GT-96100A Advanced Communication Controller
Revision 1.0
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In relation to
Figure 34, the following points should be noted:
The two request signals (High_req, Low_req) are generated by “ANDing” each of the request lines with
its respective priority attribute, and ORing the results. For example:
High_req = (req_[0] AND (req_prio[0]==high)) OR...
(req_[1] AND (req_prio[1]==high)) OR...
There is a counter associated with the priority scheme - High_cnt. The counter is used to assign differ-
ent weights to each priority level. This is a count down counter that decrements each time a high priority
request (High_req) is granted. When High_cnt expires, a slot is opened for low priority requests, and the
counter is set to its preset value.
Each time a low priority request (Low_req) is granted, High_cnt counter is preset.
10.3
Arbitration Parking
The PCI arbiter is designed to perform a default parking on the last agent granted. In order to overcome problems
that happen with some PCI devices that do not handle parking properly, there is an option to disable parking on a
per PCI master basis. This is done via the PCI_Arbiter_Configuration register PD[6:0] bits.
NOTE: In addition to disabling parking to avoid issues with some problematic devices, the user must also dis-
able parking on any unused request/grant pair. This is required to avoid possible parking on non existent
PCI masters. For example, if only 3 external agents are connected to PCI_0 arbiter (using REQ0/GNT0,
REQ1/GNT1, REQ2/GNT2 pins), then PD[6:4] should be set to 1.
10.4
PCI Arbiter Configuration Register
Table 270: PCI_0 Arbiter Configuration Register, Offset: 0x101AE0
Bits
Field
Name
Fu nction
Initial
Value
0
GCen
Gap Cycle enable
When this bit is set to 1, the PCI arbiter forces grant to be asserted only
after the PCI bus is idle. This guarantees two turn around cycles.
0
1
BDen
Broken Detection enable
Setting this bit to 1 enables the detection of broken master. A master is said
to be broken if it fails to respond to grant assertion within a window speci-
fied in BV (see BV description above).
0