PC87570 datasheet(36/168 Pages) NSC

36 / 168 page

Bus Interface Unit (BIU)

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3.3.3

Early Write Bus Cycle

If the BCFG.EWR configuration bit is 1, the BIU uses early

write bus cycles; this allows removal of the RD signal from

the memory device interface. The basic early write bus cy-

cle takes three clock cycles.

The cycle starts at T1, when the data bus is in TRI-STATE

and the address is placed on the address bus. RD is inac-

tive to indicate that this is a write bus cycle; then WR0-1 are

activated.

At the first TIW, or T2 (when there are no TIW cycles), the

data is placed on the data bus and the SELn (or SELIO) is

activated. The bus transaction is terminated at T3, when

SELn (or SELIO) becomes inactive; then WR0-1 become

inactive and the data bus is put in TRI-STATE. The address

remains valid until T3 is complete.

Thold clock cycles may follow T3, according to SZCF-

Gn.HOLD or IOCFG.HOLD (may be 0). The address and

data remains valid until the end of the last Thold cycle. The

data is put in TRI-STATE in the clock cycle after the last

Thold or T3 (if no Thold cycle is configured). See Figures 3-1,

3-2 and 3-3.

If a read bus cycle immediately follows an Early Write bus

cycle, an idle cycle is added between the two.

SZCFGn.HOLD

≠ 0

SZCFGn.WAIT

≠ 0

SELn: inactive, WR0-1: inactive,

If SZCFGn.HOLD = 0 data put in TRI-STATE.

SZCFGn.WAIT = 0

Hold cycles completed

Address placed on A0-15,

WR(0-1): activated

Data placed

on D0-15,

SELn: active

Internal waits completed

SZCFGn.HOLD=0

Address on A0-15 invalid/changed

Data put in TRI-STATE

Data placed on D0-15,

SELn: active

Internal waits corresponding to SZCFGn.WAIT

Thold

Hold cycles according to SZCFGn.HOLD

TIW

end

begin

Figure 3-1. Early Write Bus Cycle

Note: Any reference to SZCFGn, also applies to the IOCFG Register.

Any reference to SELn, also applies to the SELIO signal.

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