3. Processor Bus Interface
100
PowerSpan II User Manual
80A1010_MA001_09
Integrated Device Technology
www.idt.com
3.4
PB Master Interface
The PowerSpan II becomes active as PB Master when:
•
PowerSpan II is accessed as a PCI target
•
one of the PowerSpan II DMA engines is processing a transfer
The operation of the PB Master is described by dividing a transaction into three different phases:
•
Address Phase: This section discusses the arbitration for the address bus, and generation of the PB
address and transfer types.
•
Data Transfer: This section describes arbitration for the data bus, and control of transaction size
and length.
•
Terminations: This section describes the terminations supported by
PowerSpan II, and exception handling.
3.4.1
Address Phase
The address phase deals with the arbitration for the address bus, and generation of the PB address and
transfer types.
3.4.1.1
Address Bus Arbitration and Tenure
The PB Master asserts Address Bus Busy (PB_ABB_) to indicate address bus ownership after it
receives a qualified bus grant for its address bus request. A qualified bus grant assumes the following:
•
address bus grant asserted
•
PB_ARTRY_ negated
•
address bus not busy
The PB Master negates PB_ABB_ for at least one clock after Address Acknowledge (PB_AACK_) has
been asserted by the slave. This is true even if the arbiter parked the bus on PowerSpan II. For example,
in
the bus is parked at the PowerSpan II (PB_BG[1]_ is asserted throughout),
PB_ABB_ is negated the first positive clock edge after sampling PB_AACK_.
The PB Master operates in a multi-processor, cache-coherent PowerPC environment that requires
correct implementation of the
window of opportunity
. The following PB Master behavior supports the
window of opportunity
:
•
respond to PB_ARTRY_ in the address retry window
•
snoop PB_ARTRY_
The PowerSpan II PB Master derives equivalent Address Bus Busy information from
processor bus control signals. This allows the PowerSpan II processor bus arbiter to operate
in 60x environments that do not implement ABB. The PowerQUICC II uses ABB to qualify
address bus grants generated by the system arbiter.