MPC555 / MPC556
MEMORY ACCESS TIMING
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
F-1
APPENDIX F
MEMORY ACCESS TIMING
F.1 Introduction
lists all possible memory access timing to internal and external memory
combinations. The clock values show the number of clocks from the moment an ad-
dress is valid on a specific bus, until data is back on that same bus. The following as-
sumptions were used when compiling the information:
• The arbitration time was ignored. The values assume that the bus (or buses) in-
volved in a transaction was in the IDLE state when the transaction needs that bus.
• The UIMB works in a mode of 1:1. This is relevant for IMB accesses values. In
the case of 2:1 mode, the clock latency for a cycle on the IMB should be doubled.
(each IMB access takes two clocks.)
• The basic delay of external bus to U-bus is four clocks (external master case).
• All IMB accesses are assumed to be 16-bit accesses only. If 32-bit accesses are
used, then each such IMB access is split into two separate 16-bit cycles with nor-
mal IMB performance for each.
Table F-1 Memory Access Times Using Different Buses
INTERNAL
EXTERNAL RAM/
FLASH
SHOW CYCLE
FLASH
RAM
IMB
SIU
Internal
Memory
Mapped
External
Non-
mapped
Internal
Memory
Write
Read
RCPU Load/Store
3/4
1
NOTES:
1. “/” comes for on/off page flash access.
1
6
5
4+N
2
2. N is the number of clocks from external address valid till external data valid in the case of read cycle. In the case
of zero wait states, N = 2.
4+N
2
2
RCPU Instruction
Fetches
2-1-1-1-1...
3
3
3. Assuming BBC is parked on U-BUS.
-
-
2+N
2+N
-
1
4
4. Until address is valid on external pins
Peripheral Mode
(ony ext. master
is active)
4/5
6
7
6
Slave Mode
(both ext. & int.
CPUs are active)
5/6
7
8
7
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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