DP83936AVF datasheet(70/104 Pages) NSC

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70 Bus Interface (Continued)

7372 Slave Cycle for BMODE e 0

The system accesses the SONIC-T by driving SAS CS

SWR and RAk50l These signals will be sampled each

bus cycle but the SONIC-T will not actually start a slave

cycle until CS has been sampled low and SAS has been

sampled high CS should not be asserted low before the

falling edge of SAS as this will cause improper slave opera-

tion CS may be asserted low however before the rising

edge of SAS In this case it is suggested that SAS be driven

high within one bus clock after the falling edge of CS Be-

tween one and two bus clocks after the assertion of CS

once SAS has been driven high SMACK will be driven low

to signify that the SONIC-T has started the slave cycle Al-

though CS is an asynchronous input meeting its setup time

(as shown in

Figures 7-21 and 7-22 ) will guarantee that

SMACK which is asserted off a falling edge will be assert-

ed 1 bus clock after the falling edge that CS was clocked in

on This is assuming that the SONIC-T is not a bus master

when CS is asserted If the SONIC-T is a bus master then

when CS is asserted the SONIC-T will complete its current

master bus cycle and get off the bus temporarily (see Sec-

tion 738) In this case SMACK will be asserted 5 bus

clocks after the falling edge that CS was clocked in on This

is assuming that there were no wait states in the current

master mode access Wait states will increase the time for

SMACK to go low by the number of wait states in the cycle

If the slave access is a read cycle

(Figure 7-21) then the

data will be driven off the same edge as SMACK Ifitisa

write cycle

(Figure 7-22) then the data will be latched in

exactly 2 bus clocks after the assertion of SMACK In either

case RDYo is driven low 25 bus clocks after SMACK to

terminate the slave cycle For a read cycle the assertion of

RDYo indicates valid register data and for a write cycle the

assertion indicates that the SONIC-T has latched the data

The SONIC-T deasserts RDYi SMACK and the data if the

cycle is a read cycle at the falling edge of SAS or the rising

edge of CS depending on which is first

Note 1

The SONIC-T transfers data only on lines Dk150l during slave

mode accesses

Note 2

For multiple register accesses CS can be held low and SAS can be

used to delimit the slave cycle (this is the only case where CS may

be asserted before SAS) In this case SMACK will be driven low

due to SAS going high since CS has already been asserted Notice

that this means SMACK will not stay asserted low during the entire

time CS is low (as is the case for MREQ Section 738)

Note 3

If memory request (MREQ) follows a chip select CS it must be

asserted at least 2 bus clocks after CS is deasserted Both CS and

MREQ must not be asserted concurrently

Note 4

When CS is deasserted it must remain deasserted for at least one

bus clock

Note 5

The way in which SMACK is asserted due to CS is not the same as

the way in which SMACK is asserted due to MREQ The assertion of

SMACK is dependent upon both CS and SAS being low not just CS

This is not the same as the case for MREQ (see Section 738) The

assertion of SMACK in these two cases should not be confused

TLF12597 – 48

FIGURE 7-21 Register Read BMODE e 0

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DP83936AVF Datasheet(PDF) 70 Page - National Semiconductor (TI)

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