PowerPC™ 405 Processor Block Reference Guide www.xilinx.com 109
UG018 (v2.0) August 20, 2004 1-800-255-7778
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DCR Interface 1:2 Clocking, Latched Acknowledge

The example in Figure 2-36 assumes the following:
xThe PowerPC 405 DCR interface is clocked at half the frequency of the peripheral
containing the addressed DCR.
xThe acknowledge signal is latched and forwarded with the DCR bus as shown in
Figure 2-31, page 103.
xAfter the acknowledge signal is asserted, it is not deasserted until the appropriate
read-access or write-access request signal is deasserted.

External DCR Timing Consideration (Virtex-II Pro/ProX Only)

Users need to be aware that there is no DCR clock input to the processor block of the
Virtex-II Pro and Virtex-II ProX devices. When dealing with signals that cross CPU clock
domain and DCR clock domain, users may want to add re-synchronization flip-flops to
simply timing constraints, or set up appropriate multi-cycle/false path constraints in the
UCF file.
An example for the re-synchronization of DCR interface can be found in Xilinx Embedded
Development Kit (EDK). Please refer to the Virtex-II Pro PowerPC405 wrapper IP in the
“Processor IP Reference Guide” for details.
The Virtex-4-FX family does have a DCR clock input and does not have the
synchronization issues mentioned here.
External Interrupt Controller Interface
The PowerPC embedded-environment architecture defines two classes of interrupts:
critical and noncritical. The interrupt handler for an external critical interrupt is located at
exception-vector offset 0x0100. The interrupt handler for an external noncritical interrupt
is located at exception-vector offset 0x0200. Generally, the processor prioritizes critical
Figure 2-36: DCR Inter face 1:2 Clocking, Latched Acknowledge
Cycle
CPMC405CLOCK (Virtex-II Pro)/
CPMDCRCLK (Virtex-4 FX)
DCR (FPGA) Clock
UG018_44_032504
DCRWRITE/DCRREAD
DCRABUS[0:9]
PPC405 Outputs:
DCRDBUSOUT[0:31]
DCRDBUSIN[0:31]
DCRACK
DCR Outputs:
data0 data1
addr0 addr1
data0 data1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Note: Abbreviated signal names are used.