
MOTOROLA
Chapter 3. Signal Descriptions and Clocking
3-11
Detailed Signal Descriptions
State Meaning
AssertedThe MPC8240 has granted control of the PCI bus to a
requesting master, using the priority scheme described in
Section 8.2, òPCI Bus Arbitrationó. The MPC8240 will assert only
one GNT signal during any clock cycle.
NegatedIndicates that the MPC8240 has not granted control of the
PCI bus and external devices may not initiate a PCI transaction.
3.2.1.5.2 PCI Bus Grant (GNT[4D0])Internal Arbiter Disabled
The MPC8240 PCI arbiter is disabled by a high value on the reset conTguration pin MAA2
or by the clearing of bit 15 of the PCI arbitration control register. In this case, the GNT0
becomes the PCI bus request output for the MPC8240, and it is asserted when the
MPC8240 needs to run a PCI transaction. If the REQ0 input signal is asserted prior to the
need to run a PCI transaction, then the GNT0 signal will not assert (the bus is parked) when
a PCI transaction is to be run. Following is the state meaning for the GNT[4D0] input
signals when the internal arbiter is disabled.
State Meaning
AssertedThe MPC8240 asserts the GNT0 signal as the PCI bus
request output signal. GNT[4D1] signals do not assert in this case.
NegatedThe GNT[4D1] signals are driven high (negated) in this
mode. GNT0 is negated when the MPC8240 is not requesting control
of the PCI bus or the bus is parked on the MPC8240.
3.2.1.6 PCI Bus Request (REQ[4D0])Input
The PCI bus request (REQ[4D0]) signals are inputs on the MPC8240, and they have a
different meaning depending on whether the MPC8240 PCI arbiter is enabled or disabled.
The PCI REQ signals are point-to-point, and every master has its own REQ signal.
3.2.1.6.1 PCI Bus Request (REQ[4D0])Internal Arbiter Enabled
The MPC8240 PCI arbiter is enabled by a low value on the reset conTguration pin MAA2
or by the setting of bit 15 of the PCI arbitration control register. In this case, the REQ[4D0]
signals are used in conjunction with the GNT[4D0] signals as the arbiter for up to Tve PCI
masters. Following is the state meaning for the REQ[4D0] input signals in this case.
State Meaning
AssertedExternal devices are requesting control of the PCI bus.
The MPC8240 acts on the requests as described in Section 8.2, òPCI
Bus Arbitration.ó
NegatedIndicates that no external devices are requesting the use of
the PCI bus.
3.2.1.6.2 PCI Bus Request (REQ[4D0])Internal Arbiter Disabled
The MPC8240 PCI arbiter is disabled by a high value on the reset conTguration pin MAA2
or by the clearing of bit 15 of the PCI arbitration control register. In this case, the REQ0
becomes the PCI bus grant input for the MPC8240, and it is asserted when the external
arbiter is granting the use of the PCI bus to the MPC8240. Note that if the REQ0 input
signal is asserted prior to the need to run a PCI transaction, then the MPC8240 GNT0 signal
will not assert (the bus is parked) when a PCI transaction is to be run.