MPC555
/
MPC556
EXTERNAL BUS INTERFACE
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
9-47
causes a transfer error acknowledge. See
9.5.2.3 Single Beat Flow with Small Port
.
9.5.10.2 Termination Signals Protocol Summary
summarizes how the MPC555
/ MPC556 recognizes the termination signals
provided by the slave device that is addressed by the initiated transfer.
9.5.11 Bus Operation in External Master Modes
When an external master takes ownership of the external bus and the MPC555
/
MPC556 is programmed for external master mode operation, the external master can
access the internal space of the MPC555
/ MPC556 (see
). In an external master mode, the external master owns the bus, and the direc-
tion of most of the bus signals is inverted, relative to its direction when the MPC555
/
MPC556 owns the bus.
The external master gets ownership of the bus and asserts TS in order to initiate an
external master access. The access is directed to the internal bus only if the input ad-
dress matches the internal address space. The access is terminated with one of the
followings outputs: TA, TEA, or RETRY. If the access completes successfully, the
MPC555
/ MPC556 asserts TA, and the external master can proceed with another ex-
ternal master access or relinquish the bus. If an address or data error is detected in-
ternally, the MPC555
/ MPC556 asserts TEA for one clock. TEA should be negated
before the second rising edge after it is sampled asserted in order to avoid the detec-
tion of an error for the next bus cycle initiated. TEA is an open drain pin, and the ne-
gation timing depends on the attached pullup. The MPC555
/ MPC556 asserts the
RETRY signal for one clock in order to retry the external master access.
If the address of the external access does not match the internal memory space, the
internal memory controller can provide the chip-select and control signals for accesses
that belong to one of the memory controller regions. This feature is explained in
and
illustrate the basic flow of read and write external master
accesses.
Table 9-9 Termination Signals Protocol
TEA
TA
RETRY
Action
Asserted
X
X
Transfer error termination
Negated
Asserted
X
Normal transfer termination
Negated
Negated
Asserted
Retry transfer termination
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
.
..