MPC555
/
MPC556
EXTERNAL BUS INTERFACE
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
9-33
Figure 9-25 Bus Arbitration Timing Diagram
9.5.6.4 Internal Bus Arbiter
The MPC555
/ MPC556 can be configured at system reset to use the internal bus ar-
biter. In this case, the MPC555
/ MPC556 will be parked on the bus. The parking fea-
ture allows the MPC555
/ MPC556 to skip the bus request phase, and if BB is negated,
assert BB and initiate the transaction without waiting for BG from the arbiter.
The priority of the external device relative to the internal MPC555
/ MPC556 bus mas-
ters is programmed in the SIU module configuration register. If the external device re-
quests the bus and the MPC555
/ MPC556 does not require it, or if the external device
has higher priority than the current internal bus master, the MPC555
/ MPC556 grants
the bus to the external device.
describes the priority mechanism used by the internal arbiter.
CLKOUT
BR0
BG1
ADDR & Attr.
BG0
BR1
Master 0
“Turns On” and
Drives Signals
Master 0
Negates BB
and “Turns Off”
(Three-state Controls)
BB
TS
TA
Master 1
“Turns On” and
Drives Signals
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
.
..