JAJSQ97J may 2009 – january 2021 XIO2001
PRODUCTION DATA
Because of the shared bus nature of PCI, it is required that if the bus is idle at a given time that some device on the bus must drive some signals to stable states. These signals are the address/data lines, the command/byte enables, and a valid parity. If no devices are requesting use of the bus, it is the responsibility of the arbiter to assign ownership of the bus so that the bus signals are never floating while in idle states.
If the XIO2001 internal arbiter is enabled then there are two modes supported for bus parking. The default mode for bus parking is for the arbiter to continue to assert GNT for the last bus master. In this mode once a device has completed its transaction, the arbiter will continue to assert the GNT for that bus master and that device is required to drive a stable pattern onto the required signals. This will continue until another device requests use of the bus resulting in the arbiter removing GNT from the current bus owner grants it to the new requestor.
Alternatively, the XIO2001 can be configured to self-park. In this mode if no other devices have their REQ asserted, the XIO2001 will remove GNT from the current bus owner and drive a stable pattern onto the required lines.
It is suggested that implementations use the default mode of bus parking. The PCI Specification recommends leaving the current GNT signal asserted if no devices are asserting REQ. Some PCI bus masters will release their REQ signals after having begun a transaction, even if that transaction may require the use of the bus for an extended time. If the XIO2001 self-parks the bus, then these bus masters will have their transaction lengths limited to the latency timer setting. This may result in increased arbitration, higher overhead for transactions, and decreased bus performance.