6. Arbitration
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PowerSpan II User Manual
80A1010_MA001_09
Integrated Device Technology
www.idt.com
The current data bus grant is negated when the requesting master has qualified the grant.
Some host processors (for example, the PowerQUICC II) and other processor bus agents require the
system signal DBB_ to qualify data bus grants. The PowerSpan II PB Master does not require DBB_ to
qualify data bus grants.
6.3.2.1
Qualifying Data Bus Grant
Some processors, specifically the PowerPC 7400, must have the data bus grant qualified by the arbiter
before it is issued to the master. PowerSpan II, by default, does not qualify the data bus grant by the
PowerSpan II PB Arbiter and requires that the requesting master qualify bus grants before beginning an
data tenure.
The PowerSpan II PB Arbiter can be programmed to qualify data bus grants before issuing them by
setting the 7400_MODE bit in the
“Reset Control and Status Register” on page 324
. When the
7400_MODE bit is set to 1, the PB arbiter qualifies data bus grants before issuing them to a processor
bus master. When the 7400_MODE bit is disabled (default setting) the PB arbiter issues a data bus
grant to the processor bus master and expects that the processor bus master the at receives the grant
qualifies the grant.
The 7400_MODE bit is a power-up option.
6.3.3
Address Only Cycles
The arbiter supports address only cycles. If Transfer Type (PB_TT[3]) is sampled low during PB_TS_,
the arbiter does not grant the data bus. The use of PB_TT[3] as a data bus request means that the
PowerSpan II PB arbiter does not support the processor bus instructions eciwx and ecowx.
6.3.4
PowerSpan II Arbiter and System Boot
System boot from the PCI bus can be selected by configuring the processor bus arbiter at power-up to
ignore all external requests on PB_BR[3:1]_. This allows an external PCI master, with the PowerSpan
II PB Master, to configure the host processor memory controller and load boot code before enabling
recognition of requests on PB_BR[3:1]_.
Alternatively, at power-up the processor bus arbiter can be configured to recognize requests on
PB_BR[1]_ and ignore requests on PB_BR[3:2]_. In this case the processor connected to PB_BR[1]_
can enable recognition of requests from other masters when its system configuration tasks are
complete.
The PowerSpan II processor bus arbiter controls system boot with the M3_EN, M2_EN and M1_EN
bits in the
“PCI-1 Bus Arbiter Control Register” on page 284
(PB_ARB_CTRL), as well as the
power-up option PWRUP_BOOT.
Requesting masters are required to qualify bus grants before beginning an data tenure.