
8-6
MPC8240 Integrated Processor User's Manual
MOTOROLA
PCI Bus Arbitration
8.2.2 PCI Bus Parking
When no device is using or requesting the bus, the PCI arbiter grants the bus to a selected
device. This is known as parking the bus on the selected device. The selected device is
required to drive the AD[31D0], C/BE[0D3] and PAR signals to a stable value, preventing
these signals from oating.
The parking mode control parameter (bits 14D13) in the PCI arbitration register determines
which device the arbiter selects for parking the PCI bus. If parking mode control bits are
0b00 (or if the bus is not idle), then the bus is parked on the last master to use the bus. If the
bus is idle, and the parking mode control bits are b10, then the bus is parked on the
MPC8240; if the control bits are b01, then the bus is parked on device 0 (that is, the device
connected to GNT0).
8.2.3 Broken Master Lock-Out
The PCI bus arbiter on MPC8240 has a feature that allows it to lock out any masters that
are broken or ill-behaved. The broken master feature is controlled by programming bit 12
of the PCI arbitration control register (0b0 = enabled, 0b1 = disabled).
When the broken master feature is enabled, a granted device that does not assert FRAME
within 16 PCI clock cycles after the bus is idle will have its grant removed and subsequent
requests will be ignored until its REQ is negated for at least one clock cycle. This prevents
ill-behaved masters from monopolizing the bus. When the broken master feature is
disabled, a device that requests the bus and receives a grant never loses its grant until and
unless it begins a transaction or negates its REQ signal. Disabling the broken master feature
is not recommended.
8.2.4 Bus Lock Mode
Under certain conditions, it may be useful to have the bus remain granted to the owner of a
resource lock, see Section 8.5, òExclusive Access.ó The bus lock mode parameter (bit 11)
in the PCI arbitration control register alters the behavior of the PCI arbiter for locked bus
operations.
If the bus lock mode bit is set, then for as long as LOCK is asserted, the bus will be granted
to the locking master. Note that his feature prevents any non-exclusive accesses while
LOCK is asserted which can reduce throughput in a system. Its use should be carefully
considered.
8.2.5 Power-Saving Modes and the PCI Arbiter
In the sleep power-saving mode, the clock driving PCI_SYNC_IN can be disabled. If the
clock is disabled, the arbitration logic is not able to perform its function. System
programmers must park the bus with a device that can sustain the AD[31D0], C/BE[0D3]
and PAR signals prior to disabling the MPC8240s clock. If the bus is parked on the
MPC8240 when its clocks are stopped, then the MPC8240 will sustain the AD[31D0], C/