10. Reset and Clocking > Clocking
91
PEB383 User Manual
July 25, 2011
Integrated Device Technology, Inc.
Confidential - NDA Required
10.2.2
PCI Clocking
The PCI clocking for the PEB383 is shown in
. The PEB383 supports clock master and slave
mode, and is configured by the PCB design. The bridge drives up to four external clocks,
PCI_CLKO[3:0], which are individually enabled through the
“Clock Out Enable Function and Debug
. PCI_CLKO[4] can be used for external clock compensation.
Figure 27: PCI Clocking
10.2.2.1
Master Mode Clocking
Master mode clocking is provided by the PEB383. PCI_CLKO[4:0] is generated from PCIE_REFCLK
and a programmable PLL. The decoder sets the divider ratios for programmable PLL as a function of
PCI_M66EN, and the
“PCI Miscellaneous Clock Straps Register”
. PCI_M66EN selects 66 MHz when
high, and 33 MHz when low. The
“PCI Miscellaneous Clock Straps Register”
allows this pin to be
overwritten, and one of the following speeds used: 25, 33, 50, and 66 MHz.
Prior to the configuration of the PCI bus speed, the PCI clock is in bypass mode, which generates a
25-MHz clock on the PCI bus. After the release of reset, the PLL locks to a new frequency based on the
value of the PCI_M66EN signal (see
Table 27: PCI Clocking
PCI Bus Rate
PCI_M66EN Signal
25 MHz
Requires software configuration
a
33 MHz
0
50 MHz
Requires software configuration
66 MHz
1
PCI E_REFCLK_p
PCI E_REFCLK_n
PCI _CLKO[4:0]
PCI_M66EN
P CI_CLK
Dec oder
Logic
Internal CLK
CLK tree
100 MHz
Pr ogr am ma ble
PLL
PWRUP_P LL_B YPASS