8-34
MPC561/MPC563 Reference Manual
MOTOROLA
Clocks Unit Programming Model
10
PRQEN
Power management request enable
0 Remains in the lower frequency (defined by DFNL) even if the power management bit in
the MSR is reset (normal operational mode) or if there is a pending interrupt from the
interrupt controller
1 Switches to high frequency (defined by DFNH) when the power management bit in the
MSR is reset (normal operational mode) or there is a pending interrupt from the interrupt
controller
11
RTSEL
RTC circuit input source select. At power-on reset RTSEL receives the value of the
MODCK1 signal. Refer to
Table 8-1. Note that if the chip is operating in limp mode (BUCS
= 0), the RTSEL bit is ignored, and the backup clock is the clock source for the RT and PIT
clocks
0 OSCM clock is selected as input to RTC and PIT
1 EXTCLK clock is selected as the RTC and PIT clock source
12
BUCS
Backup clock status. This status bit indicates the current system clock source. When loss
of clock is detected and the LME bit is set, the clock source is the backup clock and this bit
is set. When the STBUC bit and LME bit are set, the system switches to the backup clock
and BUCS is set.
0 System clock is not the backup clock
1 System clock is the backup clock
13:14
EBDF[0:1]
External bus division factor. These bits define the frequency division factor between
(GCLK1 and GCLK2) and (GCLK1_50 and GCLK2_50). CLKOUT is similar to GCLK2_50.
The GCLK2_50 and GCKL1_50 are used by the external bus interface and controller in
order to interface to the external system. The EBDF bits are initialized during hard reset
using the hard reset configuration mechanism.
00 CLKOUT is GCKL2 divided by 1
01 CLKOUT is GCKL2 divided by 2
1x Reserved
Note: If EBDF > 0, an external burst access with short setup timing will corrupt any USIU
15
LME
Limp mode enable. When LME is set, the loss-of-clock monitor is enabled and any
detection of loss of clock will switch the system clock automatically to backup clock. It is
also possible to switch to the backup clock by setting the STBUC bit.
If LME is cleared, the option of using limp mode is disabled. The loss of clock detector is
not active, and any write to STBUC is ignored.
The LME bit is writable once, by software, after power-on reset, when the system clock is
not backup clock (BUCS = 0).
During power-on reset, the value of LME is determined by the MODCK[1:3] bits. (Refer to
0 Limp mode disabled
1 Limp mode enabled
16:17
EECLK[0:1]
Enable engineering clock. This field controls the output buffer voltage of the ENGCLK pin.
When both bits are set the ENGCLK pin is held in the high state. These bits can be
dynamically changed without generating spikes on the ENGCLK pin. If ENGCLK is not
connected to external circuits, set both bits (disabling ENGCLK) to minimize noise and
power dissipation. For measurement purposes the backup clock (BUCLK) can be driven
externally on the ENGCLK pin.
00 Engineering clock enabled, 2.6 V output buffer
01 Engineering clock enabled (slew rate controlled), 5 V output buffer
10 BUCLK is the output on the ENGCLK 2.6 V output buffer
11 Engineering clock disabled
Table 8-9. SCCR Bit Descriptions (continued)
Bits
Name
Description