Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 493

Powerquicc family

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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

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

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

For a comprehensive discussion of external bus interfacing, see

Freescale Semiconductor

MPC885 PowerQUICC Family Reference Manual, Rev. 2

Figure 15-45

Section 15.8, "External Master Support."

Memory Controller

shows, the CSx and