User`s manual

Am186™CC Microcontroller Router Reference Design User’s Manual
2-11
Am186
™
CC Microcontroller Clocking
NOTE: See sheet 10 of the schematics for clock information.
The Am186CC microcontroller router reference design uses an internal 2x PLL to
provide a 48-MHz system clock and the required 48-MHz USB clock. The presence
or absence of resistors R51, R52, R53, and R54 determine the CPU and USB
clocking speeds. R52 is at location F-6. R51, R53, and R54 are located at location
E-8. If the resistor is present, the associated CPU signal is pulled Low. If absent,
the signal is High. Table 2-5 and Table 2-6 list resistor population information for
CPU PLL and USB PLL clock modes. The Am186CC microcontroller router
reference design only has R54 populated.
On reset or when booting up, the CPU reads its HLDA (CLKSEL1) and PCS4
(CLKSEL2) pins for the CPU clocking speed and PCS0
(USBSEL1) and PCS1
(USBSEL2) for the USB clocking speed.
Table 2-5. CPU PLL Clock Modes
CLKSEL1 (R52) CLKSEL2 (R51) CPU PLL Mode
1 1 2x PLL (default)
1 0 4x PLL
0 1 1x PLL
0 0 PLL bypass
Table 2-6. USB PLL Clock Modes
USBSEL1 (R54) USBSEL2 (R53) USB PLL Mode
1 1 Use CPU clock, USBPLL
disabled
1 0 4x PLL
0 1 2x PLL (default)
0 0 PLL bypass
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