Functional Description

Configuration mechanism #1 uses an address register/data register format. Performing a configuration access is a two-step process. The first step is to place the address of the configuration cycle within the CONFIG_ADDRESS register. Note that this action does not generate any cycles on the PCI bus. The second step is to either read or write configuration data into the CONFIG_DATA register. If the CONFIG_ADDRESS register has been set up correctly, the Raven will pass this access on to the PCI bus as a configuration cycle.

The addresses of the CONFIG_ADDRESS and CONFIG_DATA registers are actually embedded within PCI I/O space. If the CONFIG_ADDRESS register has been set incorrectly or the access to either the CONFIG_ADDRESS or CONFIG_DATA register is not 1,2, or 4 bytes wide, the Raven will pass the access on to PCI as a normal I/O Space transfer.

The CONFIG_ADDRESS register is located at offset $CF8 from the bottom of PCI I/O space. The CONFIG_DATA register is located at offset $CFC from the bottom of PCI I/O space. The Raven address decode logic has been designed such that MSADD3 and MSOFF3 must be used for mapping to PCI Configuration (consequently I/O) space. The MSADD3/MSOFF3 register group is initialized at reset to allow PCI I/O access starting at address $80000000. The powerup location (that is, little- endian disabled) of the CONFIG_ADDRESS register is $80000CF8, and the CONFIG_DATA register is located at $80000CFC.

The CONFIG_ADDRESS register must be prefilled with four fields:

1.Register Number

2.Function Number

3.Device Number

4.Bus Number

The Register Number and Function Number are passed along to the PCI bus as portions of the lower address bits.

When performing a configuration cycle, the Raven uses the upper 20 address bits as IDSEL lines. During the address phase of a configuration cycle, only one of the upper address bits will be set. The device that has its

2

http://www.motorola.com/computer/literature

2-23

Page 93
Image 93
Motorola MVME2300 Series manual Functional Description