PC87570-ICCSLASHVUL datasheet(42/168 Pages) NSC

42 / 168 page

Bus Interface Unit (BIU)

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3.3.6

Fast Read Bus Cycle

When SZCFGn.FRE is 1, the fast read bus cycle is enabled

for zone “n”. The fast read bus cycle takes one clock cycle.

At the beginning of the T1-2 clock cycle the address is

placed on the address bus and SELn and RD are activated.

WR(0-1) are inactive, indicating a read bus cycle. At the end

of the clock cycle, the BIU samples the data. SELn and RD

deactivate in the following clock cycle, unless another read

from the same zone follows. If a write to the same zone fol-

lows, and late write is configured, SELn remains activated.

The address remains valid until the beginning of the clock

cycle after the T1-2 clock cycle.

The fast read bus cycle cannot be extended by adding wait

cycles (SZCFGn.WAIT is ignored during this bus cycle). Ad-

ditionally, hold cycles cannot be added (SZCFGn.HOLD is

also ignored). When a write bus cycle consecutively pre-

cedes a fast read bus cycle, an idle clock cycle is forced be-

tween the two. See Figure 3-12.

When the core attempts to access more bytes (i.e., a word)

than the configured bus width, the transaction is broken up

into “basic” (T1-2) bus cycles.

CLK

A0-18

SELn

D0-7

WR0-1

Bus State

T2B

BST0-2

Figure 3-10. Normal Read Bus Cycle with 0 Wait on Burst

CLK

A0-18

WR0-1

D0-7

TIW

TBW

SELn

T2B

BST0-2

Figure 3-11. Normal Read Bus Cycle with 2 Internal Waits and 1 Wait on Burst

PC87570-ICC/VUL Datasheet(PDF) 42 Page - National Semiconductor (TI)

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