7733 datasheet(736/940 Pages) RENESAS

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APPLICATIONS

7735 Group User’s Manual

17–10

(3) Precautions on memory expansion

When specifications of the 7735 Group do not match those of an external memory as described in

the following to , some considerations about the circuit are necessary:

When using an external memory which requires a long address access time (ta(AD))

When using an external memory which outputs data within an interval of tpxz(RDE-DZ) after signal

____

RDE’s

falling edge

When using an external memory which outputs data for more than an interval of tpzx(RDE-DZ)

____

after signal RDE’s rising edge

When using an external memory which requires a long address access time (ta(AD))

When an external memory requires a long address access time (ta(AD)) which does not satisfy the

7735 Group’s tsu(D-RDE), try to lower f(XIN) or extend a bus cycle by inserting a wait.

There are two methods for insertion of a wait: the software wait and the ready function. For the

software wait, refer to section “12.2 Software wait”; for the ready function, refer to section “12.3

Ready function.”

q Wait 1 (Software wait)

____ ____

____

Insert a wait equivalent to one cycle of clock

φ1 while signal RDE/WEL/WEH is at “L”-level.

q Wait 0 (Software wait)

____ ____

____

Insert a wait equivalent to one cycle of clock

φ1 while signal RDE/WEL/WEH is at “H”- and “L”-

levels.

q Ready function

Insert a wait in an arbitrary duration.

Figure 17.1.5 shows a ready generating circuit example (with no wait). In Figure 17.1.5, when

____

f(XIN) > 20.7 MHz, the setup time for the RDY input (tsu(RDY-

φ1)) is insufficient. In this case, refer to

the ready generating circuit example (with wait 1) shown in Figure 17.1.6. In Figure 17.1.6,

tsu(RDY-

φ1) is satisfied when f(XIN)

≤ 25 MHz. Note that a wait generated by the ready function is also

valid for the access to the internal area. Therefore, in Figures 17.1.5 and 17.1.6, areas where the wait

_____

is inserted are specified by using signals RSMP and CS0.

For circuits where the software wait is used, refer to Figures 17.1.2 to 17.1.5.

17.1 Memory expansion

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