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21555 Non-Transparent PCI-to-PCI Bridge User Manual 89
Parallel ROM Interface
8.7 Attaching Additional Devices to the ROM Interface
The 21555 allows additional devices to be attached to the ROM interface. Two ROM interface signals are slightly
redefined to support multiple devices by setting the Multiple Device Enable bit in the Chip Control 0 configuration
register. In this mode, the maximum ROM size is reduced because the upper address lines are used to decode device
select lines.
For this mode of operation, an external decoder is needed to decode the upper address bits into device select lines.
The pr_ale_l signal is used to enable the device select decoder, in addition to enabling the clocking of the address
registers. When pr_ale_l is low, the address registers are enabled, and when pr_ale_l is high, the device select
decoder is enabled.
Note: Signal pr_ale_l must be driven low when the serial ROM is used to disable all other device select
lines when an external decoder is used.
In addition, in this mode the pr_cs_l output pin is redefined to be a device ready input (pr_rdy). When pr_rdy is
deasserted, the device select signal (controlled by pr_ale_l) and read or write strobe assertions are extended until
pr_rdy is asserted again. The read and write strobes are deasserted upon detection of chip select deassertion (fall
time for pr_ale_l) with the hold time specified by the strobe mask.
Note: When multiple devices are attached and multiple device mode is used, signal pr_cs_l should be
pulled up through an external resistor.
Figure 18 illustrates how the interface is connected for use with multiple devices.