MPC555
/
MPC556
CLOCKS AND POWER CONTROL
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
8-23
Figure 8-11 External Power Supply Scheme
The MPC555 / MPC556 asserts the TEXP signal, if enabled, when the RTC or TB time
value matches the value programmed in the associated alarm register or when the PIT
or DEC value reaches zero. The TEXP signal is negated when the TEXPS status bit
is written to one.
The KAPWR power supply feeds the main crystal oscillator (OSCM). The condition for
the main crystal oscillator stability is that the power supply value changes slowly. The
maximum slope must be less than 5 mV per oscillation cycle (
τ
> 200-300/freq
oscm
).
8.9.3.2 Keep Alive Power Registers Lock Mechanism
The USIU timer, clocks, reset, power, decrementer, and time base registers are pow-
ered by the KAPWR supply. When the main power supply is disconnected after power-
down mode is entered, the value stored in any of these registers is preserved. If pow-
er-down mode is not entered before power disconnect, there is a chance of data loss
in these registers. To minimize the possibility of data loss, the MPC555 / MPC556 in-
cludes a key mechanism that ensures data retention as long as a register is locked.
While a register is locked, writes to this register are ignored.
Each of the registers in the KAPWR region have a key that can be in one of two states:
open or locked. At power-on reset the following keys are locked: RTC, RTSEC, RT-
Main Power
Backup
VDDSYN
3.3 V
VDD
KAPWR
Supply
MPC555
Switch
Logic
TEXP
3 V - 3.3 V
SW1
SW2
Power
Supply
VDDSRAM
3.3 V
Power Supply
VDDSRAM
O
O
O
O
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
.
..