MPC555
/
MPC556
CLOCKS AND POWER CONTROL
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
8-14
Figure 8-8 Clock Source Flow Chart
NOTES
BUCLK_ENABLE = (STBUC | LOC) & LME lock indicates loss of lock
status bit (LOCS) for all cases and loss of clock sticky bit (LOCSS)
when state 3 is active. When BUCLK_ENABLE is changed, the chip
asserts HRESET to switch the system clock to BUCLK or PLL.
At PORESET negation, if the PLL is not locked, the loss-of-clock
sticky bit (LOCSS) is asserted, and the chip should operate with BU-
CLK.
hr
es
et
_b =
1
bu
cl
k_en
abl
e =
1
ass
er
t hr
es
et
_b
buclk-enable = 1
& hreset_b = 0
LM
E =
1
po
re
se
t_b
= 0
1,BUCLK
2,BUCLK
5, osc
poreset_b = 1
LME = 1
3,BUCLK
4,osc
6,BULCK
po
re
se
t_
b
= 0
h
rese
rt_b =
0
hr
es
et_b
=
1
bu
clk
_en
able
=
1
hr
es
et
_b
=
1
bu
clk
_e
na
ble
=
0
hre
se
t_
b =
1
else
hreset_b = 0
LOCS
LME = 0
buclk_enable = 0
& hreset_b = 0
else
buclk_enable=0
& hreset_b=0
buclk_enable = 1
& hreset_b = 0
else
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
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c
.
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