Memory Controller
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
15-53
GCLK2_50 edge as programmed in the RAM word until UPWAIT is negated. This allows wait states to
be inserted as required by an external device through an external signal.
Figure 15-45
shows how the WAEN bit in the word read by the UPM and the UPWAIT signal hold the
UPM in a particular state until UPWAIT is negated. As the example in
Figure 15-45
shows, the CSx and
GPL1 states (C12 and F) and the WAEN value (CC) are frozen until UPWAIT is recognized as deasserted.
Figure 15-45. Wait Mechanism Timing for Internal and External Synchronous Masters
15.6.4.11.2
External Asynchronous Masters
For an external asynchronous master, AS is the external signal that activates the wait mechanism. The
UPM enters a wait state if AS was sampled asserted on the previous falling edge of GLCK2_50 and WAEN
= 1 in the current RAM word. In this wait state, external signals are frozen after the falling edge of
GCLK2_50, as programmed in the RAM word in which WAEN is set. This is demonstrated in the example
in
Figure 15-46
in which the CSx and GPL1 states (C12 and F) and the WAEN value (CC) are frozen until
AS is recognized as deasserted. The TA signal driven by the UPM also remains in its programmed state
until AS is negated. This allows TA to be used as an asynchronous handshake signal by programming UTA
= 0 in the same RAM word in which WAEN = 1. If this is done, TA can be used to signal that AS should
deassert (similar to DTACK in the 68000 bus).
The wait state is exited when AS is negated, at which point all external signals controlled by the UPM are
driven high asynchronously from the AS deassertion. External signals are driven in this state until the
LAST bit is set in a RAM word. The TODT bit is relevant only in words read by the UPM after AS is
negated.
For a comprehensive discussion of external bus interfacing, see
Section 15.8, “External Master Support.”
GCLK2_50
CSx
GPL1
TA
WAEN
UPWAIT
C14
C13
C11
C10
C9
C8
C7
C6
C5
C4
C3
C2
C1
GCLK1_50
CLKOUT
G
E
D
C
B
A
RAM
Word N
RAM
Word N + 1
RAM
Word N + 2
WAIT
WAIT
RAM
Word N+3
C12
F
AA
BB
CC
DD
Summary of Contents for PowerQUICC MPC870
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