external bus is requested by driving its HOLD input low. After
synchronizing this signal, the C166S will complete a current external bus cycle (if any is
active), release the external bus, and grant access to it by driving the HLDA output low.
During hold state, the C166S treats the external bus interface as follows:
• Address and data bus(es) float to tristate
• ALE is pulled low by an internal pull-down device
• Command lines are pulled high by internal pull-up devices (RD, WR/WRL, BHE/WRH)
• CSx outputs are pulled high (push/pull mode) or float to tri-state (open-drain mode)
C1
66
S
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Ma
ste
r M
o
d
e
BREQ
HLDA
HOLD
C1
66
S
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S
lav
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BREQ
HLDA
HOLD
Содержание C166S V1 SubSystem
Страница 10: ...User s Manual C166S V1 SubSystem User s Manual I 6 V 1 6 2001 08...
Страница 16: ...User s Manual C166S V1 SubSystem Introduction User s Manual 1 6 V 1 6 2001 08...
Страница 126: ...User s Manual C166S V1 SubSystem Central Processing Unit User s Manual 3 96 V 1 6 2001 08...
Страница 178: ...User s Manual C166S V1 SubSystem Memory Organization User s Manual 4 52 V 1 6 2001 08...
Страница 206: ...User s Manual C166S V1 SubSystem Instruction Set User s Manual 5 28 V 1 6 2001 08...
Страница 306: ...User s Manual C166S V1 SubSystem Detailed Instruction Set User s Manual 6 100 V 1 6 2001 08...
Страница 362: ...User s Manual C166S V1 SubSystem The External Bus Interface User s Manual 8 40 V 1 6 2001 08...
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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