Memory Controller
MPC885 PowerQUICC Family Reference Manual, Rev. 2
15-54
Freescale Semiconductor
Figure 15-46. Wait Mechanism Timing for an External Asynchronous Master
15.7
Handling Devices with Slow or Variable Access Times
The memory controller provides two ways to interface with slave devices that are very slow (access time
is greater than the maximum allowed by the user programming model) or cannot guarantee a predefined
access time (for example some FIFO, hierarchical bus interface, or dual-port memory devices).
These mechanisms are as follows:
•
The wait mechanism—Used only in accesses controlled by the UPM. MAMR[GPLA4DIS] and
MBMR[GPLB4DIS] enable this mechanism.
•
The external TA mechanism is used only in accesses controlled by the GPCM. ORx[SETA]
specifies whether TA is generated internally or externally.
The following sections describe how the two mechanisms work.
15.7.1
Hierarchical Bus Interface Example
Assume that the CPU initiates a local-bus read cycle that addresses main memory connected to the system
bus. The hierarchical bus interface accepts local bus requests and generates a read cycle on the system bus.
The programmer cannot predict when valid data can be latched by the CPU because a DMA device may
be occupying the system bus.
•
The wait solution (UPM)—The external module asserts UPWAIT to the memory controller to
indicate that data is not ready. The memory controller synchronized this signal because the wait
signal is asynchronous. As a result of the wait signal being asserted, the UPM enters a freeze mode
GCLK2_50
CSx
GPL1
TA
WAEN
AS
C11
C10
C9
C8
C7
C6
C5
C4
C3
C2
C1
GCLK1_50
CLKOUT
E
D
C
B
A
RAM
Word N
RAM
Word N + 1
RAM
Word N + 2
WAIT
WAIT
RAM
Word N+3
AA
BB
CC
DD
C12
F
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