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Clocks and Power Control
5-14
MPC801 USER’S MANUAL
MOTOROLA
5
TBS—Timebase Source
This bit determines the clock source that drives the timebase and decrementer.
0 = TB frequency source is the crystal oscillator frequency divided by 4 or 16.
1 = TB frequency source is the system clock divided by 16.
RTDIV—Real-Time Clock Divide
This bit indicates if the clock to the real-time clock and periodic interrupt timer is additionally
divided by 128. At power-on reset this bit is cleared if both MODCK[1] and MODCK[2] are
zeros. Otherwise, it is set.
0 = The real-time clock and periodic interrupt timer are divided by 4.
1 = The real-time clock and periodic interrupt timer are divided by 512.
RTSEL—Real-Time Clock Circuit Input Source Select
This bit specifies the input source to the real-time clock. At power-on reset, this bit receives
the value of the MODCK[1] bit.
0 = The real-time clock and periodic interrupt timer are divided by 4.
1 = The real-time clock and periodic interrupt timer are divided by 512.
PRQEN—Power Management Request Enable
This bit specifies whether or not the general system clock returns to the high frequency
defined by DFNH while there is a pending interrupt from the interrupt controller or POW bit
in the machine state register is clear. This bit is cleared by power-on or hard reset.
0 = The system remains in the lower frequency defined by DFNL even if there is a
pending interrupt from the interrupt controller or POW bit in the machine state
register is cleared.
1 = The system switches to high frequency defined by DFNH when there is a pending
interrupt from the interrupt controller or POW bit in the machine state register is
cleared.
Bits 11–12 and 15–16—Reserved
These bits are reserved and should be set to 0.
EBDF—External Bus Division Factor
These bits define the frequency division factor between GCLK1/GCLK2 and GCLK1_50/
GCLK2_50. CLKOUT is similar to GCLK2_50. The GCLK2_50 and GCLK1_50 are used by
the external master, bus interface, and memory controller to interface with the external
system. These bits are initialized during HRESET using the hard reset configuration
mechanism.
00 = CLKOUT is GCLK2 divided by 1.
01 = CLKOUT is GCLK2 divided by 2.
1x = Reserved.