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Functional Description 4–13
System Clocks
Figure 4–6 AlphaPC 164SX System Clocks
At system reset, the clock synthesizer is programmed to provide a 400-MHz clock to
the 21164PC
. The IRQ pins in the 21164PC are either pulled up or down to set the
internal driver to divide by 6, providing a system clock of 66 MHz (sys_clk_out1).
If an operating frequency other than 400 MHz is selected by the configuration
switches, the boot code changes the synthesizer output; the 21174 drives the correct
divide ratio onto the IRQ lines and resets the CPU. If an invalid speed setting is
selected, the system defaults to the operating speed of 400 MHz.
DIMM0
Microprocessor
Clock
Synthesizer
21164PC
Microprocessor
PCI
Slots
ISA
Slots
dram_clkax2
dram_clkbx2
dram_clkcx2
dram_clkdx2
pciclk_slot0
pciclk_slot1
pciclk_slot2
pciclk_slot3
14mhz_out
osc
sysclk
pciclk_sio
clk_in_l
clk_in_h
irq_h[3:0]
CY2308
PLL
DIMM1
DIMM2
DIMM3
21174
Bridge
14.3-MHz
Oscillator
37C669
Comb.
Controller
Page
Boundary
PAL
cyb_pal_clk